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

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Message-ID: <17900.60167.418766.280544@cargo.ozlabs.ibm.com>
Date:	Tue, 6 Mar 2007 15:16:07 +1100
From:	Paul Mackerras <paulus@...ba.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Balbir Singh <balbir@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mel@...net.ie>, npiggin@...e.de,
	clameter@...r.sgi.com, mingo@...e.hu, jschopp@...tin.ibm.com,
	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

Linus Torvalds writes:

> The point being that in the guests, hotunplug is almost useless (for 
> bigger ranges), and we're much better off just telling the virtualization 
> hosts on a per-page level whether we care about a page or not, than to 
> worry about fragmentation.

We don't have that luxury on IBM System p machines, where the
hypervisor manages memory in much larger units than a page.  Typically
the size of memory block that the hypervisor uses to manage memory is
16MB or more -- which makes sense from the point of view that if the
hypervisor had to manage individual pages, it would end up adding a
lot more overhead than it does.

Paul.
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