ATTN: Arriving @ performance numbers Multi-core CPUs

[Sarnath]

All,

I had been using the “QueryPerformanceCounter()” APIs provided by Windows to assess performance. However, this API has the multi-core CPU drawback - where two profiling points in your code may have actually executed in different CPUs. Thus, this can give spurious performance numbers (I saw -ve times on my AMD dual-core while measuring short durations).

For this purpose, it would be a good idea to use “SetThreadAffinityMask” to tie a thread to 1 CPU (should hold good for MAIN thread as well).

MSDN link: [url=“Game Timing and Multicore Processors | Microsoft Docs”]Microsoft Docs - Developer tools, technical documentation and coding examples

I am not sure if this is applicable to the “event” record API from CUDA - Some1 said these APIs too internally use “QueryPerformanceCounter()”. Any comments from NVIDIA?

Best Regards,
Sarnath

[T.B]

Unless your BIOS is broken, QueryPerformanceCounter should work nicely in multicore environments.
It’s RDTSC that causes problems on multicore by reading the clock register.

If you are going to use thread affinity, you actually may want to look into RDTSC, if you sample often. It is a lot cheaper than QueryPerformanceCounter.

[Sarnath]

T.B,

Yes you are right. But to be on safer side (against buggy drivers,bioses), MSDN recommends this.

Set that single thread to remain on a single processor by using the Windows API SetThreadAffinityMask. Typically, this is the main game thread. While QueryPerformanceCounter and QueryPerformanceFrequency typically adjust for multiple processors, bugs in the BIOS or drivers may result in these routines returning different values as the thread moves from one processor to another. So, it’s best to keep the thread on a single processor.