Re: IO-less dirty throttling V6 results available

[Jan Kara <jack@suse.cz>]

On Thu 24-02-11 23:25:09, Wu Fengguang wrote:
> The bdi base throttle bandwidth is updated based on three class of
> parameters.
> 
> (1) level of dirty pages
> 
> We try to avoid updating the base bandwidth whenever possible. The
> main update criteria are based on the level of dirty pages, when
> - the dirty pages are nearby the up or low control scope, or
> - the dirty pages are departing from the global/bdi dirty goals
> it's time to update the base bandwidth.
> 
> Because the dirty pages are fluctuating steadily, we try to avoid
> disturbing the base bandwidth when the smoothed number of dirty pages
> is within (write bandwidth / 8) distance to the goal, based on the
> fact that fluctuations are typically bounded by the write bandwidth.
> 
> (2) the position bandwidth
> 
> The position bandwidth is equal to the base bandwidth if the dirty
> number is equal to the dirty goal, and will be scaled up/down when
> the dirty pages grow larger than or drop below the goal.
> 
> When it's decided to update the base bandwidth, the delta between
> base bandwidth and position bandwidth will be calculated. The delta
> value will be scaled down at least 8 times, and the smaller delta
> value, the more it will be shrank. It's then added to the base
> bandwidth. In this way, the base bandwidth will adapt to the position
> bandwidth fast when there are large gaps, and remain stable when the
> gap is small enough. 
> 
> The delta is scaled down considerably because the position bandwidth
> is not very reliable. It fluctuates sharply when the dirty pages hit
> the up/low limits. And it takes time for the dirty pages to return to
> the goal even when the base bandwidth has be adjusted to the right
> value. So if tracking the position bandwidth closely, the base
> bandwidth could be overshot.
> 
> (3) the reference bandwidth
> 
> It's the theoretic base bandwidth! I take time to calculate it as a
> reference value of base bandwidth to eliminate the fast-convergence
> vs. steady-state-stability dilemma in pure position based control.
> It would be optimal control if used directly, however the reference
> bandwidth is not directly used as the base bandwidth because the
> numbers for calculating it are all fluctuating, and it's not
> acceptable for the base bandwidth to fluctuate in the plateau state.
> So the roughly-accurate calculated value is now used as a very useful
> double limit when updating the base bandwidth.
> 
> Now you should be able to understand the information rich
> balance_dirty_pages-pages.png graph. Here are two nice ones:
> 
> http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/dirty-throttling-v6/4G-60%25/btrfs-16dd-1M-8p-3927M-60%-2.6.38-rc6-dt6+-2011-02-24-23-14/balance_dirty_pages-pages.png
> 
> http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/dirty-throttling-v6/10HDD-JBOD-6G-6%25/xfs-1dd-1M-16p-5904M-6%25-2.6.38-rc5-dt6+-2011-02-21-20-00/balance_dirty_pages-pages.png
  Thanks for the update on your patch series :). As you probably noted,
I've created patches which implement IO-less balance_dirty_pages()
differently so we have two implementations to compare (which is a good
thing I believe). The question is how to do the comparison...

I have implemented comments, Peter had to my patches and I have finished
scripts for gathering mm statistics and processing trace output and
plotting them. Looking at your test scripts I can probably use some
of your workloads as mine are currently simpler. Currently I have some
simple dd tests running, I'll run something over NFS, SATA+USB and
hopefully several SATA drives next week.

The question is how to compare results? Any idea? Obvious metrics are
overall throughput and fairness for IO bound tasks. But then there are
more subtle things like how the algorithm behaves for tasks that are not IO
bound for most of the time (or do less IO). Any good metrics here? More
things we could compare?

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR