Re: Using bintime() in acpi_cpu_idle()?
On Sun, 29 Jul 2012, Alexander Motin wrote: With ACPI timer gradually becoming one of slowest in the system, is there some reason to use it directly in acpi_cpu_idle()? I've made a patch: http://people.freebsd.org/~mav/sleep_time.patch to use binuptime() instead. Using even HPET from system time counter (not even speaking about TSC) that significantly improves performance on some workloads if this code is not covered by MWAIT optimization in cpu_idle(). Does it work with a perverse timecounter like the i8254 work? The user is permitted to switch to any supported timecounter. There are other perverse ones: - ACPI. This seems to be unavailable if the system thinks ACPI-fast works. Bug. The user should be able to downgrade to it if ACPI-fast in fact doesn't work. Since it reads the hardware more than once, it is much slower than direct use of the hardware. - ACPI-fast. Even this is perverse. It only reads the hardware once, but goes through many software layers. binuptime() is more accurate than uncalibrated scaling. Is accuracy required? If not, the CPU ticker might work, and is faster than HPET, and and is not under user control for perverse settings. It normally reduces to readtsc() with no serializing instruction even in proposed changes. This is good enough for process times (not very good) and depends on the CPU not changing. Its calibration is very accurate (similar to timecounters) modulo bugs, but not always up to date. Bruce ___ freebsd-acpi@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-acpi To unsubscribe, send any mail to "freebsd-acpi-unsubscr...@freebsd.org"
Re: Using bintime() in acpi_cpu_idle()?
On 29.07.2012 11:37, Bruce Evans wrote: On Sun, 29 Jul 2012, Alexander Motin wrote: With ACPI timer gradually becoming one of slowest in the system, is there some reason to use it directly in acpi_cpu_idle()? I've made a patch: http://people.freebsd.org/~mav/sleep_time.patch to use binuptime() instead. Using even HPET from system time counter (not even speaking about TSC) that significantly improves performance on some workloads if this code is not covered by MWAIT optimization in cpu_idle(). Does it work with a perverse timecounter like the i8254 work? At least on my test system it does, even though predictably much slower then the others. The user is permitted to switch to any supported timecounter. There are other perverse ones: - ACPI. This seems to be unavailable if the system thinks ACPI-fast works. Bug. The user should be able to downgrade to it if ACPI-fast in fact doesn't work. Since it reads the hardware more than once, it is much slower than direct use of the hardware. - ACPI-fast. Even this is perverse. It only reads the hardware once, but goes through many software layers. binuptime() is more accurate than uncalibrated scaling. Is accuracy required? Accuracy is not required at all. +-20% is not a problem. If not, the CPU ticker might work, and is faster than HPET, and and is not under user control for perverse settings. It normally reduces to readtsc() with no serializing instruction even in proposed changes. This is good enough for process times (not very good) and depends on the CPU not changing. Its calibration is very accurate (similar to timecounters) modulo bugs, but not always up to date. Problem with ticker that it may stop during idle periods, and idle is exactly what happens here. Unlike timecounter usage here we don't need CPU synchronicity, but we need it working during deep sleeps. -- Alexander Motin ___ freebsd-acpi@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-acpi To unsubscribe, send any mail to "freebsd-acpi-unsubscr...@freebsd.org"
Re: Using bintime() in acpi_cpu_idle()?
On Sun, 29 Jul 2012, Alexander Motin wrote: On 29.07.2012 11:37, Bruce Evans wrote: ... binuptime() is more accurate than uncalibrated scaling. Is accuracy required? Accuracy is not required at all. +-20% is not a problem. If not, the CPU ticker might work, and is faster than HPET, and and is not under user control for perverse settings. It normally reduces to readtsc() with no serializing instruction even in proposed changes. This is good enough for process times (not very good) and depends on the CPU not changing. Its calibration is very accurate (similar to timecounters) modulo bugs, but not always up to date. Problem with ticker that it may stop during idle periods, and idle is exactly what happens here. Unlike timecounter usage here we don't need CPU synchronicity, but we need it working during deep sleeps. The ticker is the same as the timecounter in many cases of interest. If the TSC stops then it cannot be used for timecounting unless timecounting is reinitialized. Timecounting should be reinitialized after deep sleeps, but you say you need it to work during deep sleeps. I wouldn't trust timecounters for some time after waking up after a deep sleep. If their clock stopped then the times read might only be very out of date. If their clock didn't stop, then they might have wrapped or otherwise overflowed and the times read would be garbage. Is there any locking or ordering to prevent them being used before they are reinitialized? Bruce ___ freebsd-acpi@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-acpi To unsubscribe, send any mail to "freebsd-acpi-unsubscr...@freebsd.org"
Re: Using bintime() in acpi_cpu_idle()?
