linux-kernel - Re: [PATCH 3/3] readahead: scale max readahead size depending on memory size

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Message-ID: <20070723101146.GF11657@kernel.dk>
Date:	Mon, 23 Jul 2007 12:11:46 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Jörn Engel <joern@...fs.org>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Fengguang Wu <wfg@...l.ustc.edu.cn>, riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Tim Pepper <lnxninja@...ibm.com>,
	Chris Snook <csnook@...hat.com>
Subject: Re: [PATCH 3/3] readahead: scale max readahead size depending on
	memory size

On Mon, Jul 23 2007, Jörn Engel wrote:
> On Sun, 22 July 2007 18:44:03 +0200, Jens Axboe wrote:
> > > 
> > > >  I agree with the low point of 128k.
> > > 
> > > Perhaps that should be enforced then, because currently a system with
> > > <64M will get less.
> > 
> > I think it should remain the low point.
> 
> I believe this whole thing is fundamentally flawed.  The perfect
> readahead size depends on the device in question.  If we set a single
> system-wide value depending on memory size, it may easily be too small
> and too large at the same time.  Think hard disk and SSD.
> 
> It may make sense to have a _maximum_ readahead size which depends on
> memory size.  But the preferred size (if there is enough RAM) should
> depend solely on the device, possibly as a function of the
> bandwidth * read latency product.

The value IS the maximum read-ahead value, not an enforced minimum.

However, there's definitely room for improvement in the queue feedback.
Even for seekless devices like SSD, read-ahead may be beneficial due to
zero request -> request latency.

-- 
Jens Axboe

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