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 14:38:38 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	David Rientjes <rientjes@...gle.com>
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 14:23:44 -0800 (PST) David Rientjes <rientjes@...gle.com> wrote:

> 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?

yup.

> >  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)

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.
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