On 29.07.2012 15:26, Bruce Evans wrote: On Sun, 29 Jul 2012, Alexander Motin wrote: On 29.07.2012 11:37, Bruce Evans wrote: ... binuptime() is more accurate than uncalibrated scaling. Is accuracy required? Accuracy is not required at all. +-20% is not a problem. If not, the CPU ticker might work, and is faster than HPET, and and is not under user control for perverse settings. It normally reduces to readtsc() with no serializing instruction even in proposed changes. This is good enough for process times (not very good) and depends on the CPU not changing. Its calibration is very accurate (similar to timecounters) modulo bugs, but not always up to date. Problem with ticker that it may stop during idle periods, and idle is exactly what happens here. Unlike timecounter usage here we don't need CPU synchronicity, but we need it working during deep sleeps. The ticker is the same as the timecounter in many cases of interest. If the TSC stops then it cannot be used for timecounting unless timecounting is reinitialized. Timecounting should be reinitialized after deep sleeps, but you say you need it to work during deep sleeps. Timecounter already has detection logic to disable TSC in cases where it is unreliable. I don't want to replicate it here. I need not precise and not synchronized by reliable and fast time source. I wouldn't trust timecounters for some time after waking up after a deep sleep. If their clock stopped then the times read might only be very out of date. If their clock didn't stop, then they might have wrapped or otherwise overflowed and the times read would be garbage. Is there any locking or ordering to prevent them being used before they are reinitialized? I am not sure what reinitialization are you talking about. IIRC, there is no any waking up code for TSC. None other time counters have problems with C-states. -- Alexander Motin ___ freebsd-acpi@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-acpi To unsubscribe, send any mail to "freebsd-acpi-unsubscr...@freebsd.org"
Re: Using bintime() in acpi_cpu_idle()?
On Sun, 29 Jul 2012, Alexander Motin wrote: On 29.07.2012 15:26, Bruce Evans wrote: On Sun, 29 Jul 2012, Alexander Motin wrote: On 29.07.2012 11:37, Bruce Evans wrote: ... binuptime() is more accurate than uncalibrated scaling. Is accuracy required? Accuracy is not required at all. +-20% is not a problem. If not, the CPU ticker might work, and is faster than HPET, and and is not under user control for perverse settings. It normally reduces to readtsc() with no serializing instruction even in proposed changes. This is good enough for process times (not very good) and depends on the CPU not changing. Its calibration is very accurate (similar to timecounters) modulo bugs, but not always up to date. Problem with ticker that it may stop during idle periods, and idle is exactly what happens here. Unlike timecounter usage here we don't need CPU synchronicity, but we need it working during deep sleeps. The ticker is the same as the timecounter in many cases of interest. If the TSC stops then it cannot be used for timecounting unless timecounting is reinitialized. Timecounting should be reinitialized after deep sleeps, but you say you need it to work during deep sleeps. Timecounter already has detection logic to disable TSC in cases where it is unreliable. I don't want to replicate it here. I need not precise and not synchronized by reliable and fast time source. Yes, this logic gives exactly what you don't want (an inefficient timecounter), by preventing use of the TSC for the timecounter, although the TSC is perfectly usable for the ticker and here. I wouldn't trust timecounters for some time after waking up after a deep sleep. If their clock stopped then the times read might only be very out of date. If their clock didn't stop, then they might have wrapped or otherwise overflowed and the times read would be garbage. Is there any locking or ordering to prevent them being used before they are reinitialized? I am not sure what reinitialization are you talking about. IIRC, there is no any waking up code for TSC. None other time counters have problems with C-states. It is the timecounter code that needs reinitializing. If the TSC stops, or wraps mod 2**32, then its counts become garbage for the purpose of timecounting. Maybe it is not used for timecounting in either of these cases. But these cases shouldn't prevent its use for timecounting. The 2**32 number is because timecounters only use 32 bits of hardware counters (for efficiency). So even if the hardware has some magic to not stop the TSC while sleeping (maybe it fakes not stopping it be reloading on wakeup), it is still unusable by timecounters after sleeping for a second or 2 so that it wraps. The software needs similar faking to reload the timecounter on wakeup. This makes use of timecounters in sleep/wakeup code fragile. At boot time there is a dummy timecounter that returns bogo-times. Apparently sleeping doesn't occur before the timecounter is switched to a real one. The dummy timecounter isn't switched back to after boot time. But it probably should be, since the hardware timecounter may have stopped or wrapped. Sleeping could just set a flag to indicate this state, but then you would have to provide a fake time anyway on finding the flag set. Boot time just points to the dummy timecounter so as not to check this flag in all early timecounter "hardware" calls. Bruce ___ freebsd-acpi@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-acpi To unsubscribe, send any mail to "freebsd-acpi-unsubscr...@freebsd.org"
Re: Using bintime() in acpi_cpu_idle()?
