Re: [PATCH 4/5] writeback: per task dirty rate limit

[Wu Fengguang <fengguang.wu@intel.com>]

On Wed, Aug 10, 2011 at 02:35:06AM +0800, Peter Zijlstra wrote:
> On Sat, 2011-08-06 at 16:44 +0800, Wu Fengguang wrote:
> > 
> > Add two fields to task_struct.
> > 
> > 1) account dirtied pages in the individual tasks, for accuracy
> > 2) per-task balance_dirty_pages() call intervals, for flexibility
> > 
> > The balance_dirty_pages() call interval (ie. nr_dirtied_pause) will
> > scale near-sqrt to the safety gap between dirty pages and threshold.
> > 
> > XXX: The main problem of per-task nr_dirtied is, if 10k tasks start
> > dirtying pages at exactly the same time, each task will be assigned a
> > large initial nr_dirtied_pause, so that the dirty threshold will be
> > exceeded long before each task reached its nr_dirtied_pause and hence
> > call balance_dirty_pages(). 
> 
> Right, so why remove the per-cpu threshold? you can keep that as a bound
> on the number of out-standing dirty pages.

Right, I also have the vague feeling that the per-cpu threshold can
somehow backup the per-task threshold in case there are too many tasks.

> Loosing that bound is actually a bad thing (TM), since you could have
> configured a tight dirty limit and lock up your machine this way.

It seems good enough to only remove the 4MB upper limit for
ratelimit_pages, so that the per-cpu limit won't kick in too
frequently in typical machines.

  * Here we set ratelimit_pages to a level which ensures that when all CPUs are
  * dirtying in parallel, we cannot go more than 3% (1/32) over the dirty memory
  * thresholds before writeback cuts in.
- *
- * But the limit should not be set too high.  Because it also controls the
- * amount of memory which the balance_dirty_pages() caller has to write back.
- * If this is too large then the caller will block on the IO queue all the
- * time.  So limit it to four megabytes - the balance_dirty_pages() caller
- * will write six megabyte chunks, max.
- */
-
 void writeback_set_ratelimit(void)
 {
        ratelimit_pages = vm_total_pages / (num_online_cpus() * 32);
        if (ratelimit_pages < 16)
                ratelimit_pages = 16;
-       if (ratelimit_pages * PAGE_CACHE_SIZE > 4096 * 1024)
-               ratelimit_pages = (4096 * 1024) / PAGE_CACHE_SIZE;
 }

Thanks,
Fengguang