linux-kernel - Re: RFC: default group_isolation to 1, remove option

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Date:	Mon, 7 Mar 2011 15:34:20 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	Justin TerAvest <teravest@...gle.com>,
	Chad Talbott <ctalbott@...gle.com>,
	Nauman Rafique <nauman@...gle.com>,
	Divyesh Shah <dpshah@...gle.com>,
	lkml <linux-kernel@...r.kernel.org>,
	Gui Jianfeng <guijianfeng@...fujitsu.com>,
	Corrado Zoccolo <czoccolo@...il.com>
Subject: Re: RFC: default group_isolation to 1, remove option

On Mon, Mar 07, 2011 at 03:24:32PM -0500, Vivek Goyal wrote:

[..]
> > 
> > Or not get rid of limits completely, but do a lot more relaxed
> > accounting at the queue level still. That will not require any
> > additional tracking of io contexts etc, but still impose some limit on
> > the number of queued IOs.
> 
> A lot more relaxed limit accounting should help a bit but it after a 
> while it might happen that slow movers eat up lots of request descriptors
> and making not much of progress.
> 
> Long back I had implemented this additional notion of q->nr_group_requests
> where we defined per group number of requests allowed submitter will
> be put to sleep. I also extended it to also export per bdi per group
> congestion notion. So a flusher thread can look at the page and cgroup
> of the page and determine if respective cgroup is congested or not. If
> cgroup is congested, flusher thread can move to next inode so that it
> is not put to sleep behind a slow mover.
> 
> Completely limitless queueu will solve the problem completely. But I guess
> then we can get that complain back that flusher thread submitted too much
> of IO to device.

Also wanted to add that currently blk-throttling code implements limitless
queuing of bio. The reason I did not enforce the limit yet because of
same reason that I will run into issues with async WRITES and flusher
thread.

So once we have figured out what't the right thing to do here, I can
implement similar solution for throttling too.

One side affect of limitless bio queueing is an AIO process can queue up lots
of bios in a group and if one tries to kill the process, it waits for all the
IOs to finish and can take up a very long time depending on throttling limits
of the group.

Thanks
Vivek
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