linux-kernel - Re: [RFC] Block IO Controller V2

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Date:	Mon, 16 Nov 2009 16:32:15 -0500
From:	"Alan D. Brunelle" <Alan.Brunelle@...com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	linux-kernel@...r.kernel.org, jens.axboe@...cle.com
Subject: Re: [RFC] Block IO Controller V2 - some results

On Mon, 2009-11-16 at 16:14 -0500, Vivek Goyal wrote:
> On Mon, Nov 16, 2009 at 03:51:00PM -0500, Alan D. Brunelle wrote:
> > Hi Vivek: 
> > 
> > I'm finding some things that don't quite seem right - executive
> > summary: 
> 
> Hi Alan,
> 
> Thanks a lot for such an extensive testing and test results. I am still
> digesting the results but I thought I will make a quick note about writes.
> This patchset works only for sync IO. If you are performing buffered
> writes then you will not see any service differentiation. Providing
> support for buffered write path is in TODO list. 

Ah, I thought you meant sync I/O versus async I/O. So do you mean that
the testing should use _direct_ I/O (bypassing the cache)? 

> 
> > 
> > o  I think the apportionment algorithm doesn't work consistently well
> > for writes.
> > 
> > o  I think there are problems with significant performance loss when
> > doing random I/Os.
> 
> This concerns me. I had a quick look and as per your results, even with
> group_idle=0 you are seeing this regression. I guess this might be coming
> from the fact that we idle on sync-noidle workload per group and that
> idling becomes significant as number of groups increase.
> 
> Thanks
> Vivek

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