Re: [PATCH 5/6] mm: page_alloc: Reduce cost of dirty zone balancing

[Johannes Weiner]

On Thu, Jun 26, 2014 at 09:43:14AM +0100, Mel Gorman wrote:
> On Wed, Jun 25, 2014 at 04:35:28PM -0700, Andrew Morton wrote:
> > On Wed, 25 Jun 2014 08:58:48 +0100 Mel Gorman <mgorman@xxxxxxx> wrote:
> > 
> > > @@ -325,7 +321,14 @@ static unsigned long zone_dirty_limit(struct zone *zone)
> > >   */
> > >  bool zone_dirty_ok(struct zone *zone)
> > >  {
> > > -	unsigned long limit = zone_dirty_limit(zone);
> > > +	unsigned long limit = zone->dirty_limit_cached;
> > > +	struct task_struct *tsk = current;
> > > +
> > > +	if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk)) {
> > > +		limit = zone_dirty_limit(zone);
> > > +		zone->dirty_limit_cached = limit;
> > > +		limit += limit / 4;
> > > +	}
> > 
> > Could we get a comment in here explaining what we're doing and why
> > PF_LESS_THROTTLE and rt_task control whether we do it?
> > 
> 
>         /*
>          * The dirty limits are lifted by 1/4 for PF_LESS_THROTTLE (ie. nfsd)
>          * and real-time tasks to prioritise their allocations.
>          * PF_LESS_THROTTLE tasks may be cleaning memory and rt tasks may be
>          * blocking tasks that can clean pages.
>          */
> 
> That's fairly weak though. It would also seem reasonable to just delete
> this check and allow PF_LESS_THROTTLE and rt_tasks to fall into the slow
> path if dirty pages are already fairly distributed between zones.
> Johannes, any objection to that limit raising logic being deleted?

I copied that over from global_dirty_limits() such that the big
picture and the per-zone picture have the same view - otherwise these
tasks fall back to first fit zone allocations before global limits
start throttling dirtiers and waking up the flushers.  This increases
the probability of reclaim running into dirty pages.

Would you remove it from global_dirty_limits() as well?

On that note, I don't really understand why global_dirty_limits()
raises the *background* limit for less-throttle/rt tasks, shouldn't it
only raise the dirty limit?  Sure, the throttle point is somewhere
between the two limits, but we don't really want to defer waking up
the flushers for them.
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