QueryPerformanceCounter -- Timming code

Question

Hello --

I am calling the win/sdk function QueryPerformanceCounter to do timing -- how is
it in some cases that over 2 calls over time that the call in the future has a
lesser value than the one in the past so

start = QueryPerformanceCounter

:::  wait some time

finish = QueryPerformanceCounter


then the value of

finish - start

is less than zero -- is this possible?

Thanks

Answer 1

What kind is the machine? How much is the difference?

Assuming it is not a "normal" time rollover, you have a buggy platform, where QPC is not monotonic. Avoid these platforms or find a fix (BIOS update, etc) if you can.

-- pa

Answer 2

Hello --  Thanks for the reply. It's win/xp -- there are no more BIOS updates from the vendor MSI -- Its an old  MOBO  -- from 2006  -- I guess I could set affinity and just run on 1 cpu  -- are there any other work-a-rounds? 

Answer 3

You are probably hitting a bug on older AMD machines where the RDTSC value is not sync'd between cores.

http://support.microsoft.com/kb/909944/en-us

See Gaming Timing and Multicore Processors

Answer 4

yes -- thank for the info ... If its a hardware /  bios issue there is noting I can do about it at this time

I guess its its critical, I could force the process to run on 1 cpu 

thanks again 

Answer 5

If I remember correctly, this behavior can be seen even on one-CPU machines (I've seen it on a Geode UP system).

-- pa

Answer 6

If the RDTSC count isn't monotonically increasing on a single-CPU machine, then that's a pretty nasty hardware bug. Of course, you still have to cope with 32-bit rollover on x86 CPUs.