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There is a procs_running value in /proc/stat on Linux which shows the number of processes (threads) running.

The question is: does this value include the current process which is querying procfs or does not?

On an absolutely idle system, should we expect the value to be 0 or 1 ?

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Does procs_running field in /proc/stat include the current process which is querying procfs?

Yes, it does. The /proc/stat pseudofile is generated in fs/proc/stat.c in the Linux kernel, by calling nr_running().

The nr_running() function is defined in kernel/sched/core.c, which simply sums over all per-cpu process counters. The current process is not treated in any special way here, so it is included in the count.

On the absolutely idle system should we expect value to be 0 or 1 ?

Since a process must be reading the pseudo-file for the kernel to generate the contents, and such a process is always counted by the nr_running() kernel function, we should always expect it to be at least 1.

  • Running 4.7 and 4.8 here, and in all my systems it seems to be affected only by the current command line processes; i.e. if a simple cat, it shows 1, if with pipes, it shows the number of pipes. Odd. – Rui F Ribeiro Nov 27 '16 at 22:53
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    You are right. It is the same on older kernels such as 3.13 (Ubuntu 14.04 LTS) The question is when how does vmstat always shows 1 even if you run it through pipe – Peter Zaitsev Nov 27 '16 at 23:01
  • @PeterZaitsev: You should have included that in your question, then. In any case, which version; vmstat --version? If procps-ng, then look at the sources at proc/sysinfo.c:getstat(); specifically, find the comment // exclude vmstat itself. In other words, vmstat from procps-ng always subtracts one from the procs_running field. – Nominal Animal Nov 27 '16 at 23:32
  • I have checked with multiple versions including vmstat from procps-ng 3.3.9 Subtracting one is easy though @RuiFRibeiro brings interesting point - if you run ` cat /proc/stat | grep procs_r` you get value 2 on the idle box. My real issue is Prometheus Node Exporter seems to report value at least 4 on idle system and I'm trying to figure out a way to normalize it to match vmstat output in a way it will work on all systems – Peter Zaitsev Nov 28 '16 at 0:48
  • No, it is actually not that odd that only one process of cat /proc/stat | grep -e procs_r is running at the same time, because cating the pseudofile does not take much time at all, and the /proc/stat contents fit easily in the pipe buffer (4096 bytes by default). The shell creates the pipes first, then forks each piped command sequentially; if the cat binary is already in page cache, it is run and finishes faster than the second command in the pipe starts execution. If the second binary is started at roughly the same time, it'll block on standard input (counted in procs_blocked). ... – Nominal Animal Nov 28 '16 at 6:26
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With a little testing it looks like this INCLUDES the process which does the request. As such perhaps vmstat subtracts 1 from the number of active processes to show the real system load

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