Re: [Python-Dev] PEP 418: Add monotonic clock
On 3/27/2012 8:36 PM, Victor Stinner wrote: Scott wrote: Scott monotonic_clock = always goes forward but can be adjusted Scott steady_clock = always goes forward and cannot be adjusted I don't know if the monotonic clock should be called time.monotonic() or time.steady(). The clock speed can be adjusted by NTP, at least on Linux 2.6.28. I don't know if other clocks used by my time.monotonic() proposition can be adjusted or not. If I understand correctly, time.steady() cannot be implemented using CLOCK_MONOTONIC on Linux because CLOCK_MONOTONIC can be adjusted? Does it really matter if a monotonic speed is adjusted? You are right that CLOCK_MONOTONIC can be adjusted, so the Boost implementation is wrong. I'm not sure that CLOCK_MONOTONIC_RAW is right either due to suspend -- there doesn't appear to be a POSIX or Linux clock that is defined that meets the steady definition. I am not familiar enough with Windows or Mac to know for certain whether the Boost implementation has the correct behaviors either. With that in mind, it's certainly better that we just provide time.monotonic() for now. If platform support becomes available, then we can expose that as it becomes available in the future. In other words, at this time, I don't think time.steady() can be implemented faithfully for any platform so lets just not have it at all. In that case, I don't think time.try_monotonic() is really needed because we can emulate time.monotonic() in software if the platform is deficient. I can't imagine a scenario where you would ask for a monotonic clock and would rather have an error than have Python fill in the gap with an emulation. -- Scott Dial sc...@scottdial.com ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 418: Add monotonic clock
On 28.03.2012 06:45, Nick Coghlan wrote: On Wed, Mar 28, 2012 at 10:36 AM, Victor Stinner victor.stin...@gmail.com wrote: If QueryPerformanceCounter() is monotonic, the API can be simplified to: * time.time() = system clock * time.monotonic() = monotonic clock * time.hires() = monotonic clock or fallback to system clock time.hires() definition is exactly what I was trying to implement with time.steady(strict=True) / time.try_monotonic(). Please don't call the fallback version hires as it suggests it may be higher resolution than time.time() and that's completely the wrong idea. It's also a completely ugly name, since it's quite hard to figure out what it is supposed to stand for in the first place. Georg ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 418: Add monotonic clock
* time.time() = system clock * time.monotonic() = monotonic clock * time.hires() = monotonic clock or fallback to system clock time.hires() definition is exactly what I was trying to implement with time.steady(strict=True) / time.try_monotonic(). Please don't call the fallback version hires as it suggests it may be higher resolution than time.time() and that's completely the wrong idea. Why would it be a wrong idea? On Windows, time.monotonic() frequency is at least 1 MHz (can be GHz if it uses your CPU TSC) whereas time.time() is only updated each millisecond at the best case (each 15 ms by default if I remember correctly). On UNIX, CLOCK_MONOTONIC has the same theorical resolution than CLOCK_REALTIME (1 nanosecond thanks to the timespec structure) and I expect the same accuracy. On Mac, I don't know if mach_absolute_time() is more or as accurate than time.time(). time.hires() uses time.monotonic() if available, so if time.monotonic() has an higher resolution than time.time(), time.hires() can also be called a high-resolution clock. In practice, time.monotonic() is available on all modern platforms. If we're simplifying the idea to only promising a monotonic clock (i.e. will never go backwards within a given process, but may produce the same value for an indefinite period, and may jump forwards by arbitrarily large amounts), I don't know any monotonic clock jumping forwards by arbitrarily large amounts. Linux can change CLOCK_MONOTONIC speed, but NTP doesn't jump. then we're back to being able to enforce monotonicity even if the underlying clock jumps backwards due to system clock adjustments. Do you know a monotonic clock that goes backward? If yes, Python might workaround the clock bug directly in time.monotonic(). But I would prefer to *not* workaround OS bugs. Victor ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 418: Add monotonic clock
Scott monotonic_clock = always goes forward but can be adjusted Scott steady_clock = always goes forward and cannot be adjusted I don't know if the monotonic clock should be called time.monotonic() or time.steady(). The clock speed can be adjusted by NTP, at least on Linux 2.6.28. (...) You are right that CLOCK_MONOTONIC can be adjusted, so the Boost implementation is wrong. I'm not sure that CLOCK_MONOTONIC_RAW is right either due to suspend -- there doesn't appear to be a POSIX or Linux clock that is defined that meets the steady definition. The term adjusted should be clarified. A clock can be adjusted by setting its counter (e.g. setting the system date and time) or by changing temporary its frequency (to go faster or slower). Linux only adjusts CLOCK_MONOTONIC frequency but the clock is monotonic because it always goes forward. The monotonic property can be described as: t1=time.monotonic() t2=time.monotonic() assert t2 = t1 In that case, I don't think time.try_monotonic() is really needed because we can emulate time.monotonic() in software if the platform is deficient. time.hires() is needed when the OS doesn't provide any monotonic clock and because time.monotonic() must not use the system clock (which can jump backward). As I wrote, I don't think that Python should workaround OS bugs. If the OS monotonic clock is not monotonic, the OS should be fixed. I can't imagine a scenario where you would ask for a monotonic clock and would rather have an error than have Python fill in the gap with an emulation. Sorry, I don't understand what you mean with fill in the gap with an emulation. You would like to implement a monotonic clock based on the system clock? Victor ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 418: Add monotonic clock
On 3/28/2012 4:48 AM, Victor Stinner wrote: Scott monotonic_clock = always goes forward but can be adjusted Scott steady_clock = always goes forward and cannot be adjusted I don't know if the monotonic clock should be called time.monotonic() or time.steady(). The clock speed can be adjusted by NTP, at least on Linux 2.6.28. (...) You are right that CLOCK_MONOTONIC can be adjusted, so the Boost implementation is wrong. I'm not sure that CLOCK_MONOTONIC_RAW is right either due to suspend -- there doesn't appear to be a POSIX or Linux clock that is defined that meets the steady definition. The term adjusted should be clarified. A clock can be adjusted by setting its counter (e.g. setting the system date and time) or by changing temporary its frequency (to go faster or slower). Linux only adjusts CLOCK_MONOTONIC frequency but the clock is monotonic because it always goes forward. The monotonic property can be described as: t1=time.monotonic() t2=time.monotonic() assert t2 = t1 I agree. The point I was making is that implication of steady is that (t2-t1) is the same (given that t2 and t1 occur in time at the same relative moments), which is a guarantee that I don't see any platform providing currently. Any clock that can be adjusted in any manner is not going to meet the steady criterion. In that case, I don't think time.try_monotonic() is really needed because we can emulate time.monotonic() in software if the platform is deficient. As I wrote, I don't think that Python should workaround OS bugs. If the OS monotonic clock is not monotonic, the OS should be fixed. I sympathize with this, but if the idea is that the Python stdlib should use time.monotonic() for scheduling, then it needs to always be available. Otherwise, we are not going to use it ourselves, and what sort of example is that to set? I can't imagine a scenario where you would ask for a monotonic clock and would rather have an error than have Python fill in the gap with an emulation. Sorry, I don't understand what you mean with fill in the gap with an emulation. You would like to implement a monotonic clock based on the system clock? If time.monotonic() is only sometimes available, then I don't see the added clock being anything more than an amusement. (In this case, I'd rather just use clock_gettime() and friends directly, because I have to be platform aware anyways.) What developers want is a timer that is useful for scheduling things to happen after predictable interval in the future, so we should give them that to the best of our ability. -- Scott Dial sc...@scottdial.com ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
[Python-Dev] Bug in generator if the generator in created in a C thread
