linux-kernel - CFQ and dm-crypt

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Message-ID: <4CC439ED.8090400@dcs.fmph.uniba.sk>
Date:	Sun, 24 Oct 2010 15:51:41 +0200
From:	Richard Kralovic <Richard.Kralovic@....fmph.uniba.sk>
To:	linux-kernel@...r.kernel.org
Subject: CFQ and dm-crypt

CFQ io scheduler relies on using task_struct current to determine which
process makes the io request. On the other hand, some dm modules (such
as dm-crypt) use separate threads for doing io. As CFQ sees only these
threads, it provides a very poor performance in such a case.

IMHO the correct solution for this would be to store, for every io
request, the process that initiated it (and preserve this information
while the request is processed by device mapper). Would that be feasible?

Other possibility is to avoid using separate threads for doing io in dm
modules. The attached patch (against 2.6.36) modifies dm-crypt in this
way, what results into much better behavior of cfq (e.g., io priorities
work correctly).

Greets
	Richard

(Please CC replies to me.)

View attachment "patch" of type "text/plain" (4054 bytes)