Last updated on October 14, 2020 by Dan Nanni
When you run performance-critical HPC applications or network-heavy workload on multi-core NUMA processors, CPU/memory affinity is one important factor to consider to maximize their performance. Scheduling closely related processes on the same NUMA node can reduce slow remote memory access. On processors like Intel's Sandy Bridge processor which has an integrated PCIe controller, you want to schedule network I/O workload on the same NUMA node as the NIC card to exploit PCI-to-CPU affinity.
As part of performance tuning or troubleshooting, you may want to know on which CPU core (or NUMA node) a particular process is currently scheduled.
Here are several ways to find out which CPU core is a given Linux process or a thread is scheduled on.
If a process is explicitly pinned to a particular CPU core using commands like
taskset, you can find out the pinned CPU using the following
$ taskset -c -p <pid>
For example, if the process you are interested in has PID
$ taskset -c -p 5357
pid 5357's current affinity list: 5
The output says the process is pinned to CPU core 5.
However, if you haven't explicitly pinned the process to any CPU core, you will get something like the following as the affinity list.
pid 5357's current affinity list: 0-11
The output indicates that the process can potentially be scheduled on any CPU core from 0 to 11. So in this case,
taskset is not useful in identifying which CPU core the process is currently assigned to, and you should use other methods as described below.
ps command can tell you the CPU ID each process/thread is currently assigned to (under
$ ps -o pid,psr,comm -p <pid>
PID PSR COMMAND 5357 10 prog
The output says the process with PID
prog) is currently running on CPU core
10. If the process is not pinned, the
PSR column can keep changing over time depending on where the kernel scheduler assigns the process.
top command can also show the CPU assigned to a given process. First, launch
top command with
p option. Then press
f key, and add
Last used CPU column to the display. The currently used CPU core will appear under
$ top -p 5357
ps command, the advantage of using
top command is that you can continuously monitor how the assigned CPU changes over time.
Yet another method to check the currently used CPU of a process/thread is to use
htop from the command line. Press
<F2> key, go to
Columns, and add
The currently used CPU ID of each process will appear under
Note that all previous commands
top assign CPU core IDs
htop assigns CPU core IDs starting from