time.clock() going backwards??

[Giovanni Bajo]

Hello,

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().

I have triple-verified this, including printing the repr() of the floating
point number and verifying it was really minor than the previous value by a few
microseconds. In other words, I'm absolutely positive that it's not a mistake
on my side, but that time.clock() was really apparently "jumping backward".
This was confusing the hell out of my application, of course, and I just hacked
it so to ignore these bogus reads, and just reading off again. Since the error
was just of a few microseconds, reading time.clock() again produces a number
which was higher than what I had before, and thus OK for my application.

I was wondering if someone had experimented this behaviour before. I tried
googling but to no effect. Is it possible this to be a bug in Python itself
(maybe, shooting at the moon, in the conversion between the 64bit performance
counter and the floating point representation returned by time.clock()), or
could it be a bug in Windows itself or the mother board drivers (buf if so,
wouldn't other application start going mad)?
-- 
Giovanni Bajo