linux-kernel - Re: [PATCH] vmscan: evict use-once pages first

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Date:	Tue, 28 Apr 2009 20:36:51 -0700
From:	Elladan <elladan@...imo.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	Elladan <elladan@...imo.com>, linux-kernel@...r.kernel.org,
	peterz@...radead.org, tytso@....edu,
	kosaki.motohiro@...fujitsu.com, linux-mm@...ck.org
Subject: Re: [PATCH] vmscan: evict use-once pages first

Rik,

This patch appears to significantly improve application latency while a large
file copy runs.  I'm not seeing behavior that implies continuous bad page
replacement.

I'm still seeing some general lag, which I attribute to general filesystem
slowness.  For example, latencytop sees many events like these:

down xfs_buf_lock _xfs_buf_find xfs_buf_get_flags 1475.8 msec          5.9 %

xfs_buf_iowait xfs_buf_iostart xfs_buf_read_flags 1740.9 msec          2.6 %

Writing a page to disk                            1042.9 msec         43.7 %

It also occasionally sees long page faults:

Page fault                                        2068.3 msec         21.3 %

I guess XFS (and the elevator) is just doing a poor job managing latency
(particularly poor since all the IO on /usr/bin is on the reader disk).
Notable:

Creating block layer request                      451.4 msec         14.4 %

Thank you,
Elladan

On Tue, Apr 28, 2009 at 07:29:07PM -0400, Rik van Riel wrote:
> When the file LRU lists are dominated by streaming IO pages,
> evict those pages first, before considering evicting other
> pages.
> 
> This should be safe from deadlocks or performance problems
> because only three things can happen to an inactive file page:
> 1) referenced twice and promoted to the active list
> 2) evicted by the pageout code
> 3) under IO, after which it will get evicted or promoted
> 
> The pages freed in this way can either be reused for streaming
> IO, or allocated for something else. If the pages are used for
> streaming IO, this pageout pattern continues. Otherwise, we will
> fall back to the normal pageout pattern.
> 
> Signed-off-by: Rik van Riel <riel@...hat.com>
> 
> --- 
> Elladan, does this patch fix the issue you are seeing?
> 
> Peter, Kosaki, Ted, does this patch look good to you?
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index eac9577..4c0304e 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1489,6 +1489,21 @@ static void shrink_zone(int priority, struct zone *zone,
>  			nr[l] = scan;
>  	}
>  
> +	/*
> +	 * When the system is doing streaming IO, memory pressure here
> +	 * ensures that active file pages get deactivated, until more
> +	 * than half of the file pages are on the inactive list.
> +	 *
> +	 * Once we get to that situation, protect the system's working
> +	 * set from being evicted by disabling active file page aging
> +	 * and swapping of swap backed pages.  We still do background
> +	 * aging of anonymous pages.
> +	 */
> +	if (nr[LRU_INACTIVE_FILE] > nr[LRU_ACTIVE_FILE]) {
> +		nr[LRU_ACTIVE_FILE] = 0;
> +		nr[LRU_INACTIVE_ANON] = 0;
> +	}
> +
>  	while (nr[LRU_INACTIVE_ANON] || nr[LRU_ACTIVE_FILE] ||
>  					nr[LRU_INACTIVE_FILE]) {
>  		for_each_evictable_lru(l) {
--
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