Messages in this thread | | | Date | Fri, 10 Oct 2008 22:11:18 +0200 | From | Brice Goglin <> | Subject | Re: [PATCH] mm: use a radix-tree to make do_move_pages() complexity linear |
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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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