Reserve a processor for only one process (with already the max priority)

Question

I have used this piece of code for trying to set the -same- high priority while executing a program :

CPU_SET(CPU_NUM, &cmask);

 if (pthread_setaffinity_np(pid, sizeof(cmask), &cmask) < 0) {
      LOG_ERROR("Could not set cpu affinity to core %d", CPU_NUM);      goto exit_err;
 }
 errno = 0;
 setpriority(PRIO_PROCESS, 0, -19);

The purpose of the program is to perform a computation for a constant bunch (every 80 bytes) of input.

But when executing the program, the time elapsed for this computation varies from 30% to 150%.

When plotting the computation time values, I was waiting for a -quite- smooth graph were the deviation would be something like 10%-15%, but instead there is more than 40% !!!

So I would like to ask, if the CPU is interfering the execution of the program with an other, and if so could I force the CPU to run ONLY a specific program?

Thanks in advance !

P.S. I haven't found a thread that could answer to my question yet...

The most relevant is :) : Linux reserve a processor for a group of processes (dynamically)

score 2 · Answer 1 · edited May 23 '17 at 12:04

To try and reduce jitter some of the things you can do are:

Ensure sure you've turned off CPU scaling.
Set scheduling policy to SCHED_FIFO for that program.
Try and pin your process to a single processor if you have more than one.
Try and run as few other processes at the same time while you're measuring your program.
Don't trigger sources of time related non-determinism (e.g. disk I/O).

It is probably useful to skim through How to build a Linux RT application because accurate measurement is the same domain - it's possible to be more extreme though:

Ensure your program doesn't use dynamic memory allocations.
Use a realtime Linux kernel.
Prevent Linux from scheduling non-specific userspace programs on a given CPU.
Even disable timer ticks on a given CPU (CONFIG_TASK_ISOLATION).

Modern desktop/server processors are so complicated that trying to precisely measure a single program's execution time with low variance is extremely hard. Things like the various caches and pipeline starting states can perturb execution times in any number of ways so there are always going to be limits.

Reserve a processor for only one process (with already the max priority)

1 Answers1