Messages in this thread | | | Date | Wed, 23 Sep 2009 10:49:58 +0800 | From | Wu Fengguang <> | Subject | Re: regression in page writeback |
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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.
Thanks, Fengguang
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