linux-kernel - Re: [PATCH 1/2] vmscan: shrink_slab() require number of lru

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Date:	Mon, 28 Jun 2010 10:32:00 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	kosaki.motohiro@...fujitsu.com,
	LKML <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mel@....ul.ie>, Rik van Riel <riel@...hat.com>,
	Minchan Kim <minchan.kim@...il.com>,
	Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH 1/2] vmscan: shrink_slab() require number of lru_pages, not page order

> On Fri, 25 Jun 2010, KOSAKI Motohiro wrote:
> 
> > Fix simple argument error. Usually 'order' is very small value than
> > lru_pages. then it can makes unnecessary icache dropping.
> 
> This is going to reduce the delta that is added to shrinker->nr
> significantly thereby increasing the number of times that shrink_slab() is
> called.

Yup. But,

Smaller shrink -> only makes retry
Bigger shrink  -> makes unnecessary icache/dcache drop. it can bring
                  mysterious low performance.


> What does the "lru_pages" parameter do in shrink_slab()? Looks
> like its only role is as a divison factor in a complex calculation of
> pages to be scanned.

Yes.
scanned/lru_pages ratio define basic shrink_slab puressure strength.

So, If you intentionally need bigger slab pressure, bigger scanned parameter
passing is better rather than mysterious 'order' parameter.

> 
> do_try_to_free_pages passes 0 as "lru_pages" to shrink_slab() when trying
> to do cgroup lru scans. Why is that?

?
When cgroup lru scans, do_try_to_free_pages() don't call shrink_slab().


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