Failing to understand getrusage()

[Kostik Belousov]

On Tue, Mar 07, 2006 at 02:15:56AM +0300, Yar Tikhiy wrote:
> On Thu, Mar 02, 2006 at 11:50:29PM +0000, Nick Barnes wrote:
> > At 2006-03-02 22:24:17+0000, Nik Clayton writes:
> > > I'm failing to understand how getrusage() works, which is a bit perplexing,
> > > because it doesn't seem like it would be terribly complicated.
> > 
> > ru_maxrss is the maximum resident set size, not the heap size.
> > malloc(big) doesn't grow the resident set.  Touching the memory you
> > have allocated will grow the resident set.  Try this:
> > 
> >         getrusage(RUSAGE_SELF, &ru);
> >         printf("%lu\n", ru.ru_maxrss);
> >         p = malloc(SIZE);
> >         assert(p)
> >         getrusage(RUSAGE_SELF, &ru);
> >         printf("%lu\n", ru.ru_maxrss);
> >         for (i=0; i<SIZE; ++i) {
> >                 p[i] = 0;
> >         }
> >         getrusage(RUSAGE_SELF, &ru);
> >         printf("%lu\n", ru.ru_maxrss);
> 
> Well, there was a call to memset() in the original Nik's program
> while your code just does the same by itself.
> 
> Personally, I'd like to say a "me too".  /me too fails to see why
> in a quiet, idle system ru_maxrss is very unpredictable over numerous
> runs of the test program, both before and after the malloc+memset.
> Filling the memory with a non-zero value doesn't matter.  Is it the
> Heizenberg daemon at work? :-)

I think that this is a statclock in work :). Just add some busy loops
before each calls to getrusage like

	for (x = 0; x < 0x1000000; x++)
			getpid();

and you would get statisically stable results:

deviant% ./1mb
before: 424, after: 1548
deviant% ./1mb
before: 424, after: 1548

See,
% sysctl kern.clockrate
kern.clockrate: { hz = 1000, tick = 1000, profhz = 666, stathz = 133 }

133 Hz is very slow on 3GHz machine, and curproc->p_stats->p_ru is
updated on statclock tick, see sys/kern/kern_clock.c.

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