linux-kernel - Re: [PATCH RFC] mm/swap: automatic tuning for swapin readahead

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Date:	Mon, 8 Oct 2012 15:09:58 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Konstantin Khlebnikov <khlebnikov@...nvz.org>
cc:	Shaohua Li <shli@...nel.org>, Rik van Riel <riel@...hat.com>,
	Minchan Kim <minchan@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Wu Fengguang <fengguang.wu@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH RFC] mm/swap: automatic tuning for swapin readahead

On Thu, 4 Oct 2012, Konstantin Khlebnikov wrote:

> Here results of my test. Workload isn't very realistic, but at least it
> threaded: compiling linux-3.6 with defconfig in 16 threads on tmpfs,
> 512mb ram, dualcore cpu, ordinary hard disk. (test script in attachment)
> 
> average results for ten runs:
> 
> 		RA=3	RA=0	RA=1	RA=2	RA=4	Hugh	Shaohua
> real time	500	542	528	519	500	523	522
> user time	738	737	735	737	739	737	739
> sys time	93	93	91	92	96	92	93
> pgmajfault	62918	110533	92454	78221	54342	86601	77229
> pgpgin	2070372	795228	1034046	1471010	3177192	1154532	1599388
> pgpgout	2597278	2022037	2110020	2350380	2802670	2286671	2526570
> pswpin	462747	138873	202148	310969	739431	232710	341320
> pswpout	646363	502599	524613	584731	697797	568784	628677
> 
> So, last two columns shows mostly equal results: +4.6% and +4.4% in
> comparison to vanilla kernel with RA=3, but your version shows more stable
> results (std-error 2.7% against 4.8%) (all this numbers in huge table in
> attachment)

Thanks for doing this, Konstantin, but I'm stuck for anything much to say!
Shaohua and I are both about 4.5% bad for this particular test, but I'm
more consistently bad - hurrah!

I suspect (not a convincing argument) that if the test were just slightly
different (a little more or a little less memory, SSD instead of hard
disk, diskcache instead of tmpfs), then it would come out differently.

Did you draw any conclusions from the numbers you found?

I haven't done any more on this in the last few days, except to verify
that once an anon_vma is judged random with Shaohua's, then it appears
to be condemned to no-readahead ever after.

That's probably something that a hack like I had in mine would fix,
but that addition might change its balance further (and increase vma
or anon_vma size) - not tried yet.

All I want to do right now, is suggest to Andrew that he hold Shaohua's
patch back from 3.7 for the moment: I'll send a response to Sep 7th's
mm-commits mail to suggest that - but no great disaster if he ignores me.

Hugh

> 
> Numbers from your tests formatted into table for better readability
> 				
> HDD		Vanilla	Shaohua	RA=3	RA=0	RA=4
> SEQ, ANON	73921	76210	75611	121542	77950
> SEQ, SHMEM	73601	73176	73855	118322	73534
> RND, ANON	895392	831243	871569	841680	863871
> RND, SHMEM	1058375	1053486	827935	756489	834804
> 
> SDD		Vanilla	Shaohua	RA=3	RA=0	RA=4
> SEQ, ANON	24634	24198	24673	70018	21125
> SEQ, SHMEM	24959	24932	25052	69678	21387
> RND, ANON	43014	26146	28075	25901	28686
> RND, SHMEM	45349	45215	28249	24332	28226
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