Hi, bugs.python.org is down so I'm reporting the bug here :-) We have a crash in our product when tracing is enabled by sys.settrace() and threading.settrace(). If a Python generator is created in a C thread, calling the generator later in another thread may crash if Python tracing is enabled. - the C thread calls PyGILState_Ensure() which creates a temporary Python thread state - a generator is created, the generator has a reference to a Python frame which keeps a reference to the temporary Python thread state - the C thread calls PyGILState_Releases() which destroys the temporary Python thread state - when the generator is called later in another thread, call_trace() reads the Python thread state from the generator frame, which is the destroyed frame = it does crash on a pointer dereference if the memory was reallocated (by malloc()) and the data were erased To reproduce the crash, unpack the attached generator_frame_bug.tar.gz, compile the C module using python setup.py build and then run PYTHONPATH=$(ls -d build/lib*/) python test.py (or just python test.py if you installed the _test module). You may need to use Valgrind to see the error, or call memset(tstate, 0xFF, sizeof(*tstate)) before free(tstate) in tstate_delete_common(). Calling the generator should update its reference to the Python state thread in its frame. The generator may also clears frame-f_tstate (to detect bugs earlier), as it does for frame-f_back (to avoid a reference cycle). Attached patch implements this fix for Python 3.3. Victor generator_frame_bug.tar.gz Description: GNU Zip compressed data diff -r 51016ff7f8c9 Objects/genobject.c --- a/Objects/genobject.c Sun Mar 25 22:41:16 2012 -0400 +++ b/Objects/genobject.c Wed Mar 28 12:44:07 2012 +0200 @@ -76,6 +76,7 @@ gen_send_ex(PyGenObject *gen, PyObject * /* Generators always return to their most recent caller, not * necessarily their creator. */ +f-f_tstate = tstate; Py_XINCREF(tstate-frame); assert(f-f_back == NULL); f-f_back = tstate-frame; @@ -89,6 +90,8 @@ gen_send_ex(PyGenObject *gen, PyObject * * cycle. */ assert(f-f_back == tstate-frame); Py_CLEAR(f-f_back); +/* Clear the borrowed reference to the thread state */ +f-f_tstate = NULL; /* If the generator just returned (as opposed to yielding), signal * that the generator is exhausted. */ ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 418: Add monotonic clock
In that case, I don't think time.try_monotonic() is really needed because we can emulate time.monotonic() in software if the platform is deficient. As I wrote, I don't think that Python should workaround OS bugs. If the OS monotonic clock is not monotonic, the OS should be fixed. I sympathize with this, but if the idea is that the Python stdlib should use time.monotonic() for scheduling, then it needs to always be available. Otherwise, we are not going to use it ourselves, and what sort of example is that to set? There is time.hires() if you need a monotonic clock with a fallback to the system clock. Victor ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 418: Add monotonic clock
Georg Brandl wrote: On 28.03.2012 06:45, Nick Coghlan wrote: On Wed, Mar 28, 2012 at 10:36 AM, Victor Stinner victor.stin...@gmail.com wrote: If QueryPerformanceCounter() is monotonic, the API can be simplified to: * time.time() = system clock * time.monotonic() = monotonic clock * time.hires() = monotonic clock or fallback to system clock time.hires() definition is exactly what I was trying to implement with time.steady(strict=True) / time.try_monotonic(). Please don't call the fallback version hires as it suggests it may be higher resolution than time.time() and that's completely the wrong idea. It's also a completely ugly name, since it's quite hard to figure out what it is supposed to stand for in the first place. Precisely. I always read hires as the verb hires (as in he hires a car to go on holiday) rather than HIgh RESolution. -1 on hires, it's a horrible name. And misleading as well, because on Linux, it isn't any more high res than time.time(). +1 on Nick's suggestion of try_monotonic. It is clear and obvious and doesn't mislead. I don't have an opinion as to what the implementation of try_monotonic should be. Whether it should fall back to time.time, time.clock, or something else, I don't know. But it is a clear and obvious solution for the use-case of I prefer the monotonic clock, if it is available, otherwise I'll take my chances with a best-effect clock. -- Steven ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 418: Add monotonic clock
On Wed, 28 Mar 2012 23:05:59 +1100, Steven D'Aprano st...@pearwood.info wrote: +1 on Nick's suggestion of try_monotonic. It is clear and obvious and doesn't mislead. How about monotonicest. (No, this is not really a serious suggestion.) However, time.steadiest might actually work. --David ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 418: Add monotonic clock
On 03/28/2012 01:56 PM, R. David Murray wrote: On Wed, 28 Mar 2012 23:05:59 +1100, Steven D'Apranost...@pearwood.info wrote: +1 on Nick's suggestion of try_monotonic. It is clear and obvious and doesn't mislead. How about monotonicest. (No, this is not really a serious suggestion.) monotonish. Thus honoring the Principle Of Least Monotonishment, //arry/ ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 418: Add monotonic clock
time.monotonic(): The uneventful and colorless function. On Mar 28, 2012 9:30 PM, Larry Hastings la...@hastings.org wrote: On 03/28/2012 01:56 PM, R. David Murray wrote: On Wed, 28 Mar 2012 23:05:59 +1100, Steven D'Apranost...@pearwood.info wrote: +1 on Nick's suggestion of try_monotonic. It is clear and obvious and doesn't mislead. How about monotonicest. (No, this is not really a serious suggestion.) monotonish. Thus honoring the Principle Of Least Monotonishment, //arry/ __**_ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/**mailman/listinfo/python-devhttp://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/**mailman/options/python-dev/** anacrolix%40gmail.comhttp://mail.python.org/mailman/options/python-dev/anacrolix%40gmail.com ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 418: Add monotonic clock
No, that would be time.monotonous(). This is time.monotonic(), the function that can only play a single note at a time. Uh, I mean time.monophonic(). Hmm, this is harder than it looks. On 28 March 2012 14:48, Matt Joiner anacro...@gmail.com wrote: time.monotonic(): The uneventful and colorless function. On Mar 28, 2012 9:30 PM, Larry Hastings la...@hastings.org wrote: On 03/28/2012 01:56 PM, R. David Murray wrote: On Wed, 28 Mar 2012 23:05:59 +1100, Steven D'Apranost...@pearwood.info wrote: +1 on Nick's suggestion of try_monotonic. It is clear and obvious and doesn't mislead. How about monotonicest. (No, this is not really a serious suggestion.) monotonish. Thus honoring the Principle Of Least Monotonishment, //arry/ __**_ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/**mailman/listinfo/python-devhttp://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/**mailman/options/python-dev/** anacrolix%40gmail.comhttp://mail.python.org/mailman/options/python-dev/anacrolix%40gmail.com ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/me%40jonathanfrench.net ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 418: Add monotonic clock
Victor, I have completely lost track of the details of this discussion. Could you (with help from others who contributed) try to compile a table showing, for each platform (Windows/Mac/Linux/BSD) which clocks (or variations) we are considering, and for each of those: - a link for the reference documentation - what their typical accuracy is (barring jumps) - what they do when the civil time is made to jump (forward or back) by the user - how they are affected by small tweaks to the civil time by NTP - what they do if the system is suspended and resumed - whether they can be shared between processes running on the same machine - whether they may fail or be unsupported under some circumstances I have a feeling that if I saw such a table it would be much easier to decide. I assume much of this has already been said at one point in this thread, but it's impossible to have an overview at the moment. If someone has more questions they'd like to see answered please add to the list. -- --Guido van Rossum (python.org/~guido) ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 418: Add monotonic clock
