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Mon Nov 2 19:10:12 CET 2009 · https://mail.python.org/pipermail/python-dev/2009-November/093559.html

Martin v. Löwis skrev:
>> I did, and it does nothing of what I suggested. I am sure I can make the
>> Windows GIL in ceval_gil.h and the mutex in thread_nt.h at lot more precise and
>> efficient.
>>     
>
> Hmm. I'm skeptical that your code makes it more accurate, and I
> completely fail to see that it makes it more efficient (by what
> measurement of efficiency?)
>
> Also, why would making it more accurate make it better? IIUC,
> accuracy is completely irrelevant here, though efficiency
> (low overhead) does matter.
>
>   
>> This is the kind of code I was talking about, from ceval_gil.h:
>>
>> r = WaitForMultipleObjects(2, objects, TRUE, milliseconds);
>>
>> I would turn on multimedia timer (it is not on by default), and replace
>> this
>> call with a loop, approximately like this:
>>
>> for (;;) {
>>   r = WaitForMultipleObjects(2, objects, TRUE, 0);
>>   /* blah blah blah */     QueryPerformanceCounter(&cnt);    if (cnt >
>> timeout) break;
>>   Sleep(0);
>> }
>>
>> And the timeout "milliseconds" would now be computed from querying the
>> performance counter, instead of unreliably by the Windows NT kernel.
>>     
>
> Hmm. This creates a busy wait loop; if you add larger sleep values,
> then it loses accuracy.
>
>   
Actually an usleep lookes like this, and the call to the wait function 
must go into the for loop. But no, it's not a busy sleep.

static int inited = 0;
static __int64 hz;
static double dhz;
const double sleep_granularity = 2.0E10-3;

void usleep( long us )
{
    __int64 cnt, end;
    double diff;
    if (!inited) {
        timeBeginPeriod(1);
        QueryPerformanceFrequency((LARGE_INTEGER*)&hz);
        dhz = (double)hz;
        inited = 1;
    }      
    QueryPerformanceCounter((LARGE_INTEGER*)&cnt);
    end = cnt + (__int64)(1.0E10-6 * (double)(us) * dhz);
    for (;;) {
        QueryPerformanceCounter((LARGE_INTEGER*)&cnt);
        if (cnt >= end) break;
        diff = (double)(end - cnt)/dhz;
        if (diff > sleep_granularity)
            Sleep((DWORD)(diff - sleep_granularity));
        else
            Sleep(0);  
    }
}


> Why not just call timeBeginPeriod, and then rely on the higher clock
> rate for WaitForMultipleObjects?
>
>   
That is what I suggested when Antoine said 1-2 ms was enough.


Sturla

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[Python-Dev] Reworking the GIL