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 [off-list ref]
SUSE Labs, CR

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>