On 30.07.2012 07:33, Bruce Evans wrote: On Sun, 29 Jul 2012, Alexander Motin wrote: On 29.07.2012 15:26, Bruce Evans wrote: On Sun, 29 Jul 2012, Alexander Motin wrote: On 29.07.2012 11:37, Bruce Evans wrote: ... binuptime() is more accurate than uncalibrated scaling. Is accuracy required? Accuracy is not required at all. +-20% is not a problem. If not, the CPU ticker might work, and is faster than HPET, and and is not under user control for perverse settings. It normally reduces to readtsc() with no serializing instruction even in proposed changes. This is good enough for process times (not very good) and depends on the CPU not changing. Its calibration is very accurate (similar to timecounters) modulo bugs, but not always up to date. Problem with ticker that it may stop during idle periods, and idle is exactly what happens here. Unlike timecounter usage here we don't need CPU synchronicity, but we need it working during deep sleeps. The ticker is the same as the timecounter in many cases of interest. If the TSC stops then it cannot be used for timecounting unless timecounting is reinitialized. Timecounting should be reinitialized after deep sleeps, but you say you need it to work during deep sleeps. Timecounter already has detection logic to disable TSC in cases where it is unreliable. I don't want to replicate it here. I need not precise and not synchronized by reliable and fast time source. Yes, this logic gives exactly what you don't want (an inefficient timecounter), by preventing use of the TSC for the timecounter, although the TSC is perfectly usable for the ticker and here. Can you teach me how to use ticker that is not ticking? If TSC was considered unusable for timecounter for reasons unrelated to SMP, how can I use it as ticker. I wouldn't trust timecounters for some time after waking up after a deep sleep. If their clock stopped then the times read might only be very out of date. If their clock didn't stop, then they might have wrapped or otherwise overflowed and the times read would be garbage. Is there any locking or ordering to prevent them being used before they are reinitialized? I am not sure what reinitialization are you talking about. IIRC, there is no any waking up code for TSC. None other time counters have problems with C-states. It is the timecounter code that needs reinitializing. If the TSC stops, or wraps mod 2**32, then its counts become garbage for the purpose of timecounting. Maybe it is not used for timecounting in either of these cases. But these cases shouldn't prevent its use for timecounting. The 2**32 number is because timecounters only use 32 bits of hardware counters (for efficiency). So even if the hardware has some magic to not stop the TSC while sleeping (maybe it fakes not stopping it be reloading on wakeup), it is still unusable by timecounters after sleeping for a second or 2 so that it wraps. The software needs similar faking to reload the timecounter on wakeup. This makes use of timecounters in sleep/wakeup code fragile. At this moment I am not talking about S-states sleeping for hours. I am talking about C-states for milliseconds. It means that TSC may stop and start 10K times each second or even more. Attempt to save and restore its state will consume so much resources, that probably make it useless. What's about wrap after 2 seconds, I would be happy to make CPU sleep for so long, but now 100ms is all I can hope even on idle system. At boot time there is a dummy timecounter that returns bogo-times. Apparently sleeping doesn't occur before the timecounter is switched to a real one. The dummy timecounter isn't switched back to after boot time. But it probably should be, since the hardware timecounter may have stopped or wrapped. Sleeping could just set a flag to indicate this state, but then you would have to provide a fake time anyway on finding the flag set. Boot time just points to the dummy timecounter so as not to check this flag in all early timecounter "hardware" calls. And how dummy timecounter that counts something, but not time, can help me to measure sleep time? -- Alexander Motin ___ freebsd-acpi@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-acpi To unsubscribe, send any mail to "freebsd-acpi-unsubscr...@freebsd.org"
