linux-kernel - Re: iozone write 50% regression in kernel 2.6.24-rc1

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Date:	Tue, 13 Nov 2007 19:32:09 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
Cc:	Benny Halevy <bhalevy@...asas.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	aneesh.kumar@...ux.vnet.ibm.com
Subject: Re: iozone write 50% regression in kernel 2.6.24-rc1


On Tue, 2007-11-13 at 16:34 +0800, Zhang, Yanmin wrote:

> My new bisect captured 7c9e69faa28027913ee059c285a5ea8382e24b5d
> which caused the regression of iozone following run (3rd/4th... run after mounting
> the ext3 partition).

Linus just reverted that commit with commit:

commit 0b832a4b93932103d73c0c3f35ef1153e288327b
Author: Linus Torvalds <torvalds@...dy.linux-foundation.org>
Date:   Tue Nov 13 08:07:31 2007 -0800

    Revert "ext2/ext3/ext4: add block bitmap validation"

    This reverts commit 7c9e69faa28027913ee059c285a5ea8382e24b5d, fixing up
    conflicts in fs/ext4/balloc.c manually.

    The cost of doing the bitmap validation on each lookup - even when the
    bitmap is cached - is absolutely prohibitive.  We could, and probably
    should, do it only when adding the bitmap to the buffer cache.  However,
    right now we are better off just reverting it.

    Peter Zijlstra measured the cost of this extra validation as a 85%
    decrease in cached iozone, and while I had a patch that took it down to
    just 17% by not being _quite_ so stupid in the validation, it was still
    a big slowdown that could have been avoided by just doing it right.



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