linux-kernel - Re: [PATCH] mm: use a radix-tree to make do_move

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Date:	Fri, 10 Oct 2008 15:32:41 -0500
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Brice Goglin <Brice.Goglin@...ia.fr>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	nathalie.furmento@...ri.fr
Subject: Re: [PATCH] mm: use a radix-tree to make do_move_pages() complexity
 linear

Brice Goglin wrote:

> Just try sys_move_pages() on a 10-100MB buffer, you'll get something
> like 50MB/s on a recent Opteron machine. This throughput decreases
> significantly with the number of pages. With this patch, we get about
> 350MB/s and the throughput is stable when the migrated buffer gets
> larger. I don't have detailled numbers at hand, I'll send them by monday.

Migration throughput is optimal for sys_move_pages() and the cpuset migration.
Some comparison would be useful.

With 100MB you have ~250k pages which will require a vmalloc of 4MB for the
struct pm struct array to control the migration of each individual page.

Would it be possible to restructure this in such a way that we work in chunks
of 100 or so pages each so that we can avoid the vmalloc?

We also could do a kmalloc for each individual struct pm_struct with the radix
tree which would also avoid the vmalloc but still keep the need to allocate
4MB for temporary struct pm_structs.

Or get rid of the pm_struct altogether by storing the address of the node
vector somewhere and retrieve the node as needed from the array. This would
allow storing the struct page * pointers in the radix tree.
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