linux-kernel - Re: Change in default vm_dirty

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Date:	Thu, 21 Jun 2007 17:53:20 -0500
From:	Matt Mackall <mpm@...enic.com>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>, davej@...hat.com,
	tim.c.chen@...ux.intel.com, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org
Subject: Re: Change in default vm_dirty_ratio

On Wed, Jun 20, 2007 at 11:20:59AM +0200, Jens Axboe wrote:
> On Wed, Jun 20 2007, Peter Zijlstra wrote:
> > On Wed, 2007-06-20 at 11:14 +0200, Jens Axboe wrote:
> > > On Wed, Jun 20 2007, Andrew Morton wrote:
> > > > Perhaps our queues are too long - if the VFS _does_ back off, it'll take
> > > > some time for that to have an effect.
> > > > 
> > > > Perhaps the fact that the queue size knows nothing about the _size_ of the
> > > > requests in the queue is a problem.
> > > 
> > > It's complicated, the size may not matter a lot. 128 sequential 512kb IO
> > > may complete faster than 128 random 4kb IO's.
> > 
> > Yes, is there any way a queue could be limited to a certain amount of
> > 'completion time' ?
> 
> Not easily, we'd need some sort of disk profile for that to be remotely
> reliable.

Perhaps we want to throw some sliding window algorithms at it. We can
bound requests and total I/O and if requests get retired too slowly we
can shrink the windows. Alternately, we can grow the window if we're
retiring things within our desired timeframe.

-- 
Mathematics is the supreme nostalgia of our time.
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