linux-kernel - Re: deadline unfairness

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Message-Id: <200803241744.35298.bernd-schubert@gmx.de>
Date:	Mon, 24 Mar 2008 17:44:35 +0100
From:	Bernd Schubert <bernd-schubert@....de>
To:	Aaron Carroll <aaronc@...ato.unsw.edu.au>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: deadline unfairness

Hallo Aron,

On Saturday 22 March 2008, Bernd Schubert wrote:
> Hello Aron,
>
> On Saturday 22 March 2008, Aaron Carroll wrote:
> > Bernd Schubert wrote:
> > > Hello,
> > >
> > > some it seems the deadline scheduler is rather unfair. Below is an
> > > example of md-raid6 initialization of md3, md4 and md5. All three
> > > md-devices do share the same blockdevices (we have patched md to allow
> > > parallel rebuild of shared block devices, since for us the cpu is the
> > > bottleneck and not the block device).
> > >
> > > All rebuilds started basically at the same time, as you can see, md3 is
> > > already done and now md4 rebuilds substantially faster than md5.
> > > [..]
> > > This is basically with a 2.6.22 kernel + lustre + md-backports, but
> > > nothing done to the scheduler.
> >
> > Hi Bernd,
> >
> > There is a deadline bug in pre-2.6.24 kernels where lower-sector requests
> > can starve higher-sector requests; you might be hitting this bug.  It was
> > fixed by commit: 6f5d8aa6382eef2b26032c88656270bdae7f0c42
>
> thanks a lot for your help! I will build a new kernel later on today and
> then report back if it helps. Commit
> dfb3d72a9aa519672c9ae06f0d2f93eccb35482f also looks useful...

After applying both patches it looks much better now.


Thanks again for your help,
Bernd


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