linux-kernel - Re: [RFC PATCH] block: Fix bio merge induced high I/O latency

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Message-ID: <c384c5ea0901170850q4c72d8bcpc76eb635950aaa2e@mail.gmail.com>
Date:	Sat, 17 Jan 2009 17:50:59 +0100
From:	"Leon Woestenberg" <leon.woestenberg@...il.com>
To:	"Mathieu Desnoyers" <mathieu.desnoyers@...ymtl.ca>
Cc:	"Jens Axboe" <axboe@...nel.dk>,
	"Andrea Arcangeli" <andrea@...e.de>, akpm@...ux-foundation.org,
	"Ingo Molnar" <mingo@...e.hu>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, ltt-dev@...ts.casi.polymtl.ca,
	"Thomas Gleixner" <tglx@...utronix.de>
Subject: Re: [RFC PATCH] block: Fix bio merge induced high I/O latency

Hello Mathieu et al,

On Sat, Jan 17, 2009 at 5:26 PM, Mathieu Desnoyers
<mathieu.desnoyers@...ymtl.ca> wrote:
> A long standing I/O regression (since 2.6.18, still there today) has hit
> Slashdot recently :
> http://bugzilla.kernel.org/show_bug.cgi?id=12309

Are you sure you are solving the *actual* problem?

The bugzilla entry shows a bisect attempt that leads to a patch
involving negative clock jumps.
http://bugzilla.kernel.org/show_bug.cgi?id=12309#c29

with a corrected link to the bisect patch:
http://bugzilla.kernel.org/show_bug.cgi?id=12309#c30

Wouldn't a negative clock jump be very influential to the
(time-driven) I/O schedulers and be a more probable cause?

Regards,
-- 
Leon

p.s. Added Thomas to the CC list as his name is on the patch Signed-off-by list.
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