Shell – How to see how many context switches a process makes

linuxprocessshell

I want to see if my process makes a lot of context switches. I also want to see how manpulating task groups affects the number of context switches.

Best Answer

You can view information about your process's context switches in /proc/<pid>/status.

$ pid=307
$ grep ctxt /proc/$pid/status
voluntary_ctxt_switches:        41
nonvoluntary_ctxt_switches:     16

To see these numbers updating continuously, run

$ # Update twice a second.
$ watch -n.5 grep ctxt /proc/$pid/status

To get just the numbers, run

$ grep ctxt /proc/$pid/status | awk '{ print $2 }'

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Linux – How to renice all threads (and children) of one process on Linux

You can use /proc/$PID/task to find all threads of a given process, therefore you can use

$ ls /proc/$PID/task | xargs renice $PRIO

to renice all threads belonging to a given process.

Same way /proc/$PID/task/$PID/children can be used to find all child processes (or /proc/$PID/task/*/children if you want all child processes of all threads of a given process).

$ cat /proc/$PID/task/$PID/children | xargs renice $PRIO
$ cat /proc/$PID/task/*/children | xargs renice $PRIO

Record time of every process or thread context switch

I don't have an answer but you might find one amongst the tools, examples and resources written or listed by Brendan Gregg on the perf command and Linux kernel ftrace and debugfs.

On my Raspberry Pi these tools were in package perf-tools-unstable. The perf command was actually in /usr/bin/perf_3.16.

Of interest may be this discussion and context-switch benchmark by Benoit Sigoure, and the lat_ctx test from the fairly old lmbench suite.

They may need some work to run on the Pi, for example with tsuna/contextswitch I edited timectxswws.c get_iterations() to while (iterations * ws_pages * 4096UL < 4294967295UL) {, and removed -march=native -mno-avx from the Makefile.

Using perf record for 10 seconds on the Pi over ssh whilst simultaneously doing while sleep .1;do echo hi;done in another ssh:

sudo timeout -10 perf_3.16 record -e context-switches -a
sudo perf_3.16 script -f time,pid,comm | less

gives output like this

           sleep 29341 2703976.560357: 
         swapper     0 2703976.562160: 
    kworker/u8:2 29163 2703976.564901: 
         swapper     0 2703976.565737: 
            echo 29342 2703976.565768: 
     migration/3    19 2703976.567549: 
           sleep 29343 2703976.570212: 
     kworker/0:0 28906 2703976.588613: 
     rcu_preempt     7 2703976.609261: 
           sleep 29343 2703976.670674: 
            bash 29066 2703976.671654: 
            echo 29344 2703976.675065: 
            sshd 29065 2703976.675454: 
         swapper     0 2703976.677757:

presumably showing when a context-switch event happened, for which process.

Best Answer

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Linux – How to renice all threads (and children) of one process on Linux

Record time of every process or thread context switch

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