linux-kernel - Re: [REPORT] cfs-v4 vs sd-0.44

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Date:	Sat, 21 Apr 2007 18:18:18 +0200
From:	Willy Tarreau <w@....eu>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Con Kolivas <kernel@...ivas.org>, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>, Mike Galbraith <efault@....de>,
	Arjan van de Ven <arjan@...radead.org>,
	Peter Williams <pwil3058@...pond.net.au>,
	Thomas Gleixner <tglx@...utronix.de>, caglar@...dus.org.tr,
	Gene Heskett <gene.heskett@...il.com>
Subject: Re: [REPORT] cfs-v4 vs sd-0.44

On Sat, Apr 21, 2007 at 05:46:14PM +0200, Ingo Molnar wrote:
> 
> * Willy Tarreau <w@....eu> wrote:
> 
> > I promised to perform some tests on your code. I'm short in time right 
> > now, but I observed behaviours that should be commented on.
> 
> thanks for the feedback!
> 
> > 3) CFS-v4
> > 
> >   Feels even better, mouse movements are very smooth even under high 
> >   load. I noticed that X gets reniced to -19 with this scheduler. I've 
> >   not looked at the code yet but this looked suspicious to me. I've 
> >   reniced it to 0 and it did not change any behaviour. Still very 
> >   good. The 64 ocbench share equal CPU time and show exact same 
> >   progress after 2000 iterations. The CPU load is more smoothly spread 
> >   according to vmstat, and there's no idle (see below). BUT I now 
> >   think it was wrong to let new processes start with no timeslice at 
> >   all, because it can take tens of seconds to start a new process when 
> >   only 64 ocbench are there. [...]
> 
> ok, i'll modify that portion and add back the 50%/50% parent/child CPU 
> time sharing approach again. (which CFS had in -v1) That should not 
> change the rest of your test and should improve the task startup 
> characteristics.

If you remember, with 50/50, I noticed some difficulties to fork many
processes. I think that during a fork(), the parent has a higher probability
of forking other processes than the child. So at least, we should use
something like 67/33 or 75/25 for parent/child.

There are many shell-scripts out there doing a lot of fork(), and it should
be reasonable to let them keep some CPU to continue to work.

Also, I believe that (in shells), most forked processes do not even consume
a full timeslice (eg: $(uname -n) is very fast). This means that assigning
them with a shorter one will not hurt them while preserving the shell's
performance against CPU hogs.

Willy

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