Multithreading

Restricting a job to run on specific CPUs

Suppose you have a job or a process that runs in parallel on multiple cores and you would like to limit the number of CPUs that this job has access to. The set of CPUs a process is eligible to run on is determined by the process’s “CPU affinity mask”.

Under Linux, a job or process’s affinity can be set by executing the sched_setaffinity system call. There is a standard shell wrapper (i.e. a shell script that embeds a system command or utility, that saves a set of parameters passed to that command) that can be used to to set this called taskset.

taskset can be used to both start a job or process with a specific CPU affinity, as well as change the CPU affinity of an already-running job or process. Its syntax is as follows:

• To start a new job to run on specific CPUs:

  taskset [options] mask command [argument...]
  

• To set the CPU affinity of a job with process id <pid>:

  taskset [options] -p [mask] <pid>
  

  By default, the mask is specified in hexadecimal. If you would instead like to use a numerical list of processors (from, say, 0 to N), the -c (or --cpu-list) flag can be used.

Examples:

1. To start a job with name <job> to run on specific CPUs:

   taskset --cpu-list 0-2,4 <job>
   

   This will run <job> on CPUs #0, #1, #2, and #4.

2. To retrieve the CPU affinity of a process whose process id (pid) is <pid>

   taskset --pid <pid>
   

   or in short-hand

   taskset -p <pid>
   

3. To change the CPU affinity of a process whose process id is <pid>

   taskset --cpu-list --pid 0-3 <pid>
   

   or in shorthand

   taskset -cp 0-3 <pid>
   

   which will set the job with process id <pid> to run on CPUs #0, #1, #2, and #3.