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Subject: Re: [RFC PATCH] block: Fix bio merge induced high I/O latency
From: Ben Gamari <bgamari@gmail.com>
To: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Cc: Jens Axboe <axboe@kernel.dk>, Andrea Arcangeli <andrea@suse.de>,
       akpm@linux-foundation.org, Ingo Molnar <mingo@elte.hu>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       linux-kernel@vger.kernel.org, ltt-dev@lists.casi.polymtl.ca
In-Reply-To: <20090117162657.GA31965@Krystal>
References: <20090117004439.GA11492@Krystal>
	 <20090117162657.GA31965@Krystal>
Content-Type: text/plain
Date: Sat, 17 Jan 2009 15:03:46 -0500
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On Sat, 2009-01-17 at 11:26 -0500, Mathieu Desnoyers wrote:
> This patch implements a basic test to make sure we never merge more than 128
> requests into the same request if it is the "last_merge" request. I have not
> been able to trigger the problem again with the fix applied. It might not be in
> a perfect state : there may be better solutions to the problem, but I think it
> helps pointing out where the culprit lays.

Unfortunately, it seems like the patch hasn't really fixed much. After
porting it forward to Linus' master, I haven't exhibited any difference
in real world use cases (e.g. desktop use cases while building a
kernel).

Given Jen's remarks, I suppose this isn't too surprising. Does anyone
else with greater familiarity with the block I/O subsystem have any more
ideas about the source of the slowdown? It seems like the recent patches
incorporating blktrace support into ftrace could be helpful for further
data collection, correct?

- Ben


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