New submission from STINNER Victor:
To prepare Python to add support of monotonic clock in pytime.h, I propose to
rewrite pytime.h to use a new _PyTimeSpec structure which has a resolution of 1
nanosecond on only work on integers.
Currently, pytime.h uses a _PyTime_timeval structure which has a solution of 1
microsecond and works internally on floating point numbers. Computing the
difference between two timestamps may loose precision.
The tv_nsec field of _PyTimeSpec must be in the range [0; 999999999], even for
negative numbers. For example, -1 ns is stored as (-1, 999999999). This
property makes the code more complex, especially to round correctly.
The new API is based on the idea that all timestamps must be stored as
_PyTimeSpec.
Convert a value into a _PyTimeSpec:
- _PyTimeSpec_from_double(): C double
- _PyTimeSpec_from_object(): Python int or float object
- you can also fill directly the tv_sec and tv_nsec fields
Convert a _PyTimeSpec timestamp into a value:
- _PyTimeSpec_to_time_t(): C time_t
- _PyTimeSpec_to_timeval(): C timeval (tv_sec, tv_usec), but ot directly the
structure because the exact structure is different depending on the OS and OS
version
- _PyTimeSpec_ms(): C long, number of milliseconds
Operations on _PyTimeSpec:
- _PyTimeSpec_add(): a+b
- _PyTimeSpec_sub(): a-b
I removed high-level functions like _PyTime_ObjectToTimeval(): you should now
combine _PyTimeSpec_from_object() with _PyTimeSpec_to_timeval(). I did this to
not multiply the number of functions, because I want to test all functions in
unit tests and have a short API.
I tried to enhance detecton of overflow. I didn't review carefuly my patch yet.
I'm not sure that all calls check for error and handle exceptions correctly.
Only the following functions raise an exception on error:
- _PyTimeSpec_from_object()
- _PyTimeSpec_to_time_t()
- _PyTimeSpec_to_timeval()
Other functions sets the minimum/maximum value in case of underflow/overflow.
So even if you don't check for errors, the behaviour on overflow should be
acceptable.
The new _PyTimeSpec_Init() function checks that the system clock works, so
_PyTimeSpec_get_time() and _PyTimeSpec_get_time_info() don't need to check for
error. In fact, these functions call Py_FatalError() on error, but it should
never happen. This idea was proposed in the issue #22043 to check if the
monotonic clock is working. Maybe we can avoid checking the system clock in
_PyTimeSpec_Init() and only check the monotonic clock. I hope that all
platforms have a working system clock! The oppoosite would be a huge issue.
The patch removes function which are exposed in the stable ABI. Since the
functions are private, name starting with ("_Py"), I consider that it's ok to
remove them. The functions are replaced with new functions which a have a new
name. I excluded pytime.h from the stable ABI. I don't know why private
functions were part of the stable ABI :-/
The patch comes with unit tests for each function.
I didn't implement handling of overflow and underflow in _PyTimeSpec_add() and
_PyTimeSpec_sub() yet. But I implemented detection for overflow for the simple
case.
See also:
- Issue #22043: "Use a monotonic clock to compute timeouts".
- PEP 410: "Use decimal.Decimal type for timestamps" (rejected)
----------
files: timespec.patch
keywords: patch
messages: 224452
nosy: haypo
priority: normal
severity: normal
status: open
title: Rewrite pytime.h to work on nanoseconds
versions: Python 3.4
Added file: http://bugs.python.org/file36186/timespec.patch
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<http://bugs.python.org/issue22117>
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