Failing to understand getrusage()

[Yar Tikhiy]

On Tue, Mar 07, 2006 at 12:11:56PM +0200, Kostik Belousov wrote:
> On Tue, Mar 07, 2006 at 02:15:56AM +0300, Yar Tikhiy wrote:
> > 
> > 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.

This sounds very clear and reasonable.  I shouldn't have forgotten
about the driving role of statclock in collecting all rusage stats,
including those related to memory consumption.

Thank you for resolving our doubts! :-)

-- 
Yar