On Thu, Jun 26, 2014 at 10:37:38AM -0400, Johannes Weiner wrote: > 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. All of which is fair enough and is something that should be examined on a rainy day (shouldn't take too long in Ireland). I'm not going to touch it within this series though. It's outside the scope of what I'm trying to do here -- restore performance of tiobench and bonnie++ to as close to 3.0 levels as possible. The series is tripping up enough on the fair zone and CFQ aspects as it is without increasing the scope :( -- Mel Gorman SUSE Labs -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html