linux-kernel - Re: [Patch1/4]: fake numa for x86

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Message-ID: <Pine.LNX.4.64.0611281321020.7436@skynet.skynet.ie>
Date:	Tue, 28 Nov 2006 13:24:51 +0000 (GMT)
From:	Mel Gorman <mel@....ul.ie>
To:	Rohit Seth <rohitseth@...gle.com>
Cc:	Andi Kleen <ak@...e.de>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	David Rientjes <rientjes@...washington.edu>,
	Paul Menage <menage@...gle.com>, Andrew Morton <akpm@...l.org>
Subject: Re: [Patch1/4]: fake numa for x86_64 patch

On Mon, 27 Nov 2006, Rohit Seth wrote:

> Hi Mel,
>
> On Mon, 2006-11-27 at 13:18 +0000, Mel Gorman wrote:
>> On Wed, 22 Nov 2006, Rohit Seth wrote:
>>
>>> This patch provides a IO hole size in a given address range.
>>>
>>
>> Hi,
>>
>> This patch reintroduces a function that doubles up what
>> absent_pages_in_range(start_pfn, end_pfn). I recognise you do this because
>> you are interested in hole sizes before add_active_range() is called.
>
> Right.
>
>>
>> However, what is not clear is why these patches are so specific to x86_64.
>>
>
> Specifically in the fake numa case, we want to make sure that we don't
> carve fake nodes that only have IO holes in it.  Unlike the real NUMA
> case, here we don't have SRAT etc. to know the memory layout beforehand.
>
>
>> It looks possible to do the work of functions like split_nodes_equal() in
>> an architecture-independent manner using early_node_map rather than
>> dealing with the arch-specific nodes array. That would open the
>> possibility of providing fake nodes on more than one architecture in the
>> future.
>
> The functions like splti_nodes_equal etc. can be abstracted out to arch
> independent part.  I think the only API it needs from arch dependent
> part is to find out how much real RAM is present in range without have
> to first do add_active_range.
>

That is a problem because the ranges must be registered with 
add_active_range() to work out how much real RAM is present.

> Though as a first step, let us fix the x86_64 (as it doesn't boot when
> you have sizeable chunk of IO hole and nodes > 4).
>

Ok.

> I'm also not sure if other archs actually want to have this
> functionality.
>

It's possible that the containers people are interested in the possibility 
of setting up fake nodes as part of a memory controller.

>> What I think can be done is that you register memory as normal and then
>> split up the nodes into fake nodes. This would remove the need for having
>> e820_hole_size() reintroduced.
>
> Are you saying first let the system find out real numa topology and then
> build fake numa on top of it?
>

Yes, there is nothing stopping you altering the early_node_map[] before 
free_area_init_node() initialises the node_mem_map. If you do hit a 
problem, it'll be because x86_64 allocates it's own node_mem_map with 
CONFIG_FLAT_NODE_MEM_MAP is set. Is that set when setting up fake nodes?

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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