linux-kernel - Re: [PATCH] virtual sched

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Date:	Thu, 19 Jul 2007 19:20:21 +0000
From:	Jan Glauber <jang@...ux.vnet.ibm.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	LKML <linux-kernel@...r.kernel.org>, vatsa@...ux.vnet.ibm.com,
	mschwid2@...ux.vnet.ibm.com, efault@....de,
	dmitry.adamushko@...il.com, paulus@...ba.org, anton@...ba.org
Subject: Re: [PATCH] virtual sched_clock() for s390

On Thu, 2007-07-19 at 18:00 +0200, Ingo Molnar wrote:
> * Jeremy Fitzhardinge <jeremy@...p.org> wrote:
> 
> > >  /*
> > > - * Monotonic_clock - returns # of nanoseconds passed since time_init()
> > > + * Scheduler clock - returns current time in nanosec units.
> > > + * Now based on virtual cpu time to only account time the guest
> > > + * was actually running.
> > 
> > Runn*ing*?  Does it include time the VCPU spends idle/blocked?  If 
> > not, then the scheduler won't be able to tell how long a process has 
> > been asleep.  Maybe this doesn't matter (I had this problem in a 
> > version of Xen's sched_clock, and I can't say I saw an ill effects 
> > from it).

No, it does not include idle time, if we're going idle the cpu timer
gets stopped.

> CFS does measure time elapsed across task-sleep periods (and does 
> something similar to what the old scheduler's 'sleep average' 
> interactivity mechanism did), but that mechanism measures "time spent 
> running during sleep", not "time spent idling".
> 
> still, CFS needs time measurement across idle periods as well, for 
> another purpose: to be able to do precise task statistics for /proc. 
> (for top, ps, etc.) So it's still true that sched_clock() should include 
> idle periods too.

I'm not sure, s390 already has an implemetation for precise accounting
in the architecture code, does CFS also improve accounting data? 

Jan

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