linux-kernel - Re: [PATCH 11/11] blkcg: implement per-blkg request allocation

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Message-ID: <20120427150217.GK27486@google.com>
Date:	Fri, 27 Apr 2012 08:02:17 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Jeff Moyer <jmoyer@...hat.com>
Cc:	axboe@...nel.dk, vgoyal@...hat.com, ctalbott@...gle.com,
	rni@...gle.com, linux-kernel@...r.kernel.org,
	cgroups@...r.kernel.org, containers@...ts.linux-foundation.org,
	fengguang.wu@...el.com, hughd@...gle.com, akpm@...ux-foundation.org
Subject: Re: [PATCH 11/11] blkcg: implement per-blkg request allocation

Hello,

On Fri, Apr 27, 2012 at 10:54:01AM -0400, Jeff Moyer wrote:
> > This patch implements per-blkg request_list.  Each blkg has its own
> > request_list and any IO allocates its request from the matching blkg
> > making blkcgs completely isolated in terms of request allocation.
> 
> So, nr_requests is now actually nr_requests * # of blk cgroups.  Is that
> right?  Are you at all concerned about the amount of memory that can be
> tied up as the number of cgroups increases?

Yeah, I thought about it and I don't think there's a single good
solution here.  The other extreme would be splitting nr_requests by
the number of cgroups but that seems even worse - each cgroup should
be able to hit maximum throughput.  Given that a lot of workloads tend
to regulate themselves before hitting nr_requests, I think it's best
to leave it as-is and treat each cgroup as having separate channel for
now.  It's a configurable parameter after all.

Thanks.

-- 
tejun
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