How to limit a process to run on One CPU core in Linux

This write-up explains different ways to restrict a program to run on a specific CPU core(s). This might be required for some high priority tasks/programs to run dedicatedly on few core on a multi-core/processor systems. This is used to achieve performance benefits from multi-processor systems.

sched-setaffinity()

• Linux System Call
• A process’s CPU affinity mask determines the set of CPUs on which it is eligible to run.
• Dedicating one CPU to a particular process can be acheved as:
  • setting the affinity mask of that process to specify a single CPU.
  • and setting the affinity mask of all other processes to exclude that CPU.
• Restricting a process to run on a single CPU also avoids the performance cost caused by the cache invalidation that occurs when a process ceases to execute on one CPU and then recommences execution on a different CPU.
• These restrictions on the actual set of CPUs on which the process will run are silently imposed by the kernel.
• The affinity mask is actually a per-thread attribute that can be adjusted independently for each of the threads in a thread group.
  • use -a option with taskset command to add affinity mask to all threads of the process.
• A child created via fork(2) inherits its parent’s CPU affinity mask. The affinity mask is preserved across an execve(2).

Python 3: sched_setaffinity()

• Python 3 os module supports sched-setaffinity() method
• Example usage:

  >>> import os
  >>> os.sched_getaffinity(0)
  {0, 1, 2, 3}
  ----------------
  
  >>> os.sched_setaffinity(0, {1, 3})
  >>> os.sched_getaffinity(0)
  {1, 3}
  ----------------
  
  >>> x = {i for i in range(10)}
  >>> x
  {0, 1, 2, 3, 4, 5, 6, 7, 8, 9}
  ----------------
  
  >>> os.sched_setaffinity(0, x)
  >>> os.sched_getaffinity(0)
  {0, 1, 2, 3}
  

Reference

• sched_setaffinity(2)
• Python 3: sched_setaffinity()
• Why does multiprocessing use only a single core after I import numpy?

taskset

• Linux Command
• set or retrieve a process’s *CPU affinity*.
• Works on a running process and can also launch a new process.
• CPU affinity is a scheduler property that “bonds” a process to a given set of CPUs on the system.
• The Linux scheduler will honor the given CPU affinity and the process will not run on any other CPUs.
• Note that the Linux scheduler also supports natural CPU affinity:
  • the scheduler attempts to keep processes on the same CPU as long as practical for performance reasons.
  • Therefore, forcing a specific CPU affinity is useful only in certain applications.
• The CPU affinity is represented as a bitmask,
  • with the lowest order bit corresponding to the first logical CPU.
  • and the highest order bit corresponding to the last logical CPU.
• Not all CPUs may exist on a given system but a mask may specify more CPUs than are present.
• A retrieved mask will reflect only the bits that correspond to CPUs physically on the system.
• Examples:

  taskset -c 0 mycommand --option    # start a command with the given affinity.
  -------------------

  taskset -c -pa 0 1234              # set the affinity of a running process.
                                     # -a for applying affinity mask to all the threads of the process.
  -------------------

  0x00000001
     is processor #0,
  -------------------

  0x00000003
     is processors #0 and #1,
  -------------------

  0xFFFFFFFF
     is processors #0 through #31,
  -------------------

  32
     is processors #1, #4, and #5,
  -------------------

  --cpu-list 0-2,6
  or
  - c 0-2,6
     is processors #0, #1, #2, and #6.
  -------------------

  --cpu-list 0-10:2
  or
  -c 0-10:2
     is processors #0, #2, #4, #6, #8 and #10. The suffix ":N"
     specifies stride in the range, for example 0-10:3 is
     interpreted as 0,3,6,9 list.

Reference

• taskset

Reference

• How to limit a process to one CPU core in Linux?