On Thu, Nov 18, 2010 at 06:10:48PM +0900, KAMEZAWA Hiroyuki wrote: > On Wed, 17 Nov 2010 16:22:48 +0000 > Mel Gorman <mel@xxxxxxxxx> wrote: > > > The end of the LRU stores the oldest known page. Compaction on the other > > hand always starts scanning from the start of the zone. This patch uses > > the LRU to hint to compaction where it should start scanning from. This > > means that compaction will at least start with some old pages reducing > > the impact on running processes and reducing the amount of scanning. The > > check it makes is racy as the LRU lock is not taken but it should be > > harmless as we are not manipulating the lists without the lock. > > > > Signed-off-by: Mel Gorman <mel@xxxxxxxxx> > > Hmm, does this patch make a noticable difference ? To scanning rates - yes. > Isn't it better to start scan from the biggest free chunk in a zone ? > Not necessarily. The biggest free chunk does not necessarily contain old pages so one could stall a process by migrating a very active page. The same applies for selecting the pageblock with the oldest LRU page of course but it is less likely. I prototyped a a patch that constantly used the buddy lists to select the next pageblock to migrate from. The problem was that it was possible for it to infinite loop because it could migrate from the same block more than once in a migration cycle. To resolve that, I'd have to keep track of visited pageblocks but I didn't want to require additional memory unless it was absolutly necessary. I think the concept can be perfected and its impact would be a reduction of scanning rates but it's not something that is anywhere near merging yet. -- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom policy in Canada: sign http://dissolvethecrtc.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>