Question

I am trying to do some performance measurements using Intels RDTSC, and it is quite odd the variations I get during different testruns. In most cases my benchmark in C needs 3000000 Mio cycles, however, exactly the same execution can in some cases take 5000000, almost double as much. I tried to have no intense workloads running in parallel so that I get good performance estimations. Anyone an idea where this huge timing variations can come from? I know that interrupts and stuff can happening, but I did not expect such huge variations in timing!

PS.: I am running it on a Pentium processor with Linux running on it.

Thanks for feedback, John

Answer 1

I think the answer is in:

I tried to have no intense workloads running in parallel

You have insufficient control over this in a modern OS.

Answer 2

According to this Wikipedia article, the RDTSC (time stamp counter) cannot be used reliably for benchmarking on multi-core systems. There is no promise that all cores have the same value in the time stamp register.

On Linux, it is better to use the POSIX clock_gettime function.

Answer 3

You have to take the cache of most modern processors into account. Maybe another process evicts your program s cache content in the case where you measured the long running time. As Henk pointed out, lots of stuff happen in a modern OS that you can t control that much.

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