[Python-Dev] Use QueryPerformanceCounter() for time.monotonic() and/or time.highres()?

M.-A. Lemburg mal at egenix.com
Tue Apr 3 14:26:25 CEST 2012


Victor Stinner wrote:
>>> You seem to have missed the episode where I explained that caching the last
>>> value in order to avoid going backwards doesn't work -- at least not if the
>>> cached value is internal to the API implementation.
>>>
>> Yes, and I can't find it by briefly searching my mail.  I haven't had the energy to follow every bit of this discussion because it has become completely insane.
> 
> I'm trying to complete the PEP, but I didn't add this part yet.
> 
>> Of course we cannot promise not moving backwards, since there is a 64 bit wraparound some years in the future.
> 
> Some years? I computed 584.5 years, so it should not occur in
> practice. 32-bit wraparound is a common issue which occurs in practice
> on Windows (49.7 days wraparound), and I propose a workaround in the
> PEP (already implemented in the related issue).
> 
>> Here's actual code from production:
>>
>> BOOL WINAPI QueryPerformanceCounterCCP( LARGE_INTEGER* li )
>> {
>>        static LARGE_INTEGER last = {0};
>>        BOOL ok = QueryPerformanceCounter(li);
>>        if( !ok )
>>        {
>>                return FALSE;
>>        }
> 
> Did you already see it failing in practice? Python ignores the return
> value and only uses the counter value.
> 
>> Even negative delta values of time are usually  harmless on the application level.
>>  A curiosity, but harmless.
> 
> It depends on your usecase. For a scheduler or to implement a timeout,
> it does matter. For a benchmark, it's not an issue because you usually
> repeat a test at least 3 times. Most advanced benchmarked tools gives
> a confidence factor to check if the benchmark ran fine or not.
> 
>>  I am offering empirical evidence here from hundreds of thousands of computers
>> over six years: For timing and benchmarking, QPC is good enough, and will only
>> be as precise as the hardware and operating system permits, which in practice
>> is good enough.
> 
> The PEP contains also different proofs that QPC is not steady,
> especially on virtual machines.

I'm not sure I understand what you are after here, Victor. For benchmarks
it really doesn't matter if one or two runs fail due to the timer having
a problem: you just repeat the run and ignore the false results (you
have such issues in all empirical studies). You're making things
needlessly complicated here.

Regarding the approach to try to cover all timing requirements into
a single time.steady() API, I'm not convinced that this is good
approach. Different applications have different needs, so it's
better to provide interfaces to what the OS has to offer and
let the application decide what's best.

If an application wants to have a monotonic clock, it should use
time.monotonic(). The OS doesn't provide it, you get an AttributeError
and revert to some other function, depending on your needs.

Having a time.steady() API make this decision for you, is not
going to make your application more portable, since the choice
will inevitably be wrong in some cases (e.g. going from
CLOCK_MONOTONIC to time.time() as fallback).

BTW: You might also want to take a look at the systimes.py module
in pybench. We've been through discussions related to
benchmark timing in 2006 already and that module summarizes
the best practice outcome :-)

-- 
Marc-Andre Lemburg
eGenix.com

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