How to set CPU affinity in FreeBSD to allocate specific cores to specific processes

If your hardware has multiple CPUs, you can set CPU affinity to run certain processes on specific cores. In FreeBSD, the cpuset command is used for this purpose. I have a test server on AWS with two CPU cores. First, let’s verify the hardware by running the following command:

sysctl -a | egrep -i 'hw.machine|hw.model|hw.ncpu'

So you can see I have two cpu s on this machine , its upto 2 cores ie, 0 and 1 are the cpu’s. So from the two cpu cores, I want to run all my system default processes on first cpu and only my openvpn on second cpu .

To set all the default processes on core 0

cpuset -s1 -l 0  (the last 0 is the cpu number)

To make this change permanent even after reboot , add the below line to the /etc/rc.conf file

/usr/bin/cpuset -s1 -l 0

To set the openvpn process on CPU2 , first you need to find out the openvpn process id using the ps command .

ps ax | grep openvpn

This will show the process id of your openvpn process . Then run the below command to move the process to cpu 1

cpuset -C -c -l 1 -p openvpnpid

So to get this working even after the reboot , either you need to run some startup script to find the process id and then execute the above command or run it manually . This is because every time system reboots or the service stops the process id changes , so thats something you need to figure out .

So now if you run a top command you will be able to see all my system processes are running on CPU 0 and the openvpn process on core 1