On Fri, Jul 23, 2010 at 05:45:15PM +0800, Wu Fengguang wrote: > On Thu, Jul 22, 2010 at 06:48:23PM +0800, Mel Gorman wrote: > > On Thu, Jul 22, 2010 at 05:21:55PM +0800, Wu Fengguang wrote: > > > > I guess this new patch is more problem oriented and acceptable: > > > > > > > > --- linux-next.orig/mm/vmscan.c 2010-07-22 16:36:58.000000000 +0800 > > > > +++ linux-next/mm/vmscan.c 2010-07-22 16:39:57.000000000 +0800 > > > > @@ -1217,7 +1217,8 @@ static unsigned long shrink_inactive_lis > > > > count_vm_events(PGDEACTIVATE, nr_active); > > > > > > > > nr_freed += shrink_page_list(&page_list, sc, > > > > - PAGEOUT_IO_SYNC); > > > > + priority < DEF_PRIORITY / 3 ? > > > > + PAGEOUT_IO_SYNC : PAGEOUT_IO_ASYNC); > > > > } > > > > > > > > nr_reclaimed += nr_freed; > > > > > > This one looks better: > > > --- > > > vmscan: raise the bar to PAGEOUT_IO_SYNC stalls > > > > > > Fix "system goes totally unresponsive with many dirty/writeback pages" > > > problem: > > > > > > http://lkml.org/lkml/2010/4/4/86 > > > > > > The root cause is, wait_on_page_writeback() is called too early in the > > > direct reclaim path, which blocks many random/unrelated processes when > > > some slow (USB stick) writeback is on the way. > > > > > > > So, what's the bet if lumpy reclaim is a factor that it's > > high-order-but-low-cost such as fork() that are getting caught by this since > > [78dc583d: vmscan: low order lumpy reclaim also should use PAGEOUT_IO_SYNC] > > was introduced? > > Sorry I'm a bit confused by your wording.. > After reading the thread, I realised that fork() stalling could be a factor. That commit allows lumpy reclaim and PAGEOUT_IO_SYNC to be used for high-order allocations such as those used by fork(). It might have been an oversight to allow order-1 to use PAGEOUT_IO_SYNC too easily. > > That could manifest to the user as stalls creating new processes when under > > heavy IO. I would be surprised it would freeze the entire system but certainly > > any new work would feel very slow. > > > > > A simple dd can easily create a big range of dirty pages in the LRU > > > list. Therefore priority can easily go below (DEF_PRIORITY - 2) in a > > > typical desktop, which triggers the lumpy reclaim mode and hence > > > wait_on_page_writeback(). > > > > > > > which triggers the lumpy reclaim mode for high-order allocations. > > Exactly. Changelog updated. > > > lumpy reclaim mode is not something that is triggered just because priority > > is high. > > Right. > > > I think there is a second possibility for causing stalls as well that is > > unrelated to lumpy reclaim. Once dirty_limit is reached, new page faults may > > also result in stalls. If it is taking a long time to writeback dirty data, > > random processes could be getting stalled just because they happened to dirty > > data at the wrong time. This would be the case if the main dirtying process > > (e.g. dd) is not calling sync and dropping pages it's no longer using. > > The dirty_limit throttling will slow down the dirty process to the > writeback throughput. If a process is dirtying files on sda (HDD), > it will be throttled at 80MB/s. If another process is dirtying files > on sdb (USB 1.1), it will be throttled at 1MB/s. > It will slow down the dirty process doing the dd, but can it also slow down other processes that just happened to dirty pages at the wrong time. > So dirty throttling will slow things down. However the slow down > should be smooth (a series of 100ms stalls instead of a sudden 10s > stall), and won't impact random processes (which does no read/write IO > at all). > Ok. > > > In Andreas' case, 512MB/1024 = 512KB, this is way too low comparing to > > > the 22MB writeback and 190MB dirty pages. There can easily be a > > > continuous range of 512KB dirty/writeback pages in the LRU, which will > > > trigger the wait logic. > > > > > > To make it worse, when there are 50MB writeback pages and USB 1.1 is > > > writing them in 1MB/s, wait_on_page_writeback() may stuck for up to 50 > > > seconds. > > > > > > So only enter sync write&wait when priority goes below DEF_PRIORITY/3, > > > or 6.25% LRU. As the default dirty throttle ratio is 20%, sync write&wait > > > will hardly be triggered by pure dirty pages. > > > > > > Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx> > > > --- > > > mm/vmscan.c | 4 ++-- > > > 1 file changed, 2 insertions(+), 2 deletions(-) > > > > > > --- linux-next.orig/mm/vmscan.c 2010-07-22 16:36:58.000000000 +0800 > > > +++ linux-next/mm/vmscan.c 2010-07-22 17:03:47.000000000 +0800 > > > @@ -1206,7 +1206,7 @@ static unsigned long shrink_inactive_lis > > > * but that should be acceptable to the caller > > > */ > > > if (nr_freed < nr_taken && !current_is_kswapd() && > > > - sc->lumpy_reclaim_mode) { > > > + sc->lumpy_reclaim_mode && priority < DEF_PRIORITY / 3) { > > > congestion_wait(BLK_RW_ASYNC, HZ/10); > > > > > > > This will also delay waiting on congestion for really high-order > > allocations such as huge pages, some video decoder and the like which > > really should be stalling. > > I absolutely agree that high order allocators should be somehow throttled. > > However given that one can easily create a large _continuous_ range of > dirty LRU pages, let someone bumping all the way through the range > sounds a bit cruel.. > > > How about the following compile-tested diff? > > It takes the cost of the high-order allocation into account and the > > priority when deciding whether to synchronously wait or not. > > Very nice patch. Thanks! > Will you be picking it up or should I? The changelog should be more or less the same as yours and consider it Signed-off-by: Mel Gorman <mel@xxxxxxxxx> It'd be nice if the original tester is still knocking around and willing to confirm the patch resolves his/her problem. I am running this patch on my desktop at the moment and it does feel a little smoother but it might be my imagination. I had trouble with odd stalls that I never pinned down and was attributing to the machine being commonly heavily loaded but I haven't noticed them today. It also needs an Acked-by or Reviewed-by from Kosaki Motohiro as it alters logic he introduced in commit [78dc583: vmscan: low order lumpy reclaim also should use PAGEOUT_IO_SYNC] Thanks > <SNIP> -- 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/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>