On Wed, Mar 28, 2012 at 8:56 PM, Victor Stinner victor.stin...@gmail.com wrote: In that case, I don't think time.try_monotonic() is really needed because we can emulate time.monotonic() in software if the platform is deficient. As I wrote, I don't think that Python should workaround OS bugs. If the OS monotonic clock is not monotonic, the OS should be fixed. I sympathize with this, but if the idea is that the Python stdlib should use time.monotonic() for scheduling, then it needs to always be available. Otherwise, we are not going to use it ourselves, and what sort of example is that to set? There is time.hires() if you need a monotonic clock with a fallback to the system clock. Completely unintuitive and unnecessary. With the GIL taking care of synchronisation issues, we can easily coerce time.time() into being a monotonic clock by the simple expedient of saving the last returned value: def _make_monotic: try: # Use underlying system monotonic clock if we can return _monotonic except NameError: _tick = time() def monotic(): _new_tick = time() if _new_tick _tick: _tick = _new_tick return _tick monotonic = _make_monotonic() Monotonicity of the result is thus ensured, even when using time.time() as a fallback. If using the system monotonic clock to get greater precision is acceptable for an application, then forcing monotonicity shouldn't be a problem either. Regards, Nick. -- Nick Coghlan | ncogh...@gmail.com | Brisbane, Australia ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 418: Add monotonic clock
On 2012-03-28, at 10:17 AM, Nick Coghlan wrote: def _make_monotic: try: # Use underlying system monotonic clock if we can return _monotonic except NameError: _tick = time() def monotic(): _new_tick = time() if _new_tick _tick: _tick = _new_tick return _tick monotonic = _make_monotonic() Monotonicity of the result is thus ensured, even when using time.time() as a fallback. What if system time jumps 1 year back? We'll have the same monotonic time returned for this whole year? I don't think we should even try to emulate any of OS-level functionality. - Yury ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 418: Add monotonic clock
On Wed, Mar 28, 2012 at 7:17 AM, Nick Coghlan ncogh...@gmail.com wrote: On Wed, Mar 28, 2012 at 8:56 PM, Victor Stinner victor.stin...@gmail.com wrote: In that case, I don't think time.try_monotonic() is really needed because we can emulate time.monotonic() in software if the platform is deficient. As I wrote, I don't think that Python should workaround OS bugs. If the OS monotonic clock is not monotonic, the OS should be fixed. I sympathize with this, but if the idea is that the Python stdlib should use time.monotonic() for scheduling, then it needs to always be available. Otherwise, we are not going to use it ourselves, and what sort of example is that to set? There is time.hires() if you need a monotonic clock with a fallback to the system clock. Completely unintuitive and unnecessary. With the GIL taking care of synchronisation issues, we can easily coerce time.time() into being a monotonic clock by the simple expedient of saving the last returned value: def _make_monotic: try: # Use underlying system monotonic clock if we can return _monotonic except NameError: _tick = time() def monotic(): _new_tick = time() if _new_tick _tick: _tick = _new_tick return _tick monotonic = _make_monotonic() Monotonicity of the result is thus ensured, even when using time.time() as a fallback. If using the system monotonic clock to get greater precision is acceptable for an application, then forcing monotonicity shouldn't be a problem either. That's a pretty obvious trick. But why don't the kernels do this if monotonicity is so important? I'm sure there are also downsides, e.g. if the clock is accidentally set forward by an hour and then back again, you wouldn't have a useful clock for an hour. And the cache is not shared between processes so different processes wouldn't see the same clock value (I presume that most of these clocks have state in the kernel that isn't bound to any particular process -- AFAIK only clock() does that, and only on Unixy systems). -- --Guido van Rossum (python.org/~guido) ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 418: Add monotonic clock
On Wed, Mar 28, 2012 at 6:40 PM, Victor Stinner victor.stin...@gmail.com wrote: If we're simplifying the idea to only promising a monotonic clock (i.e. will never go backwards within a given process, but may produce the same value for an indefinite period, and may jump forwards by arbitrarily large amounts), I don't know any monotonic clock jumping forwards by arbitrarily large amounts. Linux can change CLOCK_MONOTONIC speed, but NTP doesn't jump. If I understood Glyph's explanation correctly, then if your application is running in a VM and the VM is getting its clock data from the underlying hypervisor, then suspending and resuming the VM may result in forward jumping of the monotonic clocks in the guest OS. I believe suspending and hibernating may cause similar problems for even a non-virtualised OS that is getting its time data from a real-time clock chip that keeps running even when the main CPU goes to sleep. (If I *misunderstood* Glyph's explanation, then he may have only been talking about the latter case) Monotonicity is fairly easy to guarantee - you just remember the last value you returned and ensure you never return a lower value than that for the lifetime of the process. The only complication is thread synchronisation, and the GIL (or a dedicated lock for Jython/IronPython) can deal with that. Steadiness, on the other hand, requires a real world time reference and is thus really the domain of specialised hardware like atomic clocks and GPS units rather than software that can be suspended and resumed later without changing its internal state. There's a reason comms station operators pay substantial chunks of money for time frequency reference devices [1]. This is why I now think we only need one new clock function: time.monotonic(). It will be the system monotonic clock if one is available, otherwise it will be our own equivalent wrapper around time.time() that just caches the last value returned to ensure the result never goes backwards. With time.monotonic() guaranteed to always be available, there's no need for a separate function that falls back to an unconditioned time.time() result. Regards, Nick. [1] For example: http://www.symmetricom.com/products/gps-solutions/gps-time-frequency-receivers/XLi/ -- Nick Coghlan | ncogh...@gmail.com | Brisbane, Australia ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 418: Add monotonic clock
On Wed, Mar 28, 2012 at 7:36 AM, Nick Coghlan ncogh...@gmail.com wrote: On Wed, Mar 28, 2012 at 6:40 PM, Victor Stinner victor.stin...@gmail.com wrote: If we're simplifying the idea to only promising a monotonic clock (i.e. will never go backwards within a given process, but may produce the same value for an indefinite period, and may jump forwards by arbitrarily large amounts), I don't know any monotonic clock jumping forwards by arbitrarily large amounts. Linux can change CLOCK_MONOTONIC speed, but NTP doesn't jump. If I understood Glyph's explanation correctly, then if your application is running in a VM and the VM is getting its clock data from the underlying hypervisor, then suspending and resuming the VM may result in forward jumping of the monotonic clocks in the guest OS. I believe suspending and hibernating may cause similar problems for even a non-virtualised OS that is getting its time data from a real-time clock chip that keeps running even when the main CPU goes to sleep. (If I *misunderstood* Glyph's explanation, then he may have only been talking about the latter case) Monotonicity is fairly easy to guarantee - you just remember the last value you returned and ensure you never return a lower value than that for the lifetime of the process. The only complication is thread synchronisation, and the GIL (or a dedicated lock for Jython/IronPython) can deal with that. Steadiness, on the other hand, requires a real world time reference and is thus really the domain of specialised hardware like atomic clocks and GPS units rather than software that can be suspended and resumed later without changing its internal state. There's a reason comms station operators pay substantial chunks of money for time frequency reference devices [1]. This is why I now think we only need one new clock function: time.monotonic(). It will be the system monotonic clock if one is available, otherwise it will be our own equivalent wrapper around time.time() that just caches the last value returned to ensure the result never goes backwards. As I said, I think the caching idea is bad. We may have to settle for semantics that are less than perfect -- presumably if you are doing benchmarking you just have to throw away a bad result that happened to be affected by a clock anomaly, and if you are using timeouts, retries are already part of life. With time.monotonic() guaranteed to always be available, there's no need for a separate function that falls back to an unconditioned time.time() result. I would love to have only one new clock function in 3.3. Regards, Nick. [1] For example: http://www.symmetricom.com/products/gps-solutions/gps-time-frequency-receivers/XLi/ -- --Guido van Rossum (python.org/~guido) ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] Bug in generator if the generator in created in a C thread
