>> 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.
Victor
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