linux-kernel - Re: [PATCH V6 ] mm readahead: Fix readahead fail for memoryless cpu and limit readahead pages

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Date:	Tue, 18 Feb 2014 14:46:01 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	Fengguang Wu <fengguang.wu@...el.com>,
	David Cohen <david.a.cohen@...ux.intel.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Damien Ramonda <damien.ramonda@...el.com>,
	Jan Kara <jack@...e.cz>, Linus <torvalds@...ux-foundation.org>,
	nacc@...ux.vnet.ibm.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH V6 ] mm readahead: Fix readahead fail for memoryless cpu
 and limit readahead pages

On Tue, 18 Feb 2014, Andrew Morton wrote:

> > I'm not sure I understand why we want to be independent of PAGE_SIZE since 
> > we're still relying on PAGE_CACHE_SIZE.  Don't you mean to do
> > 
> > #define MAX_READAHEAD	((512*PAGE_SIZE)/PAGE_CACHE_SIZE)
> 
> MAX_READAHEAD is in units of "pages".
> 
> This:
> 
> +#define MAX_READAHEAD   ((512*4096)/PAGE_CACHE_SIZE)
> 
> means "two megabytes", and is implemented in a way to ensure that
> MAX_READAHEAD=2mb on 4k pagesize as well as on 64k pagesize.  Because
> we don't want variations in PAGE_SIZE to cause alterations in readahead
> behavior.
> 

Ah, ok, so 2MB is the magic value that we limit readhead to on all 
architectures.  512 * 4096 is a strange way to write 2MB, but ok :)
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