Interesting bug. :-( It seems bugs.python.org is back up, so can you file it there too? On Wed, Mar 28, 2012 at 3:45 AM, Victor Stinner victor.stin...@gmail.com wrote: Hi, bugs.python.org is down so I'm reporting the bug here :-) We have a crash in our product when tracing is enabled by sys.settrace() and threading.settrace(). If a Python generator is created in a C thread, calling the generator later in another thread may crash if Python tracing is enabled. - the C thread calls PyGILState_Ensure() which creates a temporary Python thread state - a generator is created, the generator has a reference to a Python frame which keeps a reference to the temporary Python thread state - the C thread calls PyGILState_Releases() which destroys the temporary Python thread state - when the generator is called later in another thread, call_trace() reads the Python thread state from the generator frame, which is the destroyed frame = it does crash on a pointer dereference if the memory was reallocated (by malloc()) and the data were erased To reproduce the crash, unpack the attached generator_frame_bug.tar.gz, compile the C module using python setup.py build and then run PYTHONPATH=$(ls -d build/lib*/) python test.py (or just python test.py if you installed the _test module). You may need to use Valgrind to see the error, or call memset(tstate, 0xFF, sizeof(*tstate)) before free(tstate) in tstate_delete_common(). Calling the generator should update its reference to the Python state thread in its frame. The generator may also clears frame-f_tstate (to detect bugs earlier), as it does for frame-f_back (to avoid a reference cycle). Attached patch implements this fix for Python 3.3. Victor ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/guido%40python.org -- --Guido van Rossum (python.org/~guido) ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 418: Add monotonic clock
On Thu, Mar 29, 2012 at 12:27 AM, Yury Selivanov yselivanov...@gmail.com wrote: What if system time jumps 1 year back? We'll have the same monotonic time returned for this whole year? I don't think we should even try to emulate any of OS-level functionality. You have to keep in mind the alternative here: falling back to an *unconditioned* time.time() value (which is the status quo, and necessary to preserve backwards compatibility). That will break just as badly in that scenario and is precisely the reason that the OS level monotonic functionality is desirable in the first place. I'd be quite happy with a solution that made the OS level monotonic clock part of the public API, with the caveat that it may not be available. Then the necessary trio of functions would be: time.time(): existing system clock, always available time.os_monotonic(): OS level monotonic clock, not always available time.monotonic(): always available, same as os_monotonic if it exists, otherwise uses a time() based emulation that may not be consistent across processes and may mark time for extended periods if the underlying OS clock is forced to jump back a long way. I think that naming scheme is more elegant than using monotonic() for the OS level monotonicity and try_monotonic() for the fallback version, but I'd be OK with the latter approach, too. Regards, Nick. -- Nick Coghlan | ncogh...@gmail.com | Brisbane, Australia ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 418: Add monotonic clock
On 2012-03-28, at 10:36 AM, Nick Coghlan wrote: Monotonicity is fairly easy to guarantee - you just remember the last value you returned and ensure you never return a lower value than that for the lifetime of the process. As I said in my previous mail - I don't think we should ever do that. Time may jump back and forth, and with your approach it will result in monotonic() being completely unusable. If time jumps back for N minutes, or years, that leads to completely broken expectations for timeouts for N minutes or years correspondingly (and that's just the timeouts case, I'm sure that there are much more critical time-related use-cases.) If monotonic() will utilize such hack, you add nothing usable in stdlib. Every serious framework or library will have to re-implement it using only OS-level functions, and *FAIL* if the OS doesn't support monotonic time. Fail, because such framework can't guarantee that it will work correctly. So I think time module should have only one new function: monotonic(), and this function should be only available if OS provides the underlying functionality. No need for steady(), try_monotonic() and other hacks. Each module can decide if its dependancy on monotonic is critical or not, and if it is not, you can always have: try: from time import monotonic as _time except ImportError: from time import time as _time That's how lots of code is written these days, like using 'epoll' if available, and fallback to 'select' if not. Why don't you try to abstract differences between them in the standard library? So I see no point in adding some loose abstractions to the stdlib now. - Yury ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 418: Add monotonic clock
On 2012-03-28, at 10:45 AM, Nick Coghlan wrote: On Thu, Mar 29, 2012 at 12:27 AM, Yury Selivanov yselivanov...@gmail.com wrote: What if system time jumps 1 year back? We'll have the same monotonic time returned for this whole year? I don't think we should even try to emulate any of OS-level functionality. You have to keep in mind the alternative here: falling back to an *unconditioned* time.time() value (which is the status quo, and necessary to preserve backwards compatibility). That will break just as badly in that scenario and is precisely the reason that the OS level monotonic functionality is desirable in the first place. Well, my argumentation is that you either have some code that depends on monotonic time and can't work without it, or you have a code that can work with any time (and only precision matters). Maybe I'm wrong. I'd be quite happy with a solution that made the OS level monotonic clock part of the public API, with the caveat that it may not be available. Then the necessary trio of functions would be: time.time(): existing system clock, always available time.os_monotonic(): OS level monotonic clock, not always available time.monotonic(): always available, same as os_monotonic if it exists, otherwise uses a time() based emulation that may not be consistent across processes and may mark time for extended periods if the underlying OS clock is forced to jump back a long way. I still don't like this 'emulation' idea. Smells bad for standard lib. Big -1 on this approach. - Yury ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 418: Add monotonic clock
On Thu, Mar 29, 2012 at 12:42 AM, Guido van Rossum gu...@python.org wrote: As I said, I think the caching idea is bad. We may have to settle for semantics that are less than perfect -- presumably if you are doing benchmarking you just have to throw away a bad result that happened to be affected by a clock anomaly, and if you are using timeouts, retries are already part of life. I agree caching doesn't solve the problems that are solved by an OS level monotonic clock, but falling back to an unmodifided time.time() result instead doesn't solve those problems either. Falling back to time.time() just gives you the status quo: time may jump forwards or backwards by an arbitrary amount between any two calls. Cached monotonicity just changes the anomalous modes to be time jumping forwards, or time standing still for an extended period of time. The only thing the caching provides is that it becomes a reasonable fallback for a function called time.monotonic() - it *is* a monotonic clock that meets the formal contract of the function, it's just nowhere near as good or effective as one the OS can provide. Forward jumping anomalies aren't as harmful, are very hard to detect in the first place and behave the same regardless of the presence of caching, so the interesting case to look at is the difference in failure modes when the system clock jumps backwards. For benchmarking, a caching clock will produce a zero result instead of a negative result. Zeros aren't quite as obviously broken as negative numbers when benchmarking, but they're still sufficiently suspicious that most benchmarking activities will flag them as anomalous. If the jump back was sufficiently small that the subsequent call still produces a higher value than the original call, then behaviour reverts to being identical. For timeouts, setting the clock back means your operation will take longer to time out than you expected. This problem will occur regardless of whether you were using cached monotonicity (such that time stands still) or the system clock (such that time actually goes backwards). In either case, your deadline will never be reached until the backwards jump has been cancelled out by the subsequent passage of time. I want the standard library to be able to replace its time.time() calls with time.monotonic(). The only way we can do that without breaking cross-platform compatibility is if time.monotonic() is guaranteed to exist, even when the platform only provides time.time(). A dumb caching fallback implementation based on time.time() is the easiest way to achieve that withou making a complete mockery of the monotonic() name. There is then a *different* use case, which is 3.3+ only code which wants to fail noisily when there's no OS level monotonic support - the application developer really does want to fail *immediately* if there's no OS level monotonic clock available, instead of crossing your fingers and hoping you don't hit a clock adjustment glitch (crossing your fingers has, I'll point out, been the *only* option for all previous versions of Python, so it clearly can't be *that* scary a prospect). So, rather than making time.monotonic() something that the *standard library can't use*, I'd prefer to address that second use case by exposing the OS level monotonic clock as time.os_monotonic() only when it's available. That way, the natural transition for old time.time() based code is to time.monotonic() (with no cross-platform support implications), but time.os_monotonic() also becomes available for the stricter use cases. Regards, Nick. -- Nick Coghlan | ncogh...@gmail.com | Brisbane, Australia ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 418: Add monotonic clock
