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

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Date:	Thu, 1 Mar 2007 19:44:27 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Balbir Singh <balbir@...ibm.com>
cc:	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

On Fri, 2 Mar 2007, Balbir Singh wrote:
>
> > My personal opinion is that while I'm not a huge fan of virtualization,
> > these kinds of things really _can_ be handled more cleanly at that layer,
> > and not in the kernel at all. Afaik, it's what IBM already does, and has
> > been doing for a while. There's no shame in looking at what already works,
> > especially if it's simpler.
> 
> Could you please clarify as to what "that layer" means - is it the
> firmware/hardware for virtualization? or does it refer to user space?

Virtualization in general. We don't know what it is - in IBM machines it's 
a hypervisor. With Xen and VMware, it's usually a hypervisor too. With 
KVM, it's obviously a host Linux kernel/user-process combination.

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.

And in hosts, we usually don't care EITHER, since it's usually done in a 
hypervisor.

> It would also be useful to have a resource controller like per-container
> RSS control (container refers to a task grouping) within the kernel or
> non-virtualized environments as well.

.. but this has again no impact on anti-fragmentation.

In other words, I really don't see a huge upside. I see *lots* of 
downsides, but upsides? Not so much. Almost everybody who wants unplug 
wants virtualization, and right now none of the "big virtualization" 
people would want to have kernel-level anti-fragmentation anyway sicne 
they'd need to do it on their own.

		Linus
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