linux-kernel - Re: [PATCH] mm: evict streaming IO cache first

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Date:	Mon, 17 Nov 2008 09:06:07 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Rik van Riel <riel@...hat.com>
cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	Gene Heskett <gene.heskett@...il.com>
Subject: Re: [PATCH] mm: evict streaming IO cache first



On Mon, 17 Nov 2008, Rik van Riel wrote:

> Linus Torvalds wrote:
> 
> > Another thing strikes me: it looks like the logic in "get_scan_ratio()" has
> > a tendency to get unbalanced - if we end up deciding that we should scan a
> > lot of anonymous pages, the scan numbers for anonymous pages will go up, and
> > we get even _more_ eager to scan those. Of course, "rotate" events will then
> > make us less likely again, but for streaming loads, you wouldn't expect to
> > see those at all.
> 
> True for streaming loads - if we scan the file list and find
> mostly pages from streaming loads, we will become more eager
> to scan the file list.

The "count adding as activity" might hide that, but it does seem a big 
iffy.

> > There seems to be another bug there wrt the "aging" - we age anon page
> > events and file page events independently, which sounds like it would make
> > the math totally nonsensical. We do that whole
> > 
> > 	anon / (anon + file)
> 
> That's an outdated comment.  Andrew had a patch to update that
> comment, but it must have gotten lost somewhere.  I'll send you
> a patch to update it.
> 
> If you look at the actual calculation, you'l see that the
> scan percentages are keyed off just swappiness and the
> rotated/scanned ratios for each page category.

Ok, that makes sense. Yes, as ratios the math looks valid.

		Linus
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