linux-kernel - Re: [PATCH 8/8] writeback: Do not sleep on the congestion queue if there are no congested BDIs or if significant congestion is not being encountered in the current zone

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Message-Id: <20100921152433.6edd6a87.akpm@linux-foundation.org>
Date:	Tue, 21 Sep 2010 15:24:33 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Mel Gorman <mel@....ul.ie>
Cc:	linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
	Linux Kernel List <linux-kernel@...r.kernel.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Minchan Kim <minchan.kim@...il.com>,
	Wu Fengguang <fengguang.wu@...el.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Subject: Re: [PATCH 8/8] writeback: Do not sleep on the congestion queue if
 there are no congested BDIs or if significant congestion is not being
 encountered in the current zone

On Tue, 21 Sep 2010 23:10:08 +0100
Mel Gorman <mel@....ul.ie> wrote:

> On Tue, Sep 21, 2010 at 02:44:13PM -0700, Andrew Morton wrote:
> > On Mon, 20 Sep 2010 10:52:39 +0100
> > Mel Gorman <mel@....ul.ie> wrote:
> > 
> > > > > This patch tracks how many pages backed by a congested BDI were found during
> > > > > scanning. If all the dirty pages encountered on a list isolated from the
> > > > > LRU belong to a congested BDI, the zone is marked congested until the zone
> > > > > reaches the high watermark.
> > > > 
> > > > High watermark, or low watermark?
> > > > 
> > > 
> > > High watermark. The check is made by kswapd.
> > > 
> > > > The terms are rather ambiguous so let's avoid them.  Maybe "full"
> > > > watermark and "empty"?
> > > > 
> > > 
> > > Unfortunately they are ambiguous to me. I know what the high watermark
> > > is but not what the full or empty watermarks are.
> > 
> > Really.  So what's the "high" watermark? 
> 
> The high watermark is the point where kswapd goes back to sleep because
> enough pages have been reclaimed. It's a proxy measure for memory pressure.
> 
> > From the above text I'm
> > thinking that you mean the high watermark is when the queue has a small
> > number of requests and the low watermark is when the queue has a large
> > number of requests.
> > 
> 
> I was expecting "zone reaches the high watermark" was the clue that I was
> talking about zone watermarks and not an IO queue but it could be better.

It was more a rant about general terminology rather than one specific case.

> I will try and clarify. How about this as a replacement paragraph?

Works for me, thanks.
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