Re: [PATCH] mm: use a radix-tree to make do_move_pages() complexity linear

[Brice Goglin <Brice.Goglin@inria.fr>]

Andrew Morton wrote:
> On Thu, 09 Oct 2008 14:32:26 +0200
> Brice Goglin <Brice.Goglin@inria.fr> wrote:
>
>   
>> Add a radix-tree in do_move_pages() to associate each page with
>> the struct page_to_node that describes its migration.
>> new_page_node() can now easily find out the page_to_node of the
>> given page instead of traversing the whole page_to_node array.
>> So the overall complexity is linear instead of quadratic.
>>
>> We still need the page_to_node array since it is allocated by the
>> caller (sys_move_page()) and used by do_pages_stat() when no target
>> nodes are given by the application. And we need room to store all
>> these page_to_node entries for do_move_pages() as well anyway.
>>
>> If a page is given twice by the application, the old code would
>> return -EBUSY (failure from the second isolate_lru_page()). Now,
>> radix_tree_insert() will return -EEXIST, and we convert it back
>> to -EBUSY to keep the user-space ABI.
>>
>> The radix-tree is emptied at the end of do_move_pages() since
>> new_page_node() doesn't know when an entry is used for the last
>> time (unmap_and_move() could try another pass later).
>> Marking pp->page as ZERO_PAGE(0) was actually never used. We now
>> set it to NULL when pp is not in the radix-tree. It is faster
>> than doing a loop of radix_tree_lookup_gang()+delete().
>>     
>
> Any O(n*n) code always catches up with us in the end.  But I do think
> that to merge this code we'd need some description of the problem which
> we fixed.
>
> Please send a description of the situation under which the current code
> performs unacceptably.  Some before-and-after quantitative measurements
> would be good.
>
> Because it could be (as far as I know) that the problem is purely
> theoretical, in which case we might not want the patch at all.
>   

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.

Brice

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