linux-kernel - Re: [PATCH 4/4] blkcg: use radix tree to index blkgs from blkcg

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Message-ID: <20120420172639.GM22419@redhat.com>
Date:	Fri, 20 Apr 2012 13:26:39 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	axboe@...nel.dk, ctalbott@...gle.com, rni@...gle.com,
	cgroups@...r.kernel.org, containers@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/4] blkcg: use radix tree to index blkgs from blkcg

On Thu, Apr 19, 2012 at 04:29:24PM -0700, Tejun Heo wrote:
> blkg lookup is currently performed by traversing linked list anchored
> at blkcg->blkg_list.  This is very unscalable and with blk-throttle
> enabled and enough request queues on the system, this can get very
> ugly quickly (blk-throttle performs look up on every bio submission).
> 
> This patch makes blkcg use radix tree to index blkgs combined with
> simple last-looked-up hint.  This is mostly identical to how icqs are
> indexed from ioc.
> 
> Note that because __blkg_lookup() may be invoked without holding queue
> lock, hint is only updated from __blkg_lookup_create().  Due to cfq's
> cfqq caching, this makes hint updates overly lazy.  This will be
> improved with scheduled blkcg aware request allocation.
> 
> Signed-off-by: Tejun Heo <tj@...nel.org>
> Cc: Vivek Goyal <vgoyal@...hat.com>

Nice to see radix tree lookup and caching hint. Helps with scalability.

Acked-by: Vivek Goyal <vgoyal@...hat.com>

Vivek
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