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 14:23:44 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	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, Raghavendra K T wrote:

> Currently max_sane_readahead() returns zero on the cpu having no local memory node
> which leads to readahead failure. Fix the readahead failure by returning
> minimum of (requested pages, 512). Users running application on a memory-less cpu
> which needs readahead such as streaming application see considerable boost in the
> performance.
> 
> Result:
> fadvise experiment with FADV_WILLNEED on a PPC machine having memoryless CPU
> with 1GB testfile ( 12 iterations) yielded around 46.66% improvement.
> 
> fadvise experiment with FADV_WILLNEED on a x240 machine with 1GB testfile
> 32GB* 4G RAM  numa machine ( 12 iterations) showed no impact on the normal
> NUMA cases w/ patch.
> 
> Kernel     Avg  Stddev
> base	7.4975	3.92%
> patched	7.4174  3.26%
> 
> Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
> [Andrew: making return value PAGE_SIZE independent]
> Signed-off-by: Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>

So this replaces 
mm-readaheadc-fix-readahead-fail-for-no-local-memory-and-limit-readahead-pages.patch 
in -mm correct?

> ---
>  I would like to thank Honza, David for their valuable suggestions and 
>  patiently reviewing the patches.
> 
>  Changes in V6:
>   - Just limit the readahead to 2MB on 4k pages system as suggested by Linus.
>  and make it independent of PAGE_SIZE. 
> 

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)

instead?
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