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

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Date:	Mon, 17 Mar 2014 10:07:23 +0800
From:	Madper Xie <cxie@...hat.com>
To:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
Cc:	Jan Kara <jack@...e.cz>, 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>, rientjes@...gle.com,
	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


Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com> writes:

> On 02/18/2014 03:19 PM, Jan Kara wrote:
>> On Tue 18-02-14 12:55:38, 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.
>>    Can you try one more thing please? Compare startup time of some big
>> executable (Firefox or LibreOffice come to my mind) for the patched and
>> normal kernel on a machine which wasn't hit by this NUMA issue. And don't
>> forget to do "echo 3 >/proc/sys/vm/drop_caches" before each test to flush
>> the caches. If this doesn't show significant differences, I'm OK with the
>> patch.
>>
>
> Thanks Honza, I checked with firefox (starting to particular point)..
> I do not see any difference. Both the case took around 14sec.
>
>   ( some time it is even faster.. may be because we do not do free page 
> calculation?. )
Hi. Just a concern. Will the performance reduce on some special storage
backend? E.g. tape.
The existent applications may using readahead for userspace I/O schedule
to decrease seeking time.
-- 
Thanks,
Madper
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