Is clock-based timing reliable when CPU frequency is variable?

Question

A common way to measure elapsed time is:

const clock_t START = clock();
// ...
const clock_t END = clock();
double T_ELAPSED = (double)(END - START) / CLOCKS_PER_SEC;

I know this is not the best way to measure real time, but I wonder if it works on a system with a variable frequency CPU. Is it just wrong?

Presumably, CLOCKS_PER_SEC is defined within a header file as a constant value. If the underlying parameter represented by the value can change and you don't change the constant accordingly then, no, your elapsed time measurement won't be correct. — sizzzzlerz, Nov 24 '10 at 16:45
If you're doing benchmarking then it's best to set the CPU frequency governor to "performance", at least temporarily, so that you can get accurate measurements, unskewed by clock speed variations. — Paul R, Nov 24 '10 at 16:48

score 1 · Answer 1 · answered Nov 24 '10 at 16:53

It is not good to use on a variable clock speed CPU.

http://support.ntp.org/bin/view/Support/KnownHardwareIssues

NTP (network time prototcol) daemon on linux had issues with it.

Most OS's have some API calls for more accurate values, an example being on windows, QueryPerformanceCounter

http://msdn.microsoft.com/en-us/library/ms644904%28VS.85%29.aspx

score 1 · Accepted Answer · answered Nov 24 '10 at 17:00

There are system architectures that change the frequency of the CPU but have a separate and constant frequency to drive a system clock. One would think that a clock() function would return a time independent of the CPU frequency but this would have to be verified on each system the code is intended to run on.

Is clock-based timing reliable when CPU frequency is variable?

2 Answers2