Re: [PATCH RFC] mm: Implement balance_dirty_pages() through waiting for flusher thread

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

On Wed, Jun 23, 2010 at 06:45:51AM +0800, Dave Chinner wrote:
> On Tue, Jun 22, 2010 at 04:38:56PM +0200, Jan Kara wrote:
> > On Tue 22-06-10 10:31:24, Christoph Hellwig wrote:
> > > On Tue, Jun 22, 2010 at 09:52:34PM +0800, Wu Fengguang wrote:
> > > > 2) most writeback will be submitted by one per-bdi-flusher, so no worry
> > > >    of cache bouncing (this also means the per CPU counter error is
> > > >    normally bounded by the batch size)
> > > 
> > > What counter are we talking about exactly?  Once balanance_dirty_pages
> >   The new per-bdi counter I'd like to introduce.
> > 
> > > stops submitting I/O the per-bdi flusher thread will in fact be
> > > the only thing submitting writeback, unless you count direct invocations
> > > of writeback_single_inode.
> >   Yes, I agree that the per-bdi flusher thread should be the only thread
> > submitting lots of IO (there is direct reclaim or kswapd if we change
> > direct reclaim but those should be negligible). So does this mean that
> > also I/O completions will be local to the CPU running per-bdi flusher
> > thread? Because the counter is incremented from the I/O completion
> > callback.
> 
> By default we set QUEUE_FLAG_SAME_COMP, which means we hand
> completions back to the submitter CPU during blk_complete_request().
> Completion processing is then handled by a softirq on the CPU
> selected for completion processing.

Good to know about that, thanks!

> This was done, IIRC, because it provided some OLTP benchmark 1-2%
> better results. It can, however, be turned off via
> /sys/block/<foo>/queue/rq_affinity, and there's no guarantee that
> the completion processing doesn't get handled off to some other CPU
> (e.g. via a workqueue) so we cannot rely on this completion
> behaviour to avoid cacheline bouncing.

If rq_affinity does not work reliably somewhere in the IO completion
path, why not trying to fix it? Otherwise all the page/mapping/zone
cachelines covered by test_set_page_writeback()/test_clear_page_writeback()
(and more other functions) will also be bounced.

Another option is to put atomic accounting into test_set_page_writeback()
ie. the IO submission path. This actually matches the current
balanance_dirty_pages() behavior. It may then block on get_request().
The down side is, get_request() blocks until queue depth goes down
from nr_congestion_on to nr_congestion_off, which is not as smooth as
the IO completion path. As a result balanance_dirty_pages() may get
delayed much more than necessary when there is only 1 waiter, and
wake up multiple waiters in bursts.

Thanks,
Fengguang