linux-kernel - Re: Understanding I/O behaviour

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Date:	Wed, 29 Aug 2007 16:40:39 +0800
From:	Fengguang Wu <wfg@...l.ustc.edu.cn>
To:	Martin Knoblauch <spamtrap@...bisoft.de>
Cc:	linux-kernel@...r.kernel.org,
	Peter zijlstra <a.p.zijlstra@...llo.nl>, mingo@...hat.com
Subject: Re: Understanding I/O behaviour - next try

On Wed, Aug 29, 2007 at 01:15:45AM -0700, Martin Knoblauch wrote:
> 
> --- Fengguang Wu <wfg@...l.ustc.edu.cn> wrote:
> 
> > You are apparently running into the sluggish kupdate-style writeback
> > problem with large files: huge amount of dirty pages are getting
> > accumulated and flushed to the disk all at once when dirty background
> > ratio is reached. The current -mm tree has some fixes for it, and
> > there are some more in my tree. Martin, I'll send you the patch if
> > you'd like to try it out.
> >
> Hi Fengguang,
> 
>  Yeah, that pretty much describes the situation we end up. Although
> "sluggish" is much to friendly if we hit the situation :-)
> 
>  Yes, I am very interested  to check out your patch. I saw your
> postings on LKML already and was already curious. Any chance you have
> something agains 2.6.22-stable? I have reasons not to move to -23 or
> -mm.

Well, they are a dozen patches from various sources.  I managed to
back-port them. It compiles and runs, however I cannot guarantee
more...

> > >  Another thing I saw during my tests is that when writing to NFS,
> > the
> > > "dirty" or "nr_dirty" numbers are always 0. Is this a conceptual
> > thing,
> > > or a bug?
> > 
> > What are the nr_unstable numbers?
> >
> 
>  Ahh. Yes, they go up to 80-90k pages. Comparable to the nr_dirty
> numbers for the disk case. Good to know.
> 
>  For NFS, the nr_writeback numbers seem surprisingly high. They also go
> to 80-90k (pages ?). In the disk case they rarely go over 12k.

Maybe the difference of throttling one single 'cp' and a dozen 'nfsd'?

Fengguang

View attachment "writeback-fixes.patch" of type "text/x-diff" (33833 bytes)