On Fri, Apr 27, 2012 at 12:57:11PM +0200, Bartlomiej Zolnierkiewicz wrote: > From: Bartlomiej Zolnierkiewicz <b.zolnierkie@xxxxxxxxxxx> > Subject: [PATCH v4] mm: compaction: handle incorrect Unmovable type pageblocks > > When Unmovable pages are freed from Unmovable type pageblock > (and some Movable type pages are left in it) waiting until > an allocation takes ownership of the block may take too long. > The type of the pageblock remains unchanged so the pageblock > cannot be used as a migration target during compaction. > > Fix it by: > > * Adding enum compact_mode (COMPACT_ASYNC_MOVABLE, > COMPACT_ASYNC_UNMOVABLE and COMPACT_SYNC) and then converting > sync field in struct compact_control to use it. > > * Scanning the Unmovable pageblocks (during COMPACT_ASYNC_UNMOVABLE > and COMPACT_SYNC compactions) and building a count based on > finding PageBuddy pages, page_count(page) == 0 or PageLRU pages. > If all pages within the Unmovable pageblock are in one of those > three sets change the whole pageblock type to Movable. > > My particular test case (on a ARM EXYNOS4 device with 512 MiB, > which means 131072 standard 4KiB pages in 'Normal' zone) is to: > - allocate 120000 pages for kernel's usage > - free every second page (60000 pages) of memory just allocated > - allocate and use 60000 pages from user space > - free remaining 60000 pages of kernel memory > (now we have fragmented memory occupied mostly by user space pages) > - try to allocate 100 order-9 (2048 KiB) pages for kernel's usage > > The results: > - with compaction disabled I get 11 successful allocations > - with compaction enabled - 14 successful allocations > - with this patch I'm able to get all 100 successful allocations > This is looking much better to me. However, I would really like to see COMPACT_ASYNC_UNMOVABLE being used by the page allocator instead of depending on kswapd to do the work. Right now as it uses COMPACT_ASYNC_MOVABLE only, I think it uses COMPACT_SYNC too easily (making latency worse). Specifically 1. Leave try_to_compact_pages() taking a sync parameter. It is up to compaction how to treat sync==false 2. When sync==false, start with ASYNC_MOVABLE. Track how many pageblocks were scanned during compaction and how many of them were MIGRATE_UNMOVABLE. If compaction ran fully (COMPACT_COMPLETE) it implies that there is not a suitable page for allocation. In this case then check how if there were enough MIGRATE_UNMOVABLE pageblocks to try a second pass in ASYNC_FULL. By keeping all the logic in compaction.c it prevents too much knowledge of compaction sneaking into page_alloc.c 3. When scanning ASYNC_FULL, *only* scan the MIGRATE_UNMOVABLE blocks as migration targets because the first pass would have scanned within MIGRATE_MOVABLE. This will reduce the cost of the second pass. -- Mel Gorman SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>