Re: regression in page writeback

[Wu Fengguang <fengguang.wu@intel.com>]

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.

> 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. That possibly explains one major factor of the
performance gains of Jens' per-bdi writeback.

Thanks,
Fengguang