time.clock() or Windows bug?

Tim Roberts timr at probo.com
Mon Jun 9 07:15:20 CEST 2008

Nick Craig-Wood <nick at craig-wood.com> wrote:
>time.clock() uses QueryPerformanceCounter under windows.  There are
>some known problems with that (eg with Dual core AMD processors).
>See http://msdn.microsoft.com/en-us/library/ms644904.aspx
>And in particular
>    On a multiprocessor computer, it should not matter which processor
>    is called. However, you can get different results on different
>    processors due to bugs in the basic input/output system (BIOS) or
>    the hardware abstraction layer (HAL). To specify processor
>    affinity for a thread, use the SetThreadAffinityMask function.

That's an extremely arrogant statement on their part, because the fault
here is entirely within Windows.

Through Windows 2000, the operating system actually synchronized the cycle
counters on the additional processors as they came out of reset at boot
time.  (The cycle counter is, after all, a writable register.)  As a
result, the cycle counters were rarely off by more than about 20 cycles.

Beginning with XP, they stopped doing that.  As a result, the cycle
counters on multiprocessor machines can vary by millions or even tens of
millions of cycles.
Tim Roberts, timr at probo.com
Providenza & Boekelheide, Inc.

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