linux-kernel - Re: [PATCH 1/4] x86_64: Switch to SPARSE

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Date:	Mon, 02 Apr 2007 13:38:39 -0700
From:	Martin Bligh <mbligh@...gle.com>
To:	Christoph Lameter <clameter@....com>
CC:	Dave Hansen <hansendc@...ibm.com>, Andi Kleen <ak@...e.de>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	linux-mm@...ck.org,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andy Whitcroft <apw@...dowen.org>
Subject: Re: [PATCH 1/4] x86_64: Switch to SPARSE_VIRTUAL

Christoph Lameter wrote:
> On Mon, 2 Apr 2007, Dave Hansen wrote:
> 
>> I completely agree, it looks like it should be faster.  The code
>> certainly has potential benefits.  But, to add this neato, apparently
>> more performant feature, we unfortunately have to add code.  Adding the
>> code has a cost: code maintenance.  This isn't a runtime cost, but it is
>> a real, honest to goodness tradeoff.
> 
> Its just the opposite. The vmemmap code is so efficient that we can remove 
> lots of other code and gops of these alternate implementations. On x86_64 
> its even superior to FLATMEM since FLATMEM still needs a memory reference 
> for the mem_map area. So if we make SPARSE standard for all 
> configurations then there is no need anymore for FLATMEM DISCONTIG etc 
> etc. Can we not cleanup all this mess? Get rid of all the gazillions 
> of #ifdefs please? This would ease code maintenance significantly. I hate 
> having to constantly navigate my way through all the alternatives.

The original plan when this was first merged was pretty much that -
for sparsemem to replace discontigmem once it was well tested. Seems
to have got stalled halfway through ;-(

Not sure we'll get away with replacing flatmem for all arches, but
we could at least get rid of discontigmem, it seems.

M.
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