Messages in this thread | | | Date | Thu, 25 Sep 2008 14:58:59 +0200 | From | Brice Goglin <> | Subject | Re: [PATCH] mm: make do_move_pages() complexity linear |
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Brice Goglin wrote: > Page migration is currently very slow because its overhead is quadratic > with the number of pages. This is caused by each single page migration > doing a linear lookup in the page array in new_page_node(). > > Since pages are stored in the array order in the pagelist and do_move_pages > process this list in order, new_page_node() can increase the "pm" pointer > to the page array so that the next iteration will find the next page in > 0 or few lookup steps. > > [...] > > +/* > + * Allocate a page on the node given as a page_to_node in private. > + * Increase private to point to the next page_to_node so that the > + * next iteration does not have to traverse the whole pm array. > + */ > static struct page *new_page_node(struct page *p, unsigned long private, > int **result) > { > - struct page_to_node *pm = (struct page_to_node *)private; > + struct page_to_node **pmptr = (struct page_to_node **)private; > + struct page_to_node *pm = *pmptr; > > while (pm->node != MAX_NUMNODES && pm->page != p) > pm++; > > + /* prepare for the next iteration */ > + *pmptr = pm + 1; > + >
Actually, this "pm+1" breaks the case where migrate_pages() calls unmap_and_move() multiple times on the same page. In this case, we need the while loop to look at pm instead of pm+1 first. So we can't cache pm+1 in private, but caching pm is ok. There will be 1 while loop instead of 0 in the regular case. Updated patch (with more comments) coming soon.
Brice
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