Re: [9front] Understanding milli-CPUs (/dev/sysstat)

[Anthony Martin <ality@pbrane.org>]

Benjamin Riefenstahl <b.riefenstahl@turtle-trading.net> once said:
> > I wrote a daily script which sends me an email via a cron job. The
> > mail contains this load value from /dev/sysstat.
>
> I see that this device is described in cons(3).  I do not see from that
> documentation what time span sysstat uses.  It seems that you need to
> write to the file to reset it and than read from it after some time.
> But it also says that the vale is "decayed over time" which sounds like
> some kind of moving average.  It does not say (unless I missed it), how
> fast this decay is (i.e. again over which time span).

/sys/src/9/port/proc.c:/^accounttime

	/* only one processor gets to compute system load averages */
	if(m->machno != 0)
		return;

	/*
	 * calculate decaying load average.
	 * if we decay by (n-1)/n then it takes
	 * n clock ticks to go from load L to .36 L once
	 * things quiet down.  it takes about 5 n clock
	 * ticks to go to zero.  so using HZ means this is
	 * approximately the load over the last second,
	 * with a tail lasting about 5 seconds.
	 */
	n = nrun;
	nrun = 0;
	n = (nrdy+n)*1000*100;
	load = ((uvlong)load*(HZ-1)+n)/HZ;
	m->load = load/100;

Nrdy is the total number of procs in the scheduler queues.
Nrun is the number of procs running on each mach (cpu).

The multiplication and accompanying division by 100 is there
to handle precision problems with high values of HZ (the clock
tick frequency).

Cheers,
  Anthony