time.clock() going backwards??

[Claudio Grondi]

Tim Roberts wrote:
> "Tim Peters" <tim.peters at gmail.com> wrote:
> 
> 
>>[Giovanni Bajo[
>>
>>>>I experimented something very strange, a few days ago. I was debugging an
>>>>application at a customer's site, and the problem turned out to be that
>>>>time.clock() was going "backwards", that is it was sometimes
>>>>(randomically) returning a floating point value which was "less than" the
>>>>value returned by the previous invokation. The computer was a pretty fast
>>>>one (P4 3Ghz I think, running Windows XP), and this happened only between
>>>>very close invokations of time.clock().
>>
>>[Terry Reed]
>>
>>>I seem to remember this being mentioned before on the list, perhaps by Tim
>>>Peters.  Perhaps he will chime in.
>>
>>No real need ;-)  BIOS or HAL bug on a multiprocessor (or maybe even
>>hyperthreaded) box is by far the most likely cause (and the only cause
>>ever identified for sure by people who followed up).  Python's C code
>>slinging QueryPerformanceCounter is exceedingly simple, and isn't a
>>suspect because of that.  It's on the edge of vague possibility that
>>Microsoft's compiler generates non-monotonic code for converting
>>64-bit integer to double:
> 
> 
> It is much simpler than that.  With a multiprocessor HAL, including on a
> dual-core or hyperthreaded system, QueryPerformanceCounter returns the raw
> cycle counter (RDTSC).  However, on Windows XP, the operating system does
> not synchronize the cycle counters on multiple processors, and they can be
> actually be millions of cycles apart.
> 
> This was a change from previous systems.  On NT4 and Win2000, the operating
> actually rewrote the cycle counters on the second (and beyond) processors
> to align them to the first processor, so the delta was usually only a dozen
> or two cycles.  XP does not appear to do that.  I think that is a huge
> mistake, since it renders QueryPerformanceCounter non-monotonic.

How does it come, that processors on same mainboard run at different 
speeds? Do they have separate clock-pulse generators?

I can remember, that (at least on very old motherboards) the clock-pulse 
generator was a separate element and the processor just used it, so I 
would expect, that even in case of multiple processors, if there were 
only one clock-pulse generator for all of them, they were not be able to 
run at different speeds.

Claudio Grondi