linux-kernel - Re: RFC - how to balance Dirty+Writeback in the face of slow writeback.

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Date:	Mon, 28 Aug 2006 11:28:30 +1000
From:	David Chinner <dgc@....com>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	Neil Brown <neilb@...e.de>, David Chinner <dgc@....com>,
	Andi Kleen <ak@...e.de>, linux-kernel@...r.kernel.org,
	akpm@...l.org
Subject: Re: RFC - how to balance Dirty+Writeback in the face of slow  writeback.

On Fri, Aug 25, 2006 at 08:37:24AM +0200, Jens Axboe wrote:
> On Fri, Aug 25 2006, Neil Brown wrote:
> 
> > I'm beginning to think that the current scheme really works very well
> > - except for a few 'bugs'(*).
> 
> It works ok, but it makes it hard to experiment with larger queue depths
> when the vm falls apart :-). It's not a big deal, though, even if the
> design isn't very nice - nr_requests is not a well defined entity. It
> can be anywhere from 512b to megabyte(s) in size. So throttling on X
> number of requests tends to be pretty vague and depends hugely on the
> workload (random vs sequential IO).

So maybe we need a different control parameter - the amount of memory we
allow to be backed up in a queue rather than the number of requests the
queue can take...

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group
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