Is there a way to suspend OS scheduling for the duration of a program?

Question

I have an assignment where I am analyzing the runtime of various sorting algorithms. I have written the code but I think it's an unfair comparison.

My code basically grabs the the clock time before and after the sorting is finished to compute the elapsed time. However, what if the OS decides to interrupt more frequently during the runtime of a specific sorting algorithm, or if it rather decides that some other background application should be given more of the time domain when it's thread comes back up?

I am not a CS major so I may not be entirely correct here, but from what I've read previously I was concerned this might have an impact on the results.

I also realize that if OS scheduling is suspended and the program hangs then there might be a serious problem; I am just wondering if it possible.

Just call the sorting algorithm a humdred times or so, and then divide the final time into 100. — mcleod_ideafix, Nov 20 '13 at 15:20
Or run the algorihm several times and keep with the least (best) time measured. — mcleod_ideafix, Nov 20 '13 at 15:20
Suspend IO scheduling? Including page-faults, I/O, everything? — Martin James, Nov 20 '13 at 15:30
Dual-boot to MS-DOS and compile/run your algorithm there? ;-) This is part joking, but part serious, as there is no scheduler there to get in your way. You'd still have to do something about interrupts though. — Digital Trauma, Nov 20 '13 at 15:45

score 4 · Answer 1 · answered Nov 20 '13 at 15:23

Normally, there's no real reason for it. The scheduler will slightly increase the execution time, but if the code runs for a few seconds, the change will be tiny.
So unless you're running heavy applications on the same computer, the amount of noise this will add to your tests is negligible.

In Linux, you can use isolcpus parameter to mark CPUs that won't be used by the scheduler. You can find information here. I'm not sure what's the minimal kernel version.
If you use it, you'll need to use sched_setaffinity, to put your theread on an isolated CPU, because the scheduler won't put it there.

score 3 · Answer 2 · edited May 23 '17 at 12:28

It is not possible, not in user space code. Otherwise, any malicious process could steal the CPU from others.

If you want precise time counting for your process only, I suggest using time command. You can read about it here: What do 'real', 'user' and 'sys' mean in the output of time(1)?

Quick answer: you are most likely interested in user time, assuming your code doesn't make a heavy use of syscalls (which would be rather strange for a sorting algorithm)

Shahbaz · Answer 3 · 2013-11-20T15:39:18.977

On an up-to-date POSIX system (basically Linux) you can use clock_gettime with CLOCK_PROCESS_CPUTIME_ID or CLOCK_THREAD_CPUTIME_ID if you make sure the process doesn't migrate between CPUs (you can set its affinity for example).

The difference in times returned by clock_gettime with those arguments results in exact time the process/thread spent executing. Only pitfall as I mentioned is process migration as the man page says:

The CLOCK_PROCESS_CPUTIME_ID and CLOCK_THREAD_CPUTIME_ID clocks are realized on many platforms using timers from the CPUs (TSC on i386, AR.ITC on Itanium). These registers may differ between CPUs and as a consequence these clocks may return bogus results if a process is migrated to another CPU.

This means that you don't really need to suspend all other processes just to measure the execution time of your program.

Is there a way to suspend OS scheduling for the duration of a program?

3 Answers3