On Mon, Jun 21, 2010 at 11:14:16PM -0700, Andrew Morton wrote:

On Tue, 22 Jun 2010 15:44:09 +1000 Dave Chinner [off-list ref] wrote:

quoted

quoted

quoted

And so on. This isn't necessarily bad - we'll throttle for longer
than we strictly need to - but the cumulative counter resolution
error gets worse as the number of CPUs doing IO completion grows.
Worst case ends up at for (num cpus * 31) + 1 pages of writeback for
just the first waiter. For an arbitrary FIFO queue of depth d, the
worst case is more like d * (num cpus * 31 + 1).

  Hmm, I don't see how the error would depend on the FIFO depth.

It's the cumulative error that depends on the FIFO depth, not the
error seen by a single waiter.

Could use the below to basically eliminate the inaccuracies.

Obviously things might get a bit expensive in certain threshold cases
but with some hysteresis that should be manageable.

That seems a lot more... unpredictable than modifying the accounting
to avoid cumulative errors.

+	/* Check to see if rough count will be sufficient for comparison */
+	if (abs(count - rhs) > (percpu_counter_batch*num_online_cpus())) {

Also, that's a big margin when we are doing equality matches for
every page IO completion. If we a large CPU count machine where
per-cpu counters actually improve performance (say 16p) then we're
going to be hitting the slow path for the last 512 pages of every
waiter. Hence I think the counter sum is compared too often to scale
with this method of comparison.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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