linux-kernel - Re: [PATCH] Document huge memory/cache overhead of memory controller in Kconfig

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Date:	Fri, 22 Feb 2008 14:00:02 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Balbir Singh <balbir@...ux.vnet.ibm.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Nick Piggin <nickpiggin@...oo.com.au>, akpm@...l.org,
	torvalds@...l.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] Document huge memory/cache overhead of memory controller in Kconfig

On Fri, Feb 22, 2008 at 05:44:47PM +0530, Balbir Singh wrote:
> 
> My concern with all the points you mentioned is that this solution might need to
> change again,

No why would it need to change again?

> depending on the factors you've mentioned. vmalloc() is good and
> straightforward, but it has these dependencies which could call for another
> rewrite of the code.

The hotplug change would not need a rewrite of anything, just
some additional code in the SRAT parser to increase __VMALLOC_RESERVE for
each hotplug region. It's likely <= 3 additional lines.

> 
> > 
> >>>> if we decided to use vmalloc space, we would need 64
> >>>> MB of vmalloc'ed memory
> >>> Yes and if you increase mem_map you need exactly the same space
> >>> in lowmem too. So increasing the vmalloc reservation for this is
> >>> equivalent. Just make sure you use highmem backed vmalloc.
> >>>
> >> I see two problems with using vmalloc. One, the reservation needs to be done
> >> across architectures. 
> > 
> > Only on 32bit. Ok hacking it into all 32bit architectures might be
> > difficult, but I assume it would be ok to rely on the architecture
> > maintainers for that and only enable it on some selected architectures
> > using Kconfig for now.
> > 
> 
> Yes, but that's not such a good idea

Waiting for the maintainers? Why not? 

I assume the memory controller would be primarily used on larger
systems anyways and except for i386 these should be mostly 64bit
these days anyways.

> > On 64bit vmalloc should be by default large enough so it could
> > be enabled for all 64bit architectures.
> > 
> >> Two, a big vmalloc chunk is not node aware, 
> > 
> > vmalloc_node()
> > 
> 
> vmalloc_node() would need to work much the same way as mem_map does. I am

would? It already is implemented and works just fine AFAIK. 

I don't understand the rest of your point.

-Andi
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