linux-kernel - Re: The performance and behaviour of the anti-fragmentation related patches

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Date:	Mon, 05 Mar 2007 09:20:10 -0600
From:	Joel Schopp <jschopp@...tin.ibm.com>
To:	Nick Piggin <npiggin@...e.de>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mel@...net.ie>, clameter@...r.sgi.com,
	mingo@...e.hu, arjan@...radead.org, mbligh@...igh.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: The performance and behaviour of the anti-fragmentation related
 patches

> But if you don't require a lot of higher order allocations anyway, then
> guest fragmentation caused by ballooning doesn't seem like much problem.

If you only need to allocate 1 page size and smaller allocations then no it's not a 
problem.  As soon as you go above that it will be.  You don't need to go all the way 
up to MAX_ORDER size to see an impact, it's just increasingly more severe as you get 
away from 1 page and towards MAX_ORDER.

> 
> If you need higher order allocations, then ballooning is bad because of
> fragmentation, so you need memory unplug, so you need higher order
> allocations. Goto 1.

Yes, it's a closed loop.  But hotplug isn't the only one that needs higher order 
allocations.  In fact it's pretty far down the list.  I look at it like this, a lot 
of users need high order allocations for better performance and things like on-demand 
hugepages.  As a bonus you get memory hot-remove.

> Balooning probably does skew memory management stats and watermarks, but
> that's just because it is implemented as a module. A couple of hooks
> should be enough to allow things to be adjusted?

That is a good idea independent of the current discussion.

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