From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Fri, 10 Oct 2008 12:50:10 -0700 From: Andrew Morton Subject: Re: [PATCH] mm: use a radix-tree to make do_move_pages() complexity linear Message-Id: <20081010125010.164bcbb8.akpm@linux-foundation.org> In-Reply-To: <48EDF9DA.7000508@inria.fr> References: <48EDF9DA.7000508@inria.fr> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: Brice Goglin Cc: linux-kernel@vger.kernel.org, cl@linux-foundation.org, linux-mm@kvack.org, nathalie.furmento@labri.fr List-ID: On Thu, 09 Oct 2008 14:32:26 +0200 Brice Goglin 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. Thanks. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org