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

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Message-Id: <1185093405.20032.205.camel@twins>
Date:	Sun, 22 Jul 2007 10:36:45 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	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 Sun, 2007-07-22 at 10:24 +0200, Jens Axboe wrote:
> On Sat, Jul 21 2007, Peter Zijlstra wrote:

> > +static __init int readahead_init(void)
> > +{
> > +	/*
> > +	 * Scale the max readahead window with system memory
> > +	 *
> > +	 *   64M:   128K
> > +	 *  128M:   180K
> > +	 *  256M:   256K
> > +	 *  512M:   360K
> > +	 *    1G:   512K
> > +	 *    2G:   724K
> > +	 *    4G:  1024K
> > +	 *    8G:  1448K
> > +	 *   16G:  2048K
> > +	 */
> > +	ra_pages = int_sqrt(totalram_pages/16);
> > +	if (ra_pages > (2 << (20 - PAGE_SHIFT)))
> > +		ra_pages = 2 << (20 - PAGE_SHIFT);
> > +
> > +	return 0;
> > +}
> 
> How did you come up with these numbers?

Well, most other places in the kernel where we scale by memory size we
use the a sqrt curve, and the specific scale was the result of some
fiddling, these numbers looked sane to me, nothing special.

Would you suggest a different set, and if so, do you have any rationale
for them?

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