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    SubjectRe: [PATCH 7/8] writeback: sync old inodes first in background writeback
    On Fri, Jul 23, 2010 at 06:57:19PM +0800, Mel Gorman wrote:
    > 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 reads much clear. Thanks! I have the same feeling, hence the
    proposed patch.

    > > > 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.

    For the case of of a heavy dirtier (dd) and concurrent light dirtiers
    (some random processes), the light dirtiers won't be easily throttled.
    task_dirty_limit() handles that case well. It will give light dirtiers
    higher threshold than heavy dirtiers so that only the latter will be
    dirty throttled.

    > > 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@intel.com>
    > > > > ---
    > > > > 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..

    Hmm. If such large range of dirty pages are approaching the end of LRU,
    it means the LRU lists are being scanned pretty fast, indicating a
    busy system and/or high memory pressure. So it seems reasonable to act
    cruel to really high order allocators -- they won't perform well under
    memory pressure after all, and only make things worse.

    > > > 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@csn.ul.ie>

    Thanks. I'll post the patch.

    > 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.

    Great. Just added CC to Andreas Mohr.

    > 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]

    And Minchan, he has been following this issue too :)

    Thanks,
    Fengguang


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