On Thu, Mar 29, 2012 at 1:02 AM, Yury Selivanov yselivanov...@gmail.com wrote: On 2012-03-28, at 10:45 AM, Nick Coghlan wrote: On Thu, Mar 29, 2012 at 12:27 AM, Yury Selivanov yselivanov...@gmail.com wrote: What if system time jumps 1 year back? We'll have the same monotonic time returned for this whole year? I don't think we should even try to emulate any of OS-level functionality. You have to keep in mind the alternative here: falling back to an *unconditioned* time.time() value (which is the status quo, and necessary to preserve backwards compatibility). That will break just as badly in that scenario and is precisely the reason that the OS level monotonic functionality is desirable in the first place. Well, my argumentation is that you either have some code that depends on monotonic time and can't work without it, or you have a code that can work with any time (and only precision matters). Maybe I'm wrong. You're wrong. The primary use case for the new time.monotonic() function is to replace *existing* uses of time.time() in the standard library (mostly related to timeouts) that are currently vulnerable to clock adjustment related bugs. This real, concrete use case has been lost in some of the abstract theoretical discussions that have been going on this thread. We can't lose sight of the fact that using a system clock that is vulnerable to clock adjustment bugs to handle timeouts and benchmarking in Python has worked just fine for 20+ years. Using a monotonic clock instead is *better*, but it's far from essential, since clock adjustments that are big enough and poorly timed enough to cause real problems are fortunately a very rare occurrence. So, the primary use case is that we want to replace many of the time.time() calls in the standard library with time.monotonic() calls. To avoid backwards compatibility problems in the cross-platform support, that means time.monotonic() *must be available on every platform that currently provides time.time()*. This is why Victor's original proposal was that time.monotonic() simply fall back to time.time() if there was no OS level monotonic clock available. The intended use cases are using time.time() *right now* and have been doing so for years, so it is clearly an acceptable fallback for those cases. People (rightly, in my opinion) objected to the idea of time.monotonic() failing to guarantee monotonicity, thus the proposal to enforce at least a basic level of monotonicity through caching of the last returned value. I agree completely that this dumb caching solution doesn't solve any of the original problems with time.time() that make a time.monotonic() function desirable, but it isn't meant to. It's only meant to provide graceful degradation to something that is *no worse than the current behaviour when using time.time() in Python 3.2* while still respecting the property of monotonicity for the new API. Yes, it's an ugly hack, but it is a necessary fallback to avoid accidental regressions in our cross-platform support. For the major platforms (i.e. *nix, Mac OS X, Windows), there *will* be an OS level monotonic clock available, thus using time.monotonic() will have the desired effect of protecting from clocks being adjusted backwards. For other platforms, the behaviour (and vulnerabilities) will be essentially unchanged from the Python 3.2 approach (i.e. using time.time() with no monotonicity guarantees at all). However, some 3.3+ applications may want to be stricter about their behaviour and either bail out completely or fall back to an unfiltered time.time() call if an OS-level monotonic clock is not available. For those, it makes sense to expose time.os_monotonic() directly (and only if it is available), thus allowing those developers to make up their own mind instead of accepting the cross-platform fallback in time.monotonic(). Yes, you can get the exact same effect with the monotonic() and try_monotonic() naming scheme, but why force the standard library (and anyone else wanting to upgrade from time.time() without harming cross-platform support) to use such an ugly name when the os_monotonic and monotonic naming scheme provides a much neater alternative? Regards, Nick. -- Nick Coghlan | ncogh...@gmail.com | Brisbane, Australia ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] Bug in generator if the generator in created in a C thread
2012/3/28 Guido van Rossum gu...@python.org: Interesting bug. :-( It seems bugs.python.org is back up, so can you file it there too? It took us weeks to track the bug. Here is the issue: http://bugs.python.org/issue14432 Victor ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 418: Add monotonic clock
On Wed, Mar 28, 2012 at 8:08 AM, Nick Coghlan ncogh...@gmail.com wrote: On Thu, Mar 29, 2012 at 12:42 AM, Guido van Rossum gu...@python.org wrote: As I said, I think the caching idea is bad. We may have to settle for semantics that are less than perfect -- presumably if you are doing benchmarking you just have to throw away a bad result that happened to be affected by a clock anomaly, and if you are using timeouts, retries are already part of life. I agree caching doesn't solve the problems that are solved by an OS level monotonic clock, but falling back to an unmodifided time.time() result instead doesn't solve those problems either. Falling back to time.time() just gives you the status quo: time may jump forwards or backwards by an arbitrary amount between any two calls. Cached monotonicity just changes the anomalous modes to be time jumping forwards, or time standing still for an extended period of time. The only thing the caching provides is that it becomes a reasonable fallback for a function called time.monotonic() - it *is* a monotonic clock that meets the formal contract of the function, it's just nowhere near as good or effective as one the OS can provide. TBH, I don't think like this focus on monotonicity as the most important feature. Forward jumping anomalies aren't as harmful, are very hard to detect in the first place and behave the same regardless of the presence of caching, so the interesting case to look at is the difference in failure modes when the system clock jumps backwards. Agreed. For benchmarking, a caching clock will produce a zero result instead of a negative result. Zeros aren't quite as obviously broken as negative numbers when benchmarking, but they're still sufficiently suspicious that most benchmarking activities will flag them as anomalous. If the jump back was sufficiently small that the subsequent call still produces a higher value than the original call, then behaviour reverts to being identical. So for benchmarking we don't care about jumps, really, and the caching version is slightly less useful. For timeouts, setting the clock back means your operation will take longer to time out than you expected. This problem will occur regardless of whether you were using cached monotonicity (such that time stands still) or the system clock (such that time actually goes backwards). In either case, your deadline will never be reached until the backwards jump has been cancelled out by the subsequent passage of time. Where in the stdlib do we actually calculate timeouts instead of using the timeouts built into the OS (e.g. select())? I think it would be nice if we could somehow use the *same* clock as the OS uses to implement timeouts. I want the standard library to be able to replace its time.time() calls with time.monotonic(). Where in the stdlib? (I'm aware of threading.py. Any other places?) The only way we can do that without breaking cross-platform compatibility is if time.monotonic() is guaranteed to exist, even when the platform only provides time.time(). A