On Tue 22-06-10 20:09:24, Dave Chinner wrote:
> On Mon, Jun 21, 2010 at 11:14:16PM -0700, Andrew Morton wrote:
> > On Tue, 22 Jun 2010 15:44:09 +1000 Dave Chinner <david@xxxxxxxxxxxxx> wrote:
> >
> > > > > 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.
On the other hand I think we will have to come up with something
more clever than what I do now because for some huge machines with
nr_cpu_ids == 256, the error of the counter is 256*9*8 = 18432 so that's
already unacceptable given the amounts we want to check (like 1536) -
already for nr_cpu_ids == 32, the error is the same as the difference we
want to check. I think we'll have to come up with some scheme whose error
is not dependent on the number of cpus or if it is dependent, it's only a
weak dependency (like a logarithm or so).
Or we could rely on the fact that IO completions for a bdi won't happen on
all CPUs and thus the error would be much more bounded. But I'm not sure
how much that is true or not.
Honza
--
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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