linux-kernel - Re: [PATCH] [RESEND] x86_64: add memory hotremove config option

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Date:	Mon, 8 Sep 2008 15:52:49 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Yasunori Goto <y-goto@...fujitsu.com>,
	Gary Hade <garyhade@...ibm.com>, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Badari Pulavarty <pbadari@...ibm.com>,
	Mel Gorman <mel@....ul.ie>, Chris McDermott <lcm@...ibm.com>,
	linux-kernel@...r.kernel.org, x86@...nel.org,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] [RESEND] x86_64: add memory hotremove config option

On Saturday 06 September 2008 18:53, Andi Kleen wrote:
> On Sat, Sep 06, 2008 at 04:06:38PM +0900, Yasunori Goto wrote:
> > > not.
> > >
> > > This means I don't see a real use case for this feature.
> >
> > I don't think its driver is almighty.
> > IIRC, balloon driver can be cause of fragmentation for 24-7 system.
>
> Sure the balloon driver can be likely improved too, it's just
> that I don't think a balloon driver should call into the function
> the original patch in the series hooked up.
>
> > In addition, I have heard that memory hotplug would be useful for
> > reducing of power consumption of DIMM.
>
> It's unclear that memory hotplug is the right model for DIMM power
> management. The problem is that DIMMs are interleaved, so you again have to
> completely free a quite large area. It's not much easier than node hotplug.
>
> > I have to admit that memory hotplug has many issues, but I would like to
>
> Let's call it "node" or "hardware" memory hot unplug, not that
> anyone confuses it with the easier VM based hot unplug or the really
> easy hotadd.
>
> > solve them step by step.
>
> The question is if they are even solvable in a useful way.
> I'm not sure it's that useful to start and then find out
> that it doesn't work anyways.

You use non-linear mappings for the kernel, so that kernel data is
not tied to a specific physical address. AFAIK, that is the only way
to really do it completely (like the fragmentation problem).

Of course, I don't think that would be a good idea to do that in the
forseeable future.
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