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Date: Wed, 22 Apr 2009 21:22:54 -0400
From: Theodore Tso <tytso@mit.edu>
To: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Andrea Righi <righi.andrea@gmail.com>, randy.dunlap@oracle.com,
       Carl Henrik Lunde <chlunde@ping.uio.no>,
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Subject: Re: [PATCH 9/9] ext3: do not throttle metadata and journal IO
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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'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.

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.......

Regards,

						- Ted