Re: deadline unfairness

[Bernd Schubert <bernd-schubert@gmx.de>]

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...

Thanks again,
Bernd