On Wed, Aug 10, 2011 at 03:16:22AM +0800, Vivek Goyal wrote:
> On Sat, Aug 06, 2011 at 04:44:52PM +0800, Wu Fengguang wrote:
> 
> [..]
> > -/*
> > - * task_dirty_limit - scale down dirty throttling threshold for one task
> > - *
> > - * task specific dirty limit:
> > - *
> > - *   dirty -= (dirty/8) * p_{t}
> > - *
> > - * To protect light/slow dirtying tasks from heavier/fast ones, we start
> > - * throttling individual tasks before reaching the bdi dirty limit.
> > - * Relatively low thresholds will be allocated to heavy dirtiers. So when
> > - * dirty pages grow large, heavy dirtiers will be throttled first, which will
> > - * effectively curb the growth of dirty pages. Light dirtiers with high enough
> > - * dirty threshold may never get throttled.
> > - */
> 
> Hi Fengguang,
> 
> So we have got rid of the notion of per task dirty limit based on their
> fraction? What replaces it.

It's simply removed :)

> I can't see any code which is replacing it.

The think time compensation feature (patch attached) will be providing
the same protection for light/slow dirtiers. With it, the slower
dirtiers won't be throttled at all, because the pause time calculated
by

        period = pages_dirtied / rate
        pause = period - think

will be <= 0.

For example, given write_bw = 100MB/s and

- 2 dd tasks that dirty pages as fast as possible
- 1 scp whose dirty rate is limited by network bandwidth 10MB/s

Then with think time compensation, the real dirty rates will be

- 2 dd tasks: (100-10)/2 = 45MB/s (each)
- 1 scp task: 10MB/s

The scp task won't be throttled by balance_dirty_pages() any more.
This is a tested feature. In the below graph, the dirty rate (the
slope of the lines) of the last 3 tasks are 2, 4, 8 MB/s

http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/dirty-throttling-v6/RATES-2-4-8/btrfs-fio-rates-128k-8p-2975M-2.6.38-rc6-dt6+-2011-03-01-20-45/balance_dirty_pages-task-bw.png

given this fio workload, which started one full speed dirtier and
four 1, 2, 4, 8 MB/s rate limited dirtiers

http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/dirty-throttling-v6/RATES-2-4-8/btrfs-fio-rates-128k-8p-2975M-2.6.38-rc6-dt6+-2011-03-01-20-45/fio-rates

> If yes, I am wondering how
> do you get fairness among tasks which share this bdi.
> 
> Also wondering what did this patch series to do make sure that tasks
> share bdi more fairly and get write_bw/N bandwidth.

Each of the N dd tasks will be rate limited by

        rate = base_rate * pos_ratio

At any time snapshot, each bdi task will see almost the same base_rate
and pos_ratio, so will be throttled almost at the same rate. This is a
strong guarantee of fairness under all situations.

Since pos_ratio is fluctuating (evenly) around 1.0, and
base_rate=bdi->dirty_ratelimit is fluctuating around (write_bw/N),
on average we get

        avg_rate = (write_bw/N) * 1.0

(I'll explain the "dirty_ratelimit = write_bw/N" magic other emails.)

The below graphs demonstrate the dirty progress of the last 3 dd tasks.
The slope of each curve is the dirty rate.

They vividly show three curves progressing at the same pace in all of
the 3 stages

- rampup stage (20-100s) 

- disturbed stage (120s-160s)
  (disturbed by starting a 1GB read dd in the middle of the tests)

- stable stage (after 160s)

And dirtied almost the same amount of pages during the test.

http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/dirty-throttling-v8/8G/xfs-10dd-4k-32p-6802M-20:10-3.0.0-next-20110802+-2011-08-06.16:26/balance_dirty_pages-task-bw.png

http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/dirty-throttling-v8/2G/xfs-10dd-4k-8p-1947M-20:10-3.0.0-next-20110802+-2011-08-06.15:49/balance_dirty_pages-task-bw.png

Thanks,
Fengguang