On Wed, Jul 28, 2010 at 05:43:41PM +0800, KOSAKI Motohiro wrote: > > On Wed, Jul 28, 2010 at 04:46:54PM +0800, Wu Fengguang wrote: > > > The wait_on_page_writeback() call inside pageout() is virtually dead code. > > > > > > shrink_inactive_list() > > > shrink_page_list(PAGEOUT_IO_ASYNC) > > > pageout(PAGEOUT_IO_ASYNC) > > > shrink_page_list(PAGEOUT_IO_SYNC) > > > pageout(PAGEOUT_IO_SYNC) > > > > > > Because shrink_page_list/pageout(PAGEOUT_IO_SYNC) is always called after > > > a preceding shrink_page_list/pageout(PAGEOUT_IO_ASYNC), the first > > > pageout(ASYNC) converts dirty pages into writeback pages, the second > > > shrink_page_list(SYNC) waits on the clean of writeback pages before > > > calling pageout(SYNC). The second shrink_page_list(SYNC) can hardly run > > > into dirty pages for pageout(SYNC) unless in some race conditions. > > > > > > > It's possible for the second call to run into dirty pages as there is a > > congestion_wait() call between the first shrink_page_list() call and the > > second. That's a big window. > > > > > And the wait page-by-page behavior of pageout(SYNC) will lead to very > > > long stall time if running into some range of dirty pages. > > > > True, but this is also lumpy reclaim which is depending on a contiguous > > range of pages. It's better for it to wait on the selected range of pages > > which is known to contain at least one old page than excessively scan and > > reclaim newer pages. > > Today, I was successful to reproduce the Andres's issue. and I disagree this > opinion. > The root cause is, congestion_wait() mean "wait until clear io congestion". but > if the system have plenty dirty pages, flusher threads are issueing IO conteniously. > So, io congestion is not cleared long time. eventually, congestion_wait(BLK_RW_ASYNC, HZ/10) > become to equivalent to sleep(HZ/10). > > I would propose followint patch instead. > > And I've found synchronous lumpy reclaim have more serious problem. I woule like to > explain it as another mail. > > Thanks. > > > > >From 0266fb2c23aef659cd4e89fccfeb464f23257b74 Mon Sep 17 00:00:00 2001 > From: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx> > Date: Tue, 27 Jul 2010 14:36:44 +0900 > Subject: [PATCH] vmscan: synchronous lumpy reclaim don't call congestion_wait() > > congestion_wait() mean "waiting for number of requests in IO queue is > under congestion threshold". > That said, if the system have plenty dirty pages, flusher thread push > new request to IO queue conteniously. So, IO queue are not cleared > congestion status for a long time. thus, congestion_wait(HZ/10) is > almostly equivalent schedule_timeout(HZ/10). > > If the system 512MB memory, DEF_PRIORITY mean 128kB scan and 4096 times > shrink_inactive_list call. 4096 times 0.1sec stall makes crazy insane > long stall. That shouldn't. Good point. Maybe more clear to say: "It takes 4096 shrink_page_list() calls to scan 512MB memory." > In the other hand, this synchronous lumpy reclaim donesn't need this > congestion_wait() at all. shrink_page_list(PAGEOUT_IO_SYNC) cause to > call wait_on_page_writeback() and it provide sufficient waiting. Agreed. Reviewed-by: Wu Fengguang <fengguang.wu@xxxxxxxxx> Thanks, Fengguang -- 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/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>