Messages in this thread | | | Date | Tue, 22 Sep 2009 18:47:26 -0700 | From | Andrew Morton <> | Subject | Re: regression in page writeback |
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On Wed, 23 Sep 2009 09:32:36 +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. > > > > 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. > > Yes, exactly if block device is not fast enough.
Actually, no.
If there's still outstanding dirty data for any of those queues, both wb_kupdate() and background_writeout() will take a teeny sleep and then will re-poll the queues.
Did that logic get broken?
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