linux-kernel - Re: [PATCH 9/9] ext3: do not throttle metadata and journal IO

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Date:	Thu, 23 Apr 2009 11:54:19 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Theodore Tso <tytso@....edu>
Cc:	Andrea Righi <righi.andrea@...il.com>, randy.dunlap@...cle.com,
	Carl Henrik Lunde <chlunde@...g.uio.no>,
	Jens Axboe <jens.axboe@...cle.com>, eric.rannaud@...il.com,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	fernando@....ntt.co.jp, dradford@...ehost.com,
	Gui@...p1.linux-foundation.org, agk@...rceware.org,
	subrata@...ux.vnet.ibm.com, Paul Menage <menage@...gle.com>,
	akpm@...ux-foundation.org, containers@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, dave@...ux.vnet.ibm.com,
	matt@...ehost.com, roberto@...it.it, ngupta@...gle.com
Subject: Re: [PATCH 9/9] ext3: do not throttle metadata and journal IO

On Wed, 22 Apr 2009 21:22:54 -0400
Theodore Tso <tytso@....edu> wrote:

> On Thu, Apr 23, 2009 at 09:05:35AM +0900, KAMEZAWA Hiroyuki wrote:
> > So, current status is.
> > 
> >   A. memcg should support dirty_ratio for its own memory reclaim.
> >      in plan.
> > 
> >   B. another cgroup can be implemnted to support cgroup_dirty_limit().
> >      But relationship with "A" should be discussed.
> >      no plan yet.
> > 
> >   C. I/O cgroup and bufferred I/O tracking system.
> >      Now under patch review.
> > 
> > And this I/O throttle is mainly for "C" discussion. 
> 
> How much testing has been done in terms of whether the I/O throttling
> actually works?  Not just, "the kernel doesn't crash", but that where
> you have one process generating a large amount of I/O load, in various
> different ways, and whether the right things happens?  If so, how has
> this been measured?

I/O control people should prove it. And they do, I think.

> 
> I'm really concerned that given some of the ways that I/O will "leak"
> out --- the via pdflush, swap writeout, etc., that without the rest of
> the pieces in place, I/O throttling by itself might not prove to be
> very effective.  Sure, if the workload is only doing direct I/O, life
> is pretty easy and it shouldn't be hard to throttle the cgroup.
> 
It's just a problem of "what we do and what we don't, now".
Andrea, Vivek, could you clarify ? As other project, I/O controller will not be
100% at first implementation. 

> But in the case where there is bufferred I/O, without write
> throttling, it's hard to see how well the I/O controller will work in
> practice.  In fact, I wouldn't be that surprised if it's possible to
> trigger the OOM killer.......
> 

yes, then, memcg should have dirty_ratio handler. And, we may have to
implement dirty-ratio controller.  So, please don't merge memcg discussion
and I/O BW throttoling. It's related to each other but different problem.

Thanks,
-Kame

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