linux-kernel - Re: [RFC] writeback and cgroup

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Message-ID: <20120405163113.GD12854@google.com>
Date:	Thu, 5 Apr 2012 09:31:13 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Fengguang Wu <fengguang.wu@...el.com>, Jan Kara <jack@...e.cz>,
	Jens Axboe <axboe@...nel.dk>, linux-mm@...ck.org,
	sjayaraman@...e.com, andrea@...terlinux.com, jmoyer@...hat.com,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	kamezawa.hiroyu@...fujitsu.com, lizefan@...wei.com,
	containers@...ts.linux-foundation.org, cgroups@...r.kernel.org,
	ctalbott@...gle.com, rni@...gle.com, lsf@...ts.linux-foundation.org
Subject: Re: [RFC] writeback and cgroup

Hey, Vivek.

On Wed, Apr 04, 2012 at 04:18:16PM -0400, Vivek Goyal wrote:
> Hey how about reconsidering my other proposal for which I had posted
> the patches. And that is keep throttling still at device level. Reads
> and direct IO get throttled asynchronously but buffered writes get
> throttled synchronously.
> 
> Advantages of this scheme.
> 
> - There are no separate knobs.
> 
> - All the IO (read, direct IO and buffered write) is controlled using
>   same set of knobs and goes in queue of same cgroup.
> 
> - Writeback logic has no knowledge of throttling. It just invokes a 
>   hook into throttling logic of device queue.
> 
> I guess this is a hybrid of active writeback throttling and back pressure
> mechanism.
> 
> But it still does not solve the NFS issue as well as for direct IO,
> filesystems still can get serialized, so metadata issue still needs to 
> be resolved. So one can argue that why not go for full "back pressure"
> method, despite it being more complex.
> 
> Here is the link, just to refresh the memory. Something to keep in mind
> while assessing alternatives.
> 
> https://lkml.org/lkml/2011/6/28/243

Hmmm... so, this only works for blk-throttle and not with the weight.
How do you manage interaction between buffered writes and direct
writes for the same cgroup?

Thanks.

-- 
tejun
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