On Thu, Jul 22, 2010 at 04:52:10PM +0800, Wu Fengguang wrote: > > Some insight on how the other writeback changes that are being floated > > around might affect the number of dirty pages reclaim encounters would also > > be helpful. > > Here is an interesting related problem about the wait_on_page_writeback() call > inside shrink_page_list(): > > http://lkml.org/lkml/2010/4/4/86 > > The problem 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. > > 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(). > Lumpy reclaim is for high-order allocations. A simple dd should not be triggering it regularly unless there was a lot of forking going on at the same time. Also, how would a random or unrelated process get blocked on writeback unless they were also doing high-order allocations? What was the source of the high-order allocations? > I proposed this patch at the time, which was confirmed to solve the problem: > > --- linux-next.orig/mm/vmscan.c 2010-06-24 14:32:03.000000000 +0800 > +++ linux-next/mm/vmscan.c 2010-07-22 16:12:34.000000000 +0800 > @@ -1650,7 +1650,7 @@ static void set_lumpy_reclaim_mode(int p > */ > if (sc->order > PAGE_ALLOC_COSTLY_ORDER) > sc->lumpy_reclaim_mode = 1; > - else if (sc->order && priority < DEF_PRIORITY - 2) > + else if (sc->order && priority < DEF_PRIORITY / 2) > sc->lumpy_reclaim_mode = 1; > else > sc->lumpy_reclaim_mode = 0; > > > However KOSAKI and Minchan raised concerns about raising the bar. > 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); > } > I'm not seeing how this helps. It delays when lumpy reclaim waits on IO to clean contiguous ranges of pages. I'll read that full thread as I wasn't aware of it before. -- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
