linux-kernel - Re: madvise failure

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Message-Id: <20090508100745.6ef9fe3c.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Fri, 8 May 2009 10:07:45 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	"Andries E. Brouwer" <Andries.Brouwer@....nl>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: madvise failure

On Thu, 7 May 2009 14:10:17 +0200
"Andries E. Brouwer" <Andries.Brouwer@....nl> wrote:

> In an application something like
> 	p = mmap(0, sz, PROT_READ, MAP_PRIVATE, fd, 0);
> 	madvise(p, sz, MADV_SEQUENTIAL);
> is done for a small number of files, each with a size of a few GB.
> A single sequential pass is done over these files - essentially a merge.
> 
> On an old machine the madvise is useful, and decreases total time.
With the same kernel (Ubuntu/2.6.27) ? old one ?

> But on a more recent machine, with more memory, the madvise makes
> things worse. There, it seems better not to reveal to the kernel
> that the data will be read sequentially.
> 
> Timing (six files of 4GB each, quadcore Intel Q9550, 16GB memory,
> kernel 2.6.27 [Ubuntu], two other processes active):
> with madvise, 7 runs: real time varying 9m10s - 37m29s,
> without madvise, 6 runs: real time fairly constant 5m45s - 5m54s.
> 
> 
Accessing each page one by one sequentially ? or 
Sequential but (may) skip some pages in each access ?

Thanks,
-Kame

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