Messages in this thread | | | Date | Wed, 8 Oct 2014 17:33:29 +0200 | From | Michal Hocko <> | Subject | Re: [patch 3/3] mm: memcontrol: fix transparent huge page allocations under pressure |
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[I do not have time to get over all points here and will be offline until Monday - will get back to the rest then]
On Tue 07-10-14 21:11:06, Johannes Weiner wrote: > On Tue, Oct 07, 2014 at 03:59:50PM +0200, Michal Hocko wrote: [...] > > I am completely missing any notes about potential excessive > > swapouts or longer reclaim stalls which are a natural side effect of direct > > reclaim with a larger target (or is this something we do not agree on?). > > Yes, we disagree here. Why is reclaiming 2MB once worse than entering > reclaim 16 times to reclaim SWAP_CLUSTER_MAX?
You can enter DEF_PRIORITY reclaim 16 times and reclaim your target but you need at least 512<<DEF_PRIORITY pages on your LRUs to do it in a single run on that priority. So especially small groups will pay more and would be subject to mentioned problems (e.g. over-reclaim).
> There is no inherent difference in reclaiming a big chunk and > reclaiming many small chunks that add up to the same size.
[...]
> > Another part that matters is the size. Memcgs might be really small and > > that changes the math. Large reclaim target will get to low prio reclaim > > and thus the excessive reclaim. > > I already addressed page size vs. memcg size before. > > However, low priority reclaim does not result in excessive reclaim. > The reclaim goal is checked every time it scanned SWAP_CLUSTER_MAX > pages, and it exits if the goal has been met. See shrink_lruvec(), > shrink_zone() etc.
Now I am confused. shrink_zone will bail out but shrink_lruvec will loop over nr[...] until they are empty and only updates the numbers to be roughly proportional once:
if (nr_reclaimed < nr_to_reclaim || scan_adjusted) continue;
/* * For kswapd and memcg, reclaim at least the number of pages * requested. Ensure that the anon and file LRUs are scanned * proportionally what was requested by get_scan_count(). We * stop reclaiming one LRU and reduce the amount scanning * proportional to the original scan target. */ [...] scan_adjusted = true;
Or do you rely on /* * It's just vindictive to attack the larger once the smaller * has gone to zero. And given the way we stop scanning the * smaller below, this makes sure that we only make one nudge * towards proportionality once we've got nr_to_reclaim. */ if (!nr_file || !nr_anon) break;
and SCAN_FILE because !inactive_file_is_low?
[...] -- Michal Hocko SUSE Labs
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