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I have quad core CPU with hyper threading enabled. So I have 8 logical cores. I want to limit my application to use only 4 cores and I want this 4 cores to be different physical cores. Which taskset options (core numbers) should I use? :

  • taskset -c 0,1,2,3 command or
  • taskset -c 0,2,4,6 command

Thank you.

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  • Just to be clear on the Linux terminology, you have 8 logical "processors" distributed among 4 physical "cores". You want to pick 4 logical processors that correspond to 4 distinct cores. Commented Jun 19, 2018 at 18:25

3 Answers 3

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You can see which cores are on which processors by taking at look at /proc/cpuinfo. For example, you will see on a machine with a dual core cpu:

processor : 0 (snip) core id : 0 cpu cores : 2

for the first core on the processor and:

processor : 1 (snip) core id : 1 cpu cores : 2

on the second.

So, generally speaking, I think you would want:

taskset -c 0,1,2,3

or:

taskset -c 4,5,6,7
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    Um. just curious. If "processor 0" corresponds to [physical] core 0, and "processor 1" corresponds to [physical] core 1…then doesn't that mean that, to have all 4 cores selected, both 0 and 1 must necessarily be selected? (That is, 0/2/4/6 would only use physical cores 0 and 2?) Commented Jan 30, 2017 at 21:29
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    In your example, processor:0 is on core:0 and process:1 is on core:1. Those two processors are on different cores, so assuming that the pattern continues, the argument to taskset should be -c 0,1,2,3. Commented Jun 19, 2018 at 18:26
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You may want to take a look at CPU Set (cset) to create a CPU shield group for your application. This way you can ensure that real cores are being used for the important work. I typically disable hyperthreading (on Nehalem systems) for my realtime and low-latency applications.

Here's a tutorial for creating groups of CPUs for specific applications/processes using cset, which is a little more organized than taskset. https://rt.wiki.kernel.org/index.php/Cpuset_management_utility/tutorial

As far as determining the core count and location assignments, take a look at: http://dag.wieers.com/blog/is-hyper-threading-enabled-on-a-linux-system

A quick run of the utility linked on a quad-core Nehalem with hyperthreading enabled shows:

[root@XXX ~/hwloc-1.0.3]# ./utils/lstopo Machine (7980MB) + Socket #0 + L3 #0 (8192KB) L2 #0 (256KB) + L1 #0 (32KB) + Core #0 PU #0 (phys=0) PU #1 (phys=4) L2 #1 (256KB) + L1 #1 (32KB) + Core #1 PU #2 (phys=1) PU #3 (phys=5) L2 #2 (256KB) + L1 #2 (32KB) + Core #2 PU #4 (phys=2) PU #5 (phys=6) L2 #3 (256KB) + L1 #3 (32KB) + Core #3 PU #6 (phys=3) PU #7 (phys=7)
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    This still does not explain if taskset uses physical or logical indexing. If I run taskset -c 0,1,2,3 on the system above, will it bind to the first two physical cores (logical PUs 0-3) or to the first logical PU on each physical core? Commented Nov 22, 2020 at 9:43
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You can use like:

taskset -c -p 0-3 pid

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