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

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Date:	Wed, 7 Sep 2011 10:37:19 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	"linux-fsdevel@...r.kernel.org" <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 Tue, Sep 06, 2011 at 10:22:48PM +0800, Peter Zijlstra wrote:
> 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?

Sounds not necessary -- (nr_dirty > freerun) is implicitly true
because there is a big break early in the loop:

        if (nr_dirty > freerun)
                break;

Thanks,
Fengguang
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