dumb caching fallback implementation based on time.time() is the easiest way to achieve that withou making a complete mockery of the monotonic() name. Yeah, so maybe it's a bad name. :-) There is then a *different* use case, which is 3.3+ only code which wants to fail noisily when there's no OS level monotonic support - the application developer really does want to fail *immediately* if there's no OS level monotonic clock available, instead of crossing your fingers and hoping you don't hit a clock adjustment glitch (crossing your fingers has, I'll point out, been the *only* option for all previous versions of Python, so it clearly can't be *that* scary a prospect). So, rather than making time.monotonic() something that the *standard library can't use*, I'd prefer to address that second use case by exposing the OS level monotonic clock as time.os_monotonic() only when it's available. That way, the natural transition for old time.time() based code is to time.monotonic() (with no cross-platform support implications), but time.os_monotonic() also becomes available for the stricter use cases. I'd be happier if the fallback function didn't try to guarantee things the underlying clock can't guarantee. I.e. I like the idea of having a function that uses some accurate OS clock if one exists but falls back to time.time() if not; I don't like the idea of that new function trying to interpret the value of time.time() in any way. Applications that need the OS clock's guarantees can call it directly. We could also offer something where you can introspect the properties of the clock (or clocks) so that an app can choose the best clock depending on its needs. To summarize my problem with the caching idea: take a simple timeout loop such as found in several places in threading.py. def wait_for(delta, eps): # Wait for delta seconds, sleeping eps seconds at a time
Re: [Python-Dev] PEP 418: Add monotonic clock
time.timeout_clock? Everyone knows what that will be for and we won't have to make silly theoretical claims about its properties and expected uses. If no one else looks before I next get to a PC I'll dig up the clock/timing source used for select and friends, and find any corresponding syscall that retrieves it for Linux. ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 418: Add monotonic clock
On Thu, Mar 29, 2012 at 1:47 AM, Guido van Rossum gu...@python.org wrote: Where in the stdlib? (I'm aware of threading.py. Any other places?) Victor had at least one other example. multiprocessing, maybe? I believe the test suite may still have a few instances as well. But now consider a caching clock, and consider that the system clock made a jump backwards *before* this function is called. The cache prevents us from seeing it, so the initial call to now() returns the highest clock value seen so far. And until the system clock has caught up with that, now() will return the same value over and over -- so WE STILL WAIT TOO LONG. Ouch. OK, I'm convinced the caching fallback is worse than just falling back to time.time() directly, which means the naming problem needs to be handled another way. My conclusion: you can't win this game by forcing the clock to return a monotonic value. A better approach might be to compute how many sleep(eps) calls we're expected to make, and to limit the loop to that -- although sleep() doesn't make any guarantees either about sleeping too short or too long. Basically, if you do sleep(1) and find that your clock didn't move (enough), you can't tell the difference between a short sleep and a clock that jumped back. And if your clock moved to much, you still don't know if the problem was with sleep() or with your clock. With your point about the problem with the naive caching mechanism acknowledged, I think we can safely assign time.monotonic() as the name of the OS provided monotonic clock. That means choosing a name for the version that falls back to time() if monotonic() isn't available so it can be safely substituted for time.time() without having to worry about platform compatibility implications. I don't like Victor's current hires (because it doesn't hint at the fallback behaviour, may actually be the same res as time.time() and reads like an unrelated English word). My own suggestion of try_monotic() would get the job done, but is hardly going to win any API beauty contests. Cheers, Nick. -- Nick Coghlan | ncogh...@gmail.com | Brisbane, Australia ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 418: Add monotonic clock
Matt Joiner wrote: time.timeout_clock? Everyone knows what that will be for and we won't have to make silly theoretical claims about its properties and expected uses. I don't. -- Steven ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 418: Add monotonic clock
On Thu, Mar 29, 2012 at 1:14 AM, Nick Coghlan ncogh...@gmail.com wrote: That means choosing a name for the version that falls back to time() if monotonic() isn't available so it can be safely substituted for time.time() without having to worry about platform compatibility implications. What's wrong with time.time() again? As documented in http://docs.python.org/py3k/library/time.html it makes no guarantees, and specifically there is *no* guarantee that it will ever behave *badly*wink/. Of course, we'll have to guarantee that, if a badly-behaved clock is available, users can get access to it, so call that time._time(). ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 418: Add monotonic clock
Nick Coghlan wrote: Completely unintuitive and unnecessary. With the GIL taking care of synchronisation issues, we can easily coerce time.time() into being a monotonic clock by the simple expedient of saving the last returned value: [snip] Here's a version that doesn't suffer from the flaw of returning a long stream of constant values when the system clock jumps backwards a significant amount: class MockTime: def __init__(self): self.ticks = [1, 2, 3, 4, 2, 3, 4, 5, 7, 3, 4, 6, 7, 8, 8, 9] self.i = -1 def __call__(self): self.i += 1 return self.ticks[self.i] time = MockTime() _prev = _prev_raw = 0 def monotonic(): global _prev, _prev_raw raw = time() delta = max(0, raw - _prev_raw) _prev_raw = raw _prev += delta return _prev And in use: [monotonic() for i in range(16)] [1, 2, 3, 4, 4, 5, 6, 7, 9, 9, 10, 12, 13, 14, 14, 15] Time: [1, 2, 3, 4, 2, 3, 4, 5, 7, 3, 4, 6, 7, 8, 8, 9] Nick: [1, 2, 3, 4, 4, 4, 4, 5, 7, 7, 7, 7, 7, 8, 8, 9] Mine: [1, 2, 3, 4, 4, 5, 6, 7, 9, 9, 10, 12, 13, 14, 14, 15] Mine will get ahead of the system clock each time it jumps back, but it's a lot closer to the ideal of a *strictly* monotonically increasing clock. Assuming that the system clock will never jump backwards twice in a row, the double-caching version will never have more than two constant values in a row. -- Steven ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 418: Add monotonic clock
On Wed, Mar 28, 2012 at 12:56, Victor Stinner victor.stin...@gmail.com wrote: There is time.hires() if you need a monotonic clock with a fallback to the system clock. Does this primarily give a high resolution clock, or primarily a monotonic clock? That's not clear from either the name, or the PEP. //Lennart ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 418: Add monotonic clock
What's wrong with time.time() again? As documented in http://docs.python.org/py3k/library/time.html it makes no guarantees, and specifically there is *no* guarantee that it will ever behave *badly*wink/. Of course, we'll have to guarantee that, if a badly-behaved clock is available, users can get access to it, so call that time._time(). I'm not sure I understand your suggestion correctly, but replacing time.time() by time.monotonic() with fallback won't work, because time.monotonic() isn't wall-clock time: it can very well use an arbitrary reference point (most likely system start-up time). As for the hires() function, since there's no guarantee whatsoever that it does provide a better resolution than time.time(), this would be really misleading IMHO. ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] Bug tracker outage
Am 27.03.2012 23:11, schrieb Martin v. Löwis: Upfront hosting (Izak Burger) is going to do a Debian upgrade of the bug tracker machine soon (likely tomorrow). This may cause some outage, since there is a lot of custom stuff on the machine which may break with newer (Python) versions. I'll notify here when the upgrade is complete. The upgrade is complete. It was in fact the Postgres upgrade (to 8.4) which caused the longest blackout time. Regards, Martin ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 418: Add monotonic clock
On 2012-03-28, at 11:35 AM, Nick Coghlan wrote:
So, the primary use case is that we want to replace many of the
time.time() calls in the standard library with time.monotonic() calls.
To avoid backwards compatibility problems in the cross-platform
support, that means time.monotonic() *must be available on every
platform that currently provides time.time()*.
