time.clock() going backwards??

[Rob Williscroft]

Claudio Grondi wrote in news:ecu546$sn9$1 at newsreader2.netcologne.de in
gmane.comp.python.general: 

> Tim Roberts wrote:
>> "Tim Peters" <tim.peters at gmail.com> wrote:

>> 
>> 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 don't see any claim above that they run at different speeds, only 
that the counters are several million cycles apart, IOW running at the 
same speed but with different initial values, or more likely starting 
at different times. 

For processors that run at (say) 2GHz, several million (say 10 million) 
represents a difference of 10e6/2e9 = 0.005 seconds between when the 
processors were sufficiently powered up to start counting cycles.

Rob.
-- 
http://www.victim-prime.dsl.pipex.com/