linux-kernel - fio mmap sequential read 30% regression

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Date:	Thu, 02 Jul 2009 16:01:22 +0800
From:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Pavel Levshin <lpk@....spb.su>, wli@...ementarian.org,
	Nick Piggin <npiggin@...e.de>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: fio mmap sequential read 30% regression

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.

Yanmin


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