linux-kernel - Re: [PATCH] RFC: vmscan: add min_filelist_kbytes sysctl for protecting the working set

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Date:	Tue,  9 Nov 2010 11:49:43 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	kosaki.motohiro@...fujitsu.com,
	Mandeep Singh Baines <msb@...omium.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mel@....ul.ie>,
	Minchan Kim <minchan.kim@...il.com>,
	Johannes Weiner <hannes@...xchg.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, wad@...omium.org,
	olofj@...omium.org, hughd@...omium.org
Subject: Re: [PATCH] RFC: vmscan: add min_filelist_kbytes sysctl for protecting the working set

> On 11/03/2010 06:40 PM, Mandeep Singh Baines wrote:
> 
> > I've created a patch which takes a slightly different approach.
> > Instead of limiting how fast pages get reclaimed, the patch limits
> > how fast the active list gets scanned. This should result in the
> > active list being a better measure of the working set. I've seen
> > fairly good results with this patch and a scan inteval of 1
> > centisecond. I see no thrashing when the scan interval is non-zero.
> >
> > I've made it a tunable because I don't know what to set the scan
> > interval. The final patch could set the value based on HZ and some
> > other system parameters. Maybe relate it to sched_period?
> 
> I like your approach. For file pages it looks like it
> could work fine, since new pages always start on the
> inactive file list.
> 
> However, for anonymous pages I could see your patch
> leading to problems, because all anonymous pages start
> on the active list.  With a scan interval of 1
> centiseconds, that means there would be a limit of 3200
> pages, or 12MB of anonymous memory that can be moved to
> the inactive list a second.
> 
> I have seen systems with single SATA disks push out
> several times that to swap per second, which matters
> when someone starts up a program that is just too big
> to fit in memory and requires that something is pushed
> out.
> 
> That would reduce the size of the inactive list to
> zero, reducing our page replacement to a slow FIFO
> at best, causing false OOM kills at worst.
> 
> Staying with a default of 0 would of course not do
> anything, which would make merging the code not too
> useful.
> 
> I believe we absolutely need to preserve the ability
> to evict pages quickly, when new pages are brought
> into memory or allocated quickly.
> 
> However, speed limits are probably a very good idea
> once a cache has been reduced to a smaller size, or
> when most IO bypasses the reclaim-speed-limited cache.

Yeah.

But I doubt fixed rate limit is good thing. When playing movie case
(aka streaming I/O case), We don't want any throttle. I think.
Also, I don't like jiffies dependency. CPU hardware improvement naturally
will break such heuristics.


btw, now congestion_wait() already has jiffies dependency. but we should
kill such strange timeout eventually. I think.


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