How to find out on which core a process is running on in Linux

There are many processes in Linux which run on single core while many will switch between different core depending upon the availability.

With below two methods you can find if a process is continuously running on single core or if it is switching between all the available cores based on the availability.

Method 1

Below command can be used

# ps -o pid,psr,time,comm -p $(pgrep -x fio)
PID PSR TIME COMMAND
8453 4 00:00:00 fio
8455 8 00:00:01 fio

Here 'psr' column shows the last used core by the process. I have used pgrep to grep the pid of the process running by the name of 'fio'. Everytime I hit the same command I observe the processor to be changing

# ps -o pid,psr,time,comm -p $(pgrep -x fio)
PID PSR TIME COMMAND
8453 4 00:00:00 fio
8455 8 00:00:03 fio

# ps -o pid,psr,time,comm -p $(pgrep -x fio)
PID PSR TIME COMMAND
8453 9 00:00:00 fio
8455 8 00:00:03 fio

So to monitor this for a while I can run it in a while loop, here I have used a sleep of 5 seconds which you can increase based on the requirement

# while true; do echo -ne "`ps -o pid,psr,time,comm -p $(pgrep -x fio) `t"; date;sleep 5;done
PID PSR TIME COMMAND
8453 4 00:00:00 fio
8455 9 00:00:07 fio Fri Jul 7 18:47:13 IST 2017
PID PSR TIME COMMAND
8453 4 00:00:00 fio
8455 9 00:00:07 fio Fri Jul 7 18:47:18 IST 2017
PID PSR TIME COMMAND
8453 4 00:00:00 fio
8455 9 00:00:07 fio Fri Jul 7 18:47:23 IST 2017
PID PSR TIME COMMAND
8453 4 00:00:00 fio
8455 9 00:00:07 fio Fri Jul 7 18:47:28 IST 2017
PID PSR TIME COMMAND
8453 4 00:00:00 fio
8455 9 00:00:07 fio Fri Jul 7 18:47:33 IST 2017
PID PSR TIME COMMAND
8453 5 00:00:00 fio
8455 9 00:00:07 fio Fri Jul 7 18:47:38 IST 2017