linux-kernel - Re: [PATCH 5/5] linear reclaim core

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Date:	Mon, 11 Sep 2006 09:33:25 +0200
From:	Jörn Engel <joern@...nheim.fh-wedel.de>
To:	Andrew Morton <akpm@...l.org>
Cc:	Andy Whitcroft <apw@...dowen.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/5] linear reclaim core

On Sun, 10 September 2006 17:40:51 -0700, Andrew Morton wrote:
>
> > A) With sufficient fragmentation, all inactive pages have one active
> > neighbour, so shrink_inactive_list() will never find a cluster of the
> > required order.
> 
> Nope.  If the clump of pages has a mix of active and inactive, the above
> design would cause the active ones to be deactivated, so now the entire
> clump is eligible for treatment by shrink_inactive_list().

Ok?  More reading seems necessary before I can follow you here...

> Bear in mind that simply moving the pages to the inactive list isn't enough
> to get them reclaimed: we still do various forms of page aging and the
> pages can still be preserved due to that.  IOW, we have several different
> forms of page aging, one of which is LRU-ordering.  The above design
> compromises just one of those aging steps.

Are these different forms of page aging described in written form
somewhere?

> I'd be more concerned about higher-order atomic allocations.  If this thing
> is to work I suspect we'll need per-zone, per-order watermarks and kswapd
> will need to maintain those.

Or simply declare higher-order atomic allocations to be undesired?
Not sure how many of those we have that make sense.

> Don't think in terms of "freeing".  Think in terms of "scanning".  A lot of
> page reclaim's balancing tricks are cast in terms of pages-scanned,
> slabs-scanned, etc.

There is a related problem I'm sure you are aware of.  Trying to
shrink the dentry_cache or the various foofs_inode_caches we remove
tons of objects before a full slab (in most cases a page) is free and
can be returned.  ext3_inode_cache has 8 objects per slab,
dentry_cache has 29.  That's the equivalent of an order-3 or order-5
page allocation in terms of inefficiency.

And having just started thinking about the problem, my envisioned
solution looks fairly similar to Andy's work for high-order
allocations here.  Except that I cannot think in terms of "scanning",
afaict.

Jörn

-- 
Anything that can go wrong, will.
-- Finagle's Law
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