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realtime:documentation:howto:tools:cpu-partitioning:cpusets

cpusets

Cpusets is a Linux kernel facility and a subset of cgroups-v1, that provides a mechanism to assign a set of CPUs and NUMA memory nodes (if NUMA is available) to a set of tasks. This means in effect, that the tasks are constrained to the assigned cpusets. For a detailed documentation of cpusets, please refere to the Linux kernel documentation.

Enable cpusets

The following kernel configuration Parameter are required to enable cpusets:

  • CONFIG_CGROUPS
  • CONFIG_CPUSETS

Sysfs interface

To handle the cpusets in userspace, the functionality is available in sysfs: “/sys/fs/cgroup/cpuset/”. Each cpuset is represented by a directory in the cgroup file system containing (on top of the standard cgroup files) the following files describing that cpuset:

  • cpus: list of CPUs in that cpuset
  • mems: list of Memory Nodes in that cpuset
  • memory_migrate flag: if set, move pages to cpusets nodes
  • cpu_exclusive flag: is cpu placement exclusive?
  • mem_exclusive flag: is memory placement exclusive?
  • mem_hardwall flag: is memory allocation hardwalled
  • memory_pressure: measure of how much paging pressure in cpuset
  • memory_spread_page flag: if set, spread page cache evenly on allowed nodes
  • memory_spread_slab flag: if set, spread slab cache evenly on allowed nodes
  • sched_load_balance flag: if set, load balance within CPUs on that cpuset
  • sched_relax_domain_level: the searching range when migrating tasks

In addition, the root cpuset only has the following file:

  • memory_pressure_enabled flag: compute memory_pressure?

Realtime Example

The following bash commands show exemplary how cpusets can be used for realtime processes on a 4 CPU machine. To simplify the use of cpusets the Python tool cset can be used.

 $ mkdir /dev/cpuset/rt0
 $ echo 0 > /dev/cpuset/rt0/cpus
 $ echo 0 > /dev/cpuset/rt0/mems
 $ echo 1 > /dev/cpuset/rt0/cpu_exclusive
 $ echo $RT_PROC_PID > /dev/cpuset/rt0/tasks
 $ mkdir /dev/cpuset/system
 $ echo 1-3 > /dev/cpuset/system/cpus
 $ echo 0 > /dev/cpuset/system/mems
 $ echo 1 > /dev/cpuset/system/cpu_exclusive
 $ for pid in $(cat /dev/cpuset/tasks); do /bin/echo $pid > /dev/cpuset/system/tasks; done 
realtime/documentation/howto/tools/cpu-partitioning/cpusets.txt · Last modified: 2017/07/17 16:04 by anna-maria