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

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Message-ID: <47BD55F6.5030203@firstfloor.org>
Date:	Thu, 21 Feb 2008 11:44:06 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	balbir@...ux.vnet.ibm.com
CC:	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

> 1. We could create something similar to mem_map, we would need to handle 4

4? At least x86 mainline only has two ways now. flatmem and vmemmap.

> different ways of creating mem_map.

Well it would be only a single way to create the "aux memory controller
map" (or however it will be called). Basically just a call to single
function from a few different places.

> 2. On x86 with 64 GB ram, 

First i386 with 64GB just doesn't work, at least not with default 3:1
split. Just calculate it yourself how much of the lowmem area is left
after the 64GB mem_map is allocated. Typical rule of thumb is that 16GB
is the realistic limit for 32bit x86 kernels. Worrying about
anything more does not make much sense.

> 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.

-Andi
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