linux-kernel - Re: [PATCH 7/8] writeback: sync old inodes first in background writeback

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Date:	Mon, 26 Jul 2010 11:17:44 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Mel Gorman <mel@....ul.ie>,
	Christoph Hellwig <hch@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Dave Chinner <david@...morbit.com>,
	Chris Mason <chris.mason@...cle.com>,
	Nick Piggin <npiggin@...e.de>,
	Johannes Weiner <hannes@...xchg.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Minchan Kim <minchan.kim@...il.com>
Subject: Re: [PATCH 7/8] writeback: sync old inodes first in background
 writeback

On Mon, Jul 26, 2010 at 11:11:37AM +0800, Rik van Riel wrote:
> On 07/25/2010 11:08 PM, Wu Fengguang wrote:
> 
> > We do need some throttling under memory pressure. However stall time
> > more than 1s is not acceptable. A simple congestion_wait() may be
> > better, since it waits on _any_ IO completion (which will likely
> > release a set of PG_reclaim pages) rather than one specific IO
> > completion. This makes much smoother stall time.
> > wait_on_page_writeback() shall really be the last resort.
> > DEF_PRIORITY/3 means 1/16=6.25%, which is closer.
> 
> I agree with the max 1 second stall time, but 6.25% of
> memory could be an awful lot of pages to scan on a system
> with 1TB of memory :)

I totally ignored the 1TB systems out of this topic, because in such
systems, <PAGE_ALLOC_COSTLY_ORDER pages are easily available? :)

> Not sure what the best approach is, just pointing out
> that DEF_PRIORITY/3 may be too much for large systems...

What if DEF_PRIORITY/3 is used under PAGE_ALLOC_COSTLY_ORDER?

Thanks,
Fengguang
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