systemd core Isolation

Some applications may require a dedicated core (and/or socket etc) to run optimally. We can achieve this by first

• Setting PID 1 (systemd) affinity mask to 1, so that any child processes run only on this core
• Manually starting new processes (which require their own core) on a spare core, overriding the affinity mask

Querying the current values

Some ways of demonstrating the current affinity mask:

hex formatted mask:

$ taskset -p 1
pid 1's current affinity mask: 1

list of CPUs:

$ taskset -p -c 1
pid 1's current affinity list: 0
or
$ grep "Cpus_allowed_list" /proc/1/status
Cpus_allowed_list:      0

These all show the current values

Setting the default values

in /etc/systemd/system.conf , add this to the file (in the below example, to allow children of PID 1 - systemd, to be allocated to CPUs 0 - 9)

CPUAffinity=0 1 2 3 4 5 6 7 8 9

then

$ systemctl daemon-reexec     (kill and restart the daemon)
$ grep Cpus_allowed_list /proc/1/status
Cpus_allowed_list:      0-9
$ taskset -p 1
pid 1's current affinity mask: 3ff
$ taskset -p -c 1
pid 1's current affinity list: 0-9

Starting a process on a specific core

$ taskset -c 5 my_server

Moving an existing process to a specific core

$ ps -fu robbie | grep my_other_server 
 PID TTY          TIME CMD
 3345 ?        00:00:00 my_other_server
$ taskset -c 7 -p 3345