How to see how many context switches a process makes?

Question

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.

Answer 1

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 }'

Answer 2

pidstat(1) - Report statistics for Linux tasks. According to man pidstat it's so easy as just pidstat -w …

Answer 3

See man getrusage which will let you query the number of voluntary and involuntary context switches.

struct rusage {
           struct timeval ru_utime; /* user CPU time used */
           struct timeval ru_stime; /* system CPU time used */
           long   ru_maxrss;        /* maximum resident set size */
           long   ru_ixrss;         /* integral shared memory size */
           long   ru_idrss;         /* integral unshared data size */
           long   ru_isrss;         /* integral unshared stack size */
           long   ru_minflt;        /* page reclaims (soft page faults) */
           long   ru_majflt;        /* page faults (hard page faults) */
           long   ru_nswap;         /* swaps */
           long   ru_inblock;       /* block input operations */
           long   ru_oublock;       /* block output operations */
           long   ru_msgsnd;        /* IPC messages sent */
           long   ru_msgrcv;        /* IPC messages received */
           long   ru_nsignals;      /* signals received */
           long   ru_nvcsw;         /* voluntary context switches */
           long   ru_nivcsw;        /* involuntary context switches */
};

You can tell it to report per-thread information, like this:

struct rusage usage;
getrusage( RUSAGE_THREAD, &usage );

Just call it twice, before and after your critical section, and see if usage.ru_nivcsw value has increased or not.

Answer 4

To get a record of an entire process run, you can use the GNU time utility (don't confuse it with the bash builtin) with the -v option. Here's an example with unrelated lines of output removed:

$ `which time` -v ls
a.out  exception_finder.cpp  log.txt
    Command being timed: "ls"
               ...
    Voluntary context switches: 1
    Involuntary context switches: 2
               ...
    Exit status: 0

Answer 5

You can use, sar -w. For instance, sar -w 1 3, reports total number of context switches per second for every 1 seconds a total of 3 times.

Answer 6

Write the following script to file (ctx.sh). with ctx.sh <core> you will see all the processes running on a given core and changing nv-context switches will be highlighted. Looking at this, you will be able to identify which are the competing processes for the core.

#!/bin/bash

if [[ $# -eq 0 ]]
then
   echo "Usage:"
   echo "$0 <core>"
   exit 1
fi

if [[ -z $2 ]]
then
   watch -d -n .2 $0 $1 nw
fi

ps -Leo lastcpu:1,tid,comm | grep "^$1 " | awk '{printf $3": ";system("cut -d\" \" -f3  /proc/"$2"/task/"$2"/schedstat 2>/dev/null")}' | sort -k 1 | column -t

Answer 7

sudo perf stat -e context-switches -I 1000 PROCESS_NAME