linux-kernel - Re: [PATCH 0/5] blk-throttle: writeback and swap IO control

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Date:	Thu, 24 Feb 2011 11:18:44 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	Andrea Righi <arighi@...eler.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	Greg Thelen <gthelen@...gle.com>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Gui Jianfeng <guijianfeng@...fujitsu.com>,
	Ryo Tsuruta <ryov@...inux.co.jp>,
	Hirokazu Takahashi <taka@...inux.co.jp>,
	Jens Axboe <axboe@...nel.dk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jonathan Corbet <corbet@....net>,
	containers@...ts.linux-foundation.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 0/5] blk-throttle: writeback and swap IO control

On Thu, Feb 24, 2011 at 09:40:39AM +0900, KAMEZAWA Hiroyuki wrote:

[..]
> > > If we don't consider the swap IO, any other IO
> > > operation from our point of view will happen directly from process
> > > context (writes in memory + sync reads from the block device).
> > 
> > Why do we need to account for swap IO? Application never asked for swap
> > IO. It is kernel's decision to move soem pages to swap to free up some
> > memory. What's the point in charging those pages to application group
> > and throttle accordingly?
> > 
> 
> I think swap I/O should be controlled by memcg's dirty_ratio.
> But, IIRC, NEC guy had a requirement for this...
> 
> I think some enterprise cusotmer may want to throttle the whole speed of
> swapout I/O (not swapin)...so, they may be glad if they can limit throttle
> the I/O against a disk partition or all I/O tagged as 'swapio' rather than
> some cgroup name.

If swap is on a separate disk, then one can control put write throttling rules
on systemwide swapout. Though I still don't understand how that can help.

> 
> But I'm afraid slow swapout may consume much dirty_ratio and make things
> worse ;)

Exactly. So I think focus should be controlling things earlier and stop
applications early before they can either write too much data in page
cache etc.

Thanks
Vivek
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