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I want to measure the CPU time, not elapsed time on a thread. For example, if a thread is waiting or sleeping, it shouldn't count as CPU time because the thread is not in runnable state. So I got the following link to get CPU time. However, it seems to be capturing the elapsed time instead based on my test below (I expect cpu_time_used should be close to 0 but it is actually 2).What am I missing?

https://www.gnu.org/software/libc/manual/html_node/CPU-Time.html

#include <time.h>

clock_t start, end;
double cpu_time_used;

start = clock();
std::this_thread::sleep_for (std::chrono::seconds(2));
end = clock();
cpu_time_used = ((double) (end - start)) / CLOCKS_PER_SEC;
kaylum
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Kenneth
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1 Answers1

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Note that clock() measures in units of core-time, i.e. CLOCKS_PER_SEC clock() ticks represent one second of computation on one processor, so it indicates the amount of time used by the process over all threads. So, if there is another thread running during the sleep it will still be increasing the clock count -- if there are two threads in total the clock count will indicate 2 seconds have elapsed, like you have shown. If the sleeping thread is the only thread, you get a small amount of time, like @NateEldredge reports. On Linux you can query or setup a timer on CLOCK_THREAD_CPUTIME_ID instead, like @KamilCuk said.

Anonymous1847
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  • Ok, you are saying the block between start and end capture the CPU time of ALL THREADS that is running on ALL CORES during that time interval. Is that right? – Kenneth Nov 12 '20 at 01:07
  • @Kenneth Not just the one block of time. If there are two threads running, then the time gets counted twice, and the clock runs twice as fast (2*CLOCKS_PER_SEC increase per second). If there are three threads running, then the clock runs three times as fast. It counts the number of cycles consumed across all cores by your process. – Anonymous1847 Nov 12 '20 at 02:50