linux-kernel - Re: [PATCH 0/5] IO-less dirty throttling v8

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Date:	Wed, 10 Aug 2011 14:40:52 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Greg Thelen <gthelen@...gle.com>
Cc:	Minchan Kim <minchan.kim@...il.com>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Dave Chinner <david@...morbit.com>,
	Christoph Hellwig <hch@....de>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrea Righi <arighi@...eler.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org, linux-mm <linux-mm@...ck.org>,
	Jan Kara <jack@...e.cz>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [PATCH 0/5] IO-less dirty throttling v8

On Wed, Aug 10, 2011 at 12:41:00AM -0700, Greg Thelen wrote:

[..]
> > > However, before we have a "finished product", there is still another
> > > piece of the puzzle to be put in place - memcg-aware buffered
> > > writeback. That is, having a flusher thread do work on behalf of
> > > memcg in the IO context of the memcg. Then the IO controller just
> > > sees a stream of async writes in the context of the memcg the
> > > buffered writes came from in the first place. The block layer
> > > throttles them just like any other IO in the IO context of the
> > > memcg...
> >
> > Yes that is still a piece remaining. I was hoping that Greg Thelen will
> > be able to extend his patches to submit writes in the context of
> > per cgroup flusher/worker threads and solve this problem.
> >
> > Thanks
> > Vivek
> 
> Are you suggesting multiple flushers per bdi (one per cgroup)?  I
> thought the point of IO less was to one issue buffered writes from a
> single thread.

I think in one of the mail threads Dave Chinner mentioned this idea
of using per cgroup worker/worqueue.

Agreed that it leads back to the issue of multiple writers (but only
if multiple cgroups are there). But at the same time it simplifies
atleast two problems.

- Worker could be migrated to the cgroup we are writting for and we
  don't need the IO tracking logic. blkio controller should will
  automatically account the IO to right group.

- We don't have to worry about a single flusher thread sleeping
  on request queue because either queue or group is congested and
  this can lead other group's IO is not being submitted.

Thanks
Vivek
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