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    SubjectRe: regression in page writeback
    On Wed, Sep 23, 2009 at 11:10:12AM +0800, Li, Shaohua wrote:
    > On Wed, Sep 23, 2009 at 10:49:58AM +0800, Wu, Fengguang wrote:
    > > On Wed, Sep 23, 2009 at 10:36:22AM +0800, Andrew Morton wrote:
    > > > On Wed, 23 Sep 2009 10:26:22 +0800 Wu Fengguang <fengguang.wu@intel.com> wrote:
    > > >
    > > > > On Wed, Sep 23, 2009 at 09:59:41AM +0800, Andrew Morton wrote:
    > > > > > On Wed, 23 Sep 2009 09:45:00 +0800 Wu Fengguang <fengguang.wu@intel.com> wrote:
    > > > > >
    > > > > > > On Wed, Sep 23, 2009 at 09:28:32AM +0800, Andrew Morton wrote:
    > > > > > > > On Wed, 23 Sep 2009 09:17:58 +0800 Wu Fengguang <fengguang.wu@intel.com> wrote:
    > > > > > > >
    > > > > > > > > On Wed, Sep 23, 2009 at 08:54:52AM +0800, Andrew Morton wrote:
    > > > > > > > > > On Wed, 23 Sep 2009 08:22:20 +0800 Wu Fengguang <fengguang.wu@intel.com> wrote:
    > > > > > > > > >
    > > > > > > > > > > Jens' per-bdi writeback has another improvement. In 2.6.31, when
    > > > > > > > > > > superblocks A and B both have 100000 dirty pages, it will first
    > > > > > > > > > > exhaust A's 100000 dirty pages before going on to sync B's.
    > > > > > > > > >
    > > > > > > > > > That would only be true if someone broke 2.6.31. Did they?
    > > > > > > > > >
    > > > > > > > > > SYSCALL_DEFINE0(sync)
    > > > > > > > > > {
    > > > > > > > > > wakeup_pdflush(0);
    > > > > > > > > > sync_filesystems(0);
    > > > > > > > > > sync_filesystems(1);
    > > > > > > > > > if (unlikely(laptop_mode))
    > > > > > > > > > laptop_sync_completion();
    > > > > > > > > > return 0;
    > > > > > > > > > }
    > > > > > > > > >
    > > > > > > > > > the sync_filesystems(0) is supposed to non-blockingly start IO against
    > > > > > > > > > all devices. It used to do that correctly. But people mucked with it
    > > > > > > > > > so perhaps it no longer does.
    > > > > > > > >
    > > > > > > > > I'm referring to writeback_inodes(). Each invocation of which (to sync
    > > > > > > > > 4MB) will do the same iteration over superblocks A => B => C ... So if
    > > > > > > > > A has dirty pages, it will always be served first.
    > > > > > > > >
    > > > > > > > > So if wbc->bdi == NULL (which is true for kupdate/background sync), it
    > > > > > > > > will have to first exhaust A before going on to B and C.
    > > > > > > >
    > > > > > > > But that works OK. We fill the first device's queue, then it gets
    > > > > > > > congested and sync_sb_inodes() does nothing and we advance to the next
    > > > > > > > queue.
    > > > > > >
    > > > > > > So in common cases "exhaust" is a bit exaggerated, but A does receive
    > > > > > > much more opportunity than B. Computation resources for IO submission
    > > > > > > are unbalanced for A, and there are pointless overheads in rechecking A.
    > > > > >
    > > > > > That's unquantified handwaving. One CPU can do a *lot* of IO.
    > > > >
    > > > > Yes.. I had the impression that the writeback submission can be pretty slow.
    > > > > It should be because of the congestion_wait. Now that it is removed,
    > > > > things are going faster when queue is not full.
    > > >
    > > > What? The wait is short. The design intent there is that we repoll
    > > > all previously-congested queues well before they start to run empty.
    > >
    > > When queue is not congested (in which case congestion_wait is not
    > > necessary), the congestion_wait() degrades io submission speed to near
    > > io completion speed.
    > >
    > > > > > > > If a device has more than a queue's worth of dirty data then we'll
    > > > > > > > probably leave some of that dirty memory un-queued, so there's some
    > > > > > > > lack of concurrency in that situation.
    > > > > > >
    > > > > > > Good insight.
    > > > > >
    > > > > > It was wrong. See the other email.
    > > > >
    > > > > No your first insight is correct. Because the (unnecessary) teeny
    > > > > sleeps is independent of the A=>B=>C traversing order. Only queue
    > > > > congestion could help skip A.
    > > >
    > > > The sleeps are completely necessary! Otherwise we end up busywaiting.
    > > >
    > > > After the sleep we repoll all queues.
    > >
    > > I mean, it is not always necessary. Only when _all_ superblocks cannot
    > > writeback their inodes (eg. all in congestion), we should wait.
    > >
    > > Just before Jens' work, I had patch to convert
    > >
    > > - if (wbc.encountered_congestion || wbc.more_io)
    > > - congestion_wait(WRITE, HZ/10);
    > > - else
    > > - break;
    > >
    > > to
    > >
    > > + if (wbc->encountered_congestion && wbc->nr_to_write == MAX_WRITEBACK_PAGES)
    > > + congestion_wait(WRITE, HZ/10);
    > >
    > > Note that wbc->encountered_congestion only means "at least one bdi
    > > encountered congestion". We may still make progress in other bdis
    > > hence should not sleep.
    > Hi,
    > encountered_congestion only is checked when nr_to_write > 0, if some superblocks
    > aren't congestions, nr_to_write should be 0, right?

    Yeah, good spot! So the change only helps some corner cases.

    Thanks,
    Fengguang


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