On Thu 19-05-16 16:27:51, Minchan Kim wrote: > On Thu, May 19, 2016 at 09:03:57AM +0200, Michal Hocko wrote: > > On Thu 19-05-16 14:00:38, Minchan Kim wrote: > > > On Tue, May 17, 2016 at 11:02:54AM +0200, Michal Hocko wrote: > > > > On Tue 17-05-16 09:58:15, Michal Hocko wrote: > > > > > On Thu 28-04-16 17:19:21, Michal Hocko wrote: > > > > > > On Wed 27-04-16 14:17:20, Andrew Morton wrote: > > > > > > [...] > > > > > > > @@ -2484,7 +2485,14 @@ static void collapse_huge_page(struct mm > > > > > > > goto out; > > > > > > > } > > > > > > > > > > > > > > - __collapse_huge_page_swapin(mm, vma, address, pmd); > > > > > > > + swap = get_mm_counter(mm, MM_SWAPENTS); > > > > > > > + curr_allocstall = sum_vm_event(ALLOCSTALL); > > > > > > > + /* > > > > > > > + * When system under pressure, don't swapin readahead. > > > > > > > + * So that avoid unnecessary resource consuming. > > > > > > > + */ > > > > > > > + if (allocstall == curr_allocstall && swap != 0) > > > > > > > + __collapse_huge_page_swapin(mm, vma, address, pmd); > > > > > > > > > > > > > > anon_vma_lock_write(vma->anon_vma); > > > > > > > > > > > > > > > > > > > I have mentioned that before already but this seems like a rather weak > > > > > > heuristic. Don't we really rather teach __collapse_huge_page_swapin > > > > > > (resp. do_swap_page) do to an optimistic GFP_NOWAIT allocations and > > > > > > back off under the memory pressure? > > > > > > > > > > I gave it a try and it doesn't seem really bad. Untested and I might > > > > > have missed something really obvious but what do you think about this > > > > > approach rather than relying on ALLOCSTALL which is really weak > > > > > heuristic: > > > > > > I like this approach rather than playing with allocstall diff of vmevent > > > which can be disabled in some configuration and it's not a good indicator > > > to represent current memory pressure situation. > > > > Not only that it won't work for e.g. memcg configurations because we > > would end up reclaiming that memcg as the gfp mask tells us to do so and > > ALLOCSTALL would be quite about that. > > Right you are. I didn't consider memcg. Thanks for pointing out. > > > > > > However, I agree with Rik's requirement which doesn't want to turn over > > > page cache for collapsing THP page via swapin. So, your suggestion cannot > > > prevent it because khugepaged can consume memory through this swapin > > > operation continuously while kswapd is doing aging of LRU list in parallel. > > > IOW, fluctuation between HIGH and LOW watermark. > > > > I am not sure this is actually a problem. We have other sources of > > opportunistic allocations with some fallback and those wake up kswapd > > (they only clear __GFP_DIRECT_RECLAIM). Also this swapin should happen > > only when a certain portion of the huge page is already populated so > > I can't find any logic you mentioned "a certain portion of the huge page > is already populated" in next-20160517. What am I missing now? khugepaged_max_ptes_swap. I didn't look closer but from a quick glance this is the threshold for the optimistic swapin. > > it won't happen all the time and sounds like we would benefit from the > > reclaimed page cache in favor of the THP. > > It depends on storage speed. If a page is swapped out, it means it's not a > workingset so we might read cold page at the cost of evciting warm page. > Additionally, if the huge page was swapped out, it is likely to swap out > again because it's not a hot * 512 page. For those pages, shouldn't we > evict page cache? I think it's not a good tradeoff. This is exactly the problem of the optimistic THP swap in. We just do not know whether it is worth it. But I guess that a reasonable threshold would solve this. It is really ineffective to keep small pages when only few holes are swapped out (for what ever reasons). HPAGE_PMD_NR/8 which we use right now is not documented but I guess 64 pages sounds like a reasonable value which shouldn't cause way too much of reclaim. -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>