linux-kernel - Re: fio mmap sequential read 30% regression

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Date:	Thu, 2 Jul 2009 20:41:28 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Pavel Levshin <lpk@....spb.su>,
	"wli@...ementarian.org" <wli@...ementarian.org>,
	Nick Piggin <npiggin@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: fio mmap sequential read 30% regression

On Thu, Jul 02, 2009 at 04:01:22PM +0800, Zhang, Yanmin wrote:
> Comapring with 2.6.30's result, fio mmap sequtial read has
> about 30% regression on one of my stoakley machine with 1
> JBOD (7 SAS disks) with kernel 2.6.31-rc1.
> 
> Every disk has 2 partitions and 4 1-GB files per partition. Start
> 10 processes per disk to do mmap read sequentinally.
> 
> Bisect down to below patch.
> 
> ef00e08e26dd5d84271ef706262506b82195e752 is first bad commit
> commit ef00e08e26dd5d84271ef706262506b82195e752
> Author: Linus Torvalds <torvalds@...ux-foundation.org>
> Date:   Tue Jun 16 15:31:25 2009 -0700
> 
>     readahead: clean up and simplify the code for filemap page fault readahead
>     
>     This shouldn't really change behavior all that much, but the single rather
>     complex function with read-ahead inside a loop etc is broken up into more
>     manageable pieces.
>     
>     The behaviour is also less subtle, with the read-ahead being done up-front
>     rather than inside some subtle loop and thus avoiding the now unnecessary
>     extra state variables (ie "did_readaround" is gone).
>     
>     Fengguang: the code split in fact fixed a bug reported by Pavel Levshin:
>     the PGMAJFAULT accounting used to be bypassed when MADV_RANDOM is set, in
>     which case the original code will directly jump to no_cached_page reading.
> 
> 
> The bisect is stable.

Let me take care of this bug: it may well be caused by my other readahead patches.

Thanks,
Fengguang

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