linux-kernel - Re: [PATCH 11/18] block: add bdi flag to indicate risk of io queue underrun

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Message-ID: <1315318968.14232.6.camel@twins>
Date:	Tue, 06 Sep 2011 16:22:48 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	linux-fsdevel@...r.kernel.org, Tejun Heo <tj@...nel.org>,
	Jens Axboe <axboe@...nel.dk>,
	Li Shaohua <shaohua.li@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Kara <jack@...e.cz>, Christoph Hellwig <hch@....de>,
	Dave Chinner <david@...morbit.com>,
	Greg Thelen <gthelen@...gle.com>,
	Minchan Kim <minchan.kim@...il.com>,
	Vivek Goyal <vgoyal@...hat.com>,
	Andrea Righi <arighi@...eler.com>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 11/18] block: add bdi flag to indicate risk of io queue
 underrun

On Sun, 2011-09-04 at 09:53 +0800, Wu Fengguang wrote:
> +++ linux-next/mm/page-writeback.c      2011-08-31 14:40:58.000000000 +0800
> @@ -1067,6 +1067,9 @@ static void balance_dirty_pages(struct a
>                                      nr_dirty, bdi_thresh, bdi_dirty,
>                                      start_time);
>  
> +               if (unlikely(!dirty_exceeded && bdi_async_underrun(bdi)))
> +                       break;
> +
>                 dirty_ratelimit = bdi->dirty_ratelimit;
>                 pos_ratio = bdi_position_ratio(bdi, dirty_thresh,
>                                                background_thresh, nr_dirty,

So dirty_exceeded looks like:


1109                 dirty_exceeded = (bdi_dirty > bdi_thresh) ||
1110                                   (nr_dirty > dirty_thresh);

Would it make sense to write it as:

	if (nr_dirty > dirty_thresh || 
	    (nr_dirty > freerun && bdi_dirty > bdi_thresh))
		dirty_exceeded = 1;

So that we don't actually throttle bdi thingies when we're still in the
freerun area?

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