OK. I got your point. And also I've just realized what I dislike about
the way you want to implement the fallback. The main problem is that
I treat the situation when time jumps backward as an exception, because,
again, if you have timeouts you may get those timeouts to never be
executed. So let's make the try_monotonic() function (or whatever
name will be chosen) this way (your original code edited):
def _make_monotic():
try:
# Use underlying system monotonic clock if we can
return _monotonic
except NameError:
_tick = time()
def monotic():
nonlocal _time
_new_tick = time()
if _new_tick = _tick:
raise RuntimeError('time was adjusted backward')
_tick = _new_tick
return _new_tick
return monotonic
try_monotonic = _make_monotonic()
At least this approach tries to follow some of the python's zen.
-
Yury
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[Python-Dev] Virtualenv not portable from Python 2.7.2 to 2.7.3 (os.urandom missing)
I see this was reported as a debian bug. http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=665776 We encountered it as well. To reproduce, using virtualenv 1.7+ on Python 2.7.2 on Ubuntu, create a virtualenv. Move that virtualenv to a host with Python 2.7.3RC2 yields: jaraco@vdm-dev:~$ /usr/bin/python2.7 -V Python 2.7.3rc2 jaraco@vdm-dev:~$ env/bin/python -V Python 2.7.2 jaraco@vdm-dev:~$ env/bin/python -c import os; os.urandom() Traceback (most recent call last): File string, line 1, in module AttributeError: 'module' object has no attribute 'urandom' This bug causes Django to not start properly (under some circumstances). I reviewed the changes between v2.7.2 and 2.7 (tip) and it seems there was substantial refactoring of the os and posix modules for urandom. I still don't fully understand why the urandom method is missing (because the env includes the python 2.7.2 executable and stdlib). I suspect this change is going to cause some significant backward compatibility issues. Is there a recommended workaround? Should I file a bug? Regards, Jason smime.p7s Description: S/MIME cryptographic signature ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] Virtualenv not portable from Python 2.7.2 to 2.7.3 (os.urandom missing)
Hi Jason, On 03/28/2012 12:22 PM, Jason R. Coombs wrote: To reproduce, using virtualenv 1.7+ on Python 2.7.2 on Ubuntu, create a virtualenv. Move that virtualenv to a host with Python 2.7.3RC2 yields: jaraco@vdm-dev:~$ /usr/bin/python2.7 -V Python 2.7.3rc2 jaraco@vdm-dev:~$ env/bin/python -V Python 2.7.2 jaraco@vdm-dev:~$ env/bin/python -c import os; os.urandom() Traceback (most recent call last): File string, line 1, in module AttributeError: 'module' object has no attribute 'urandom' This bug causes Django to not start properly (under some circumstances). I reviewed the changes between v2.7.2 and 2.7 (tip) and it seems there was substantial refactoring of the os and posix modules for urandom. I still don’t fully understand why the urandom method is missing (because the env includes the python 2.7.2 executable and stdlib). In Python 2.6.8/2.7.3, urandom is built into the executable. A virtualenv doesn't contain the whole stdlib, only the bits necessary to bootstrap site.py. So the problem arises from trying to use the 2.7.3 stdlib with a 2.7.2 interpreter. I suspect this change is going to cause some significant backward compatibility issues. Is there a recommended workaround? Should I file a bug? The workaround is easy: just re-run virtualenv on that path with the new interpreter. I was made aware of this issue a few weeks ago, and added a warning to the virtualenv news page: http://www.virtualenv.org/en/latest/news.html I'm not sure where else to publicize it. Carl signature.asc Description: OpenPGP digital signature ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 418: Add monotonic clock
On 2012-03-28, at 1:55 PM, Yury Selivanov wrote: nonlocal _time nonlocal _tick obviously. P.S. And we can make it to raise an error after some N calls to time() resulting in lesser time that is stored in the _tick variable. - Yury ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
[Python-Dev] datetime module and pytz with dateutil
I figured out what pytz and dateutil are not mentioned in python docs for datetime module. It's clean why these libs is not a part of Python Libraries — but that's not clean for Docs. From my perspective at least pytz (as py3k compatible) should to be mentioned as the library which contains timezone info, supported carefully and recommended to use with datetime standard module, -- Thanks, Andrew Svetlov ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] Virtualenv not portable from Python 2.7.2 to 2.7.3 (os.urandom missing)
-Original Message- From: python-dev-bounces+jaraco=jaraco@python.org [mailto:python- dev-bounces+jaraco=jaraco@python.org] On Behalf Of Carl Meyer Sent: Wednesday, 28 March, 2012 14:48 The workaround is easy: just re-run virtualenv on that path with the new interpreter. Thanks for the quick response Carl. I appreciate all the work that's been done. I'm not sure the workaround is as simple as you say. Virtualenv doesn't replace the 'python' exe if it already exists (because it may already exist for a different minor version of Python (3.2, 2.6)). So the procedure is probably something like this: For each minor version of Python the virtualenv wraps (ls env/bin/python?.?): 1) Run env/bin/python -V. If the result starts with Python minor, remove env/bin/python. 2) Determine if that Python version uses distribute or setuptools. 3) Run virtualenv --python=pythonminor env (with --distribute if appropriate) I haven't yet tested this procedure, but I believe it's closer to what will need to be done. There are probably other factors. Unfortunately, to reliably repair the virtualenv is very difficult, so we will probably opt with re-deploying all of our virtualenvs. Will the release notes include something about this change, since it will likely have broad backward incompatibility for all existing virtualenvs? I wouldn't expect someone in operations to read the virtualenv news to find out what things a Python upgrade will break. Indeed, this update will probably be pushed out as part of standard, unattended system updates. I realize that the relationship between stdlib.os and posixmodule isn't a guaranteed interface, and the fact that it breaks with virtualenv is a weakness of virtualenv. Nevertheless, virtualenv has become the defacto technique for Python environments. Putting my sysops cap on, I might perceive this change as being unannounced (w.r.t. Python) and having significant impact on operations. I would think this impact deserves at least a note in the release notes. Regards, Jason smime.p7s Description: S/MIME cryptographic signature ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] datetime module and pytz with dateutil
+1 If pytz is py3k cabable. -1 for dateutIl. On Wednesday, March 28, 2012, Andrew Svetlov wrote: I figured out what pytz and dateutil are not mentioned in python docs for datetime module. It's clean why these libs is not a part of Python Libraries — but that's not clean for Docs. From my perspective at least pytz (as py3k compatible) should to be mentioned as the library which contains timezone info, supported carefully and recommended to use with datetime standard module, -- Thanks, Andrew Svetlov ___ Python-Dev mailing list Python-Dev@python.org javascript:; http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/guido%40python.org -- --Guido van Rossum (python.org/~guido) ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 418: Add monotonic clock
I would love to have only one new clock function in 3.3. I already added time.clock_gettime() to 3.3 :-) Victor ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 418: Add monotonic clock
On 3/28/2012 10:29 AM, Guido van Rossum wrote: On Wed, Mar 28, 2012 at 7:17 AM, Nick Coghlan ncogh...@gmail.com wrote: Completely unintuitive and unnecessary. With the GIL taking care of synchronisation issues, we can easily coerce time.time() into being a monotonic clock by the simple expedient of saving the last returned value: That's a pretty obvious trick. But why don't the kernels do this if monotonicity is so important? I'm sure there are also downsides, e.g. if the clock is accidentally set forward by an hour and then back again, you wouldn't have a useful clock for an hour. And the cache is not shared between processes so different processes wouldn't see the same clock value (I presume that most of these clocks have state in the kernel that isn't bound to any particular process -- AFAIK only clock() does that, and only on Unixy systems). What makes you think that isn't already true? I don't know what platforms that CPython compiles for that *won't* have one of the aforementioned functions available that provide a *real* monotonic clock. Surely, any platform that doesn't didn't recognize the need for it, or they would just provide a monotonic clock. That is to say, if you are a POSIX compliant system, then there is no reason to break gettimeofday() and friends when you can just implement CLOCK_MONOTONIC proper (even if it's just a trick like Nick's). I think the PEP should enumerate what platforms that CPython supports that will not benefit from a real monotonic clock. I think the number of platforms will be such a minority that the emulation makes sense. Practicality beats purity, and all. -- Scott Dial sc...@scottdial.com ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 418: Add monotonic clock
