Measuring amount of CPU time taken by a piece of code, in C on Unix/Linux

Question

Can clock() be used as a dependable API to measure time taken by CPU to execute a snippet of code? When verified usng times() / clock(), both do not seem to measure the CPU

Answer 1

I would try to correlate the time with the shell's time command, as a sanity check.

You should also consider that the compiler may be optimizing the loop. Since the memset does not depend on the loop variable the compiler will certainly be tempted to apply an optimization known as loop invariant code motion.

I would also caution that a 10MB possibly in-cache clear will really be 1.25 or 2.5 million CPU operations as memset certainly writes in 4-byte or 8-byte quantities. While I rather doubt that this could be done in less than a microsecond, as stores are a bit expensive and 100K adds some L1 cache pressure, you are talking about not much more than one operation per nanosecond, which is not that hard to sustain for a multi-GHz CPU.

One imagines that 600 nS would round off to 1 clock tick, but I would worry about that as well.