How is run queue length computed in linux proc filesystem

Question

I'm trying to obtain number of runnable processes from linux kernel. sar -q gives this information readily. However I'm trying to get this value from /proc filesystem. There is no file in /proc that gives this value directly, then how is runq-sz computed. The wiki page http://en.wikipedia.org/wiki/Load_(computing) provides some insight into how run queue length is computed based on ldavg values but it is unclear. Can someone provide more pointers on this. Cheers

Answer 1

As gcla said you cat use

cat /proc/loadavg

to read loadavarage from from kernel - but strictly speaking, it is not a queue length.

Take a look at

grep procs_running /proc/stat

and

grep procs_blocked /proc/stat

First is an actual running queue and second is a number of process blocked on disk IO. Load average is a function from sum of both.

Answer 2

here is the function in the sysstat daemon which provides the info sar prints out:

https://github.com/sysstat/sysstat/blob/master/rd_stats.c#L392

if ((fp = fopen(LOADAVG, "r")) == NULL)
        return;

/* Read load averages and queue length */
fscanf(fp, "%d.%d %d.%d %d.%d %ld/%d %*d\n",
       &load_tmp[0], &st_queue->load_avg_1,
       &load_tmp[1], &st_queue->load_avg_5,
       &load_tmp[2], &st_queue->load_avg_15,
       &st_queue->nr_running,
       &st_queue->nr_threads);

It reads from /proc/loadavg, which is populated by this kernel function

http://lxr.free-electrons.com/source/fs/proc/loadavg.c#L13

static int loadavg_proc_show(struct seq_file *m, void *v)
{
        unsigned long avnrun[3];

        get_avenrun(avnrun, FIXED_1/200, 0);

        seq_printf(m, "%lu.%02lu %lu.%02lu %lu.%02lu %ld/%d %d\n",
                LOAD_INT(avnrun[0]), LOAD_FRAC(avnrun[0]),
                LOAD_INT(avnrun[1]), LOAD_FRAC(avnrun[1]),
                LOAD_INT(avnrun[2]), LOAD_FRAC(avnrun[2]),
                nr_running(), nr_threads,
                task_active_pid_ns(current)->last_pid);
        return 0;
}

The nr_running() function provides the total of both currently running tasks and tasks that are ready to run on a CPU; it's an instantaneous measure. I believe this will line up with the sar runq-sz variable.

Graham