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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Date:	Thu, 16 Sep 2010 15:28:10 -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 Wed, 15 Sep 2010 13:27:51 +0100
Mel Gorman <mel@....ul.ie> wrote:

> If wait_iff_congested() is called with no BDI congested, the function simply
> calls cond_resched(). In the event there is significant writeback happening
> in the zone that is being reclaimed, this can be a poor decision as reclaim
> would succeed once writeback was completed. Without any backoff logic,
> younger clean pages can be reclaimed resulting in more reclaim overall and
> poor performance.

This is because cond_resched() is a no-op, and we skip around the
under-writeback pages and go off and look further along the LRU for
younger clean pages, yes?

> 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?

The terms are rather ambiguous so let's avoid them.  Maybe "full"
watermark and "empty"?

>
> ...
>
> @@ -706,6 +726,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  			goto keep;
>  
>  		VM_BUG_ON(PageActive(page));
> +		VM_BUG_ON(page_zone(page) != zone);

?

>  		sc->nr_scanned++;
>  
>
> ...
>
> @@ -903,6 +928,15 @@ keep_lumpy:
>  		VM_BUG_ON(PageLRU(page) || PageUnevictable(page));
>  	}
>  
> +	/*
> +	 * Tag a zone as congested if all the dirty pages encountered were
> +	 * backed by a congested BDI. In this case, reclaimers should just
> +	 * back off and wait for congestion to clear because further reclaim
> +	 * will encounter the same problem
> +	 */
> +	if (nr_dirty == nr_congested)
> +		zone_set_flag(zone, ZONE_CONGESTED);

The implicit "100%" there is a magic number.  hrm.


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