On Mon, Dec 05, 2011 at 02:42:54PM +0100, Ulrich Windl wrote:
> >>> Dejan Muhamedagic <[email protected]> schrieb am 05.12.2011 um 13:42 in
> Nachricht <[email protected]>:
> > Hi,
> >
> > On Thu, Dec 01, 2011 at 01:52:03PM +0100, Ulrich Windl wrote:
> > > >>> Dejan Muhamedagic <[email protected]> schrieb am 01.12.2011 um
> > > >>> 11:16 in
> > > Nachricht <[email protected]>:
> > > > Hi,
> > > >
> > > > On Thu, Dec 01, 2011 at 08:37:57AM +0100, Ulrich Windl wrote:
> > >
> > > [...]
> > > > > So what are you doing here?
> > > >
> > > > I'm not sure :) You seem to be very proficient at programming,
> > > > why don't then just take a look at the code in glue/clplumbing.
> > > > Search for exec_time.
> > >
> > > Hi!
> > > In which package? pacemaker? CRM? LRM?
> >
> > In cluster-glue.
>
> Hi!
>
> Found the implementation in lib/clplumbing/longclock.c with uses
> times(2) to get the resource usage. The units are measured in "clock
> ticks" which are an obsolete concept by now. As Linux one worked with
> "CLK_TCK == 100", that resolution is a multiple of 10ms.
>
> As it seems, getrusage(2) has a more powerful interface despite of the
> higher resolution clock.
>
> I don't know the motivation for the initial selection of times().
> Maybe it's that times() existed for more than 30 years.
That's probably one reason.
Requirements for the return value of "time_longclock()"
(which is what actually should be used)
are at least:
It must be monotonic, no jumping backwards.
It must not care for the wall clock time
(ignore any jumping around caused by "date --set" or equivalent,
so gettimeofday or similar are out).
It must not care which CPU it runs on.
It must be portable.
It should be as linear as possible wrt actually elapsed real time,
not return a diff of 100 for one elapsed second, and some time
later return a diff of 117 for the same "real time" period.
The return value of times(), (wrapped in time_longclock(), if
sizeof(clock_t) < 8) meets those requirements.
I don't see how getrusage would meet these requirements.
So if you want to hack something up with getrusage,
restrict that to a certain usage of time_longclock(),
not to the implementation of time_longclock() itself.
If you find something that fullfills those requirements,
so could possibly be used instead of times() in the
implementation of cl_times(), go ahead and suggest it.
clock_gettime with CLOCK_MONOTONIC may be a candidate.
But really: just to get some sub 10ms granularity of "exec time"?
Why even bother.
Cheers,
--
: Lars Ellenberg
: LINBIT | Your Way to High Availability
: DRBD/HA support and consulting http://www.linbit.com
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