Re: CFS review

[Arjan van de Ven <arjan@infradead.org>]

On Fri, 2007-08-03 at 06:18 +0200, Willy Tarreau wrote:
> On Thu, Aug 02, 2007 at 08:57:47PM -0700, Arjan van de Ven wrote:
> > On Thu, 2007-08-02 at 22:04 -0500, Matt Mackall wrote:
> > > On Wed, Aug 01, 2007 at 01:22:29PM +0200, Ingo Molnar wrote:
> > > > 
> > > > * Roman Zippel <zippel@linux-m68k.org> wrote:
> > > > 
> > > > > [...] e.g. in this example there are three tasks that run only for 
> > > > > about 1ms every 3ms, but they get far more time than should have 
> > > > > gotten fairly:
> > > > > 
> > > > >  4544 roman     20   0  1796  520  432 S 32.1  0.4   0:21.08 lt
> > > > >  4545 roman     20   0  1796  344  256 R 32.1  0.3   0:21.07 lt
> > > > >  4546 roman     20   0  1796  344  256 R 31.7  0.3   0:21.07 lt
> > > > >  4547 roman     20   0  1532  272  216 R  3.3  0.2   0:01.94 l
> > > > 
> > > > Mike and me have managed to reproduce similarly looking 'top' output, 
> > > > but it takes some effort: we had to deliberately run a non-TSC 
> > > > sched_clock(), CONFIG_HZ=100, !CONFIG_NO_HZ and !CONFIG_HIGH_RES_TIMERS.
> > > 
> > > ..which is pretty much the state of play for lots of non-x86 hardware.
> > 
> > question is if it's significantly worse than before. With a 100 or
> > 1000Hz timer, you can't expect perfect fairness just due to the
> > extremely rough measurement of time spent...
> 
> Well, at least we're able to *measure* that task 'l' used 3.3% and
> that tasks 'lt' used 32%.

It's not measured if you use jiffies level stuff. It's at best sampled!

>  If we're able to measure it, then that's
> already fine enough to be able to adjust future timeslices credits.
> Granted it may be rough for small periods (a few jiffies), but it
> should be fair for larger periods. Or at least it should *report*

but the testcase here uses a LOT shorter time than jiffies... not "a few
jiffies".

-- 
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