Does this primarily give a high resolution clock, or primarily a monotonic clock? That's not clear from either the name, or the PEP. I expect a better resolution from time.monotonic() than time.time(). I don't have exact numbers right now, but I began to document each OS clock in the PEP. Victor ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] datetime module and pytz with dateutil
I'm personally +1 for pytz only — dateutil is big enough and... Well, can we just point to pytz in our docs for datetime module? On Thu, Mar 29, 2012 at 12:06 AM, Guido van Rossum gu...@python.org wrote: +1 If pytz is py3k cabable. -1 for dateutIl. On Wednesday, March 28, 2012, Andrew Svetlov wrote: I figured out what pytz and dateutil are not mentioned in python docs for datetime module. It's clean why these libs is not a part of Python Libraries — but that's not clean for Docs. From my perspective at least pytz (as py3k compatible) should to be mentioned as the library which contains timezone info, supported carefully and recommended to use with datetime standard module, -- Thanks, Andrew Svetlov ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/guido%40python.org -- --Guido van Rossum (python.org/~guido) -- Thanks, Andrew Svetlov ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 418: Add monotonic clock
I think the PEP should enumerate what platforms that CPython supports that will not benefit from a real monotonic clock. I think the number of platforms will be such a minority that the emulation makes sense. Practicality beats purity, and all. The PEP lists OS monotonic clocks by platform. Windows, Mac OS X, Solaris, and UNIX (CLOCK_MONOTONIC friends) provide monotonic clocks. I don't know any platform without monotonic clock. Victor ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] Virtualenv not portable from Python 2.7.2 to 2.7.3 (os.urandom missing)
On Wed, 28 Mar 2012 20:56:30 -, Jason R. Coombs jar...@jaraco.com wrote: Will the release notes include something about this change, since it will likely have broad backward incompatibility for all existing virtualenvs? I wouldn't expect someone in operations to read the virtualenv news to find out what things a Python upgrade will break. Indeed, this update will probably be pushed out as part of standard, unattended system updates. I think it is reasonable to put something in the release notes. This change is much larger than changes we normally make in maintenance release, because it fixes a security bug. But because it is larger than normal, adding release notes like this about known breakage is, I think, a good idea. Perhaps you and Carl could collaborate on a page explaining the issue in detail, and on a brief note to include in the release notes that points to your more extensive discussion? But keep in mind I'm not the release manager; we'll need their buy in on this. --David ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 418: Add monotonic clock
On Wed, Mar 28, 2012 at 2:36 PM, Scott Dial scott+python-...@scottdial.com wrote: On 3/28/2012 10:29 AM, Guido van Rossum wrote: On Wed, Mar 28, 2012 at 7:17 AM, Nick Coghlan ncogh...@gmail.com wrote: Completely unintuitive and unnecessary. With the GIL taking care of synchronisation issues, we can easily coerce time.time() into being a monotonic clock by the simple expedient of saving the last returned value: That's a pretty obvious trick. But why don't the kernels do this if monotonicity is so important? I'm sure there are also downsides, e.g. if the clock is accidentally set forward by an hour and then back again, you wouldn't have a useful clock for an hour. And the cache is not shared between processes so different processes wouldn't see the same clock value (I presume that most of these clocks have state in the kernel that isn't bound to any particular process -- AFAIK only clock() does that, and only on Unixy systems). What makes you think that isn't already true? What does that refer to in this sentence? I don't know what platforms that CPython compiles for that *won't* have one of the aforementioned functions available that provide a *real* monotonic clock. Surely, any platform that doesn't didn't recognize the need for it, or they would just provide a monotonic clock. That is to say, if you are a POSIX compliant system, then there is no reason to break gettimeofday() and friends when you can just implement CLOCK_MONOTONIC proper (even if it's just a trick like Nick's). I think the PEP should enumerate what platforms that CPython supports that will not benefit from a real monotonic clock. I think the number of platforms will be such a minority that the emulation makes sense. Practicality beats purity, and all. -- Scott Dial sc...@scottdial.com -- --Guido van Rossum (python.org/~guido) ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
[Python-Dev] bug tracker offline again for re-indexing
Since Martin hasn't sent a note about this here I will: I noticed that text search wasn't working right on the bug tracker, and Martin has taken it offline again to re-index. --David ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] bug tracker offline again for re-indexing
Am 28.03.2012 23:55, schrieb R. David Murray: Since Martin hasn't sent a note about this here I will: I noticed that text search wasn't working right on the bug tracker, and Martin has taken it offline again to re-index. which will, unfortunately, take a few more hours to complete. Regards, Martin ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 418: Add monotonic clock
Where in the stdlib do we actually calculate timeouts instead of using the timeouts built into the OS (e.g. select())? At least in threading and queue modules. The common use case is to retry a function with a timeout if the syscall was interrupted by a signal (EINTR error). The socket module and _threading.Lock.acquire() implement such retry loop using the system clock. They should use a monotonic clock instead. I think it would be nice if we could somehow use the *same* clock as the OS uses to implement timeouts. On Linux, nanosleep() uses CLOCK_MONOTONIC whereas POSIX suggests CLOCK_REALTIME. Some functions allow to choose the clock, like pthread locks or clock_nanosleep(). I'd be happier if the fallback function didn't try to guarantee things the underlying clock can't guarantee. I.e. I like the idea of having a function that uses some accurate OS clock if one exists but falls back to time.time() if not; I don't like the idea of that new function trying to interpret the value of time.time() in any way. We may workaround some OS known bugs like: http://support.microsoft.com/?id=274323 The link contains an example how to workaround the bug. The idea of the workaround is to use two different monotonic clocks to detect leaps, with one trusted clock (GetTickCount) and one untrusted clock having an higher resolution (QueryPerformanceCounter). I don't think that the same algorithm is applicable on other OSes because other OSes usually only provide one monotonic clock, sometimes though different API. Victor ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 418: Add monotonic clock
Could you (with help from others who contributed) try to compile a table showing, for each platform (Windows/Mac/Linux/BSD) which clocks (or variations) we are considering, and for each of those: - a link for the reference documentation - what their typical accuracy is (barring jumps) - what they do when the civil time is made to jump (forward or back) by the user - how they are affected by small tweaks to the civil time by NTP - what they do if the system is suspended and resumed - whether they can be shared between processes running on the same machine - whether they may fail or be unsupported under some circumstances I have a feeling that if I saw such a table it would be much easier to decide. I assume much of this has already been said at one point in this thread, but it's impossible to have an overview at the moment. I don't know where I can get all these information, but I'm completing the PEP each time that I find a new information. It's difficult to get the accuracy of a clock and how it handles system suspend. I'm intereted if anyone has such information. Victor ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] bug tracker offline again for re-indexing
I noticed that text search wasn't working right on the bug tracker, and Martin has taken it offline again to re-index. which will, unfortunately, take a few more hours to complete. It seems to work now, so I turned it on again. Text search now uses Xapian, and recreating the Xapian index of all msg objects took a while. Regards, Martin ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
