On Fri, May 20, 2016 at 09:34:32AM +0200, Michal Hocko wrote: > On Fri 20-05-16 16:26:24, Minchan Kim wrote: > > On Fri, May 20, 2016 at 08:39:17AM +0200, Michal Hocko wrote: > > > On Fri 20-05-16 09:21:55, Minchan Kim wrote: > > > [...] > > > > I think other important thing we should consider is how the THP page is likely > > > > to be hot without split in a short time like KSM is doing double checking to > > > > merge stable page. Of course, it wouldn't be easyI to implement but I think > > > > algorithm is based on such *hotness* basically and then consider the number > > > > of max_swap_ptes. IOW, I think we should approach more conservative rather > > > > than optimistic because a page I/O overhead by wrong choice could be bigger > > > > than benefit of a few TLB hit. > > > > If we approach that way, maybe we don't need to detect memory pressure. > > > > > > > > For that way, how about raising bar for swapin allowance? > > > > I mean now we allows swapin > > > > > > > > from > > > > 64 pages below swap ptes and 1 page young in 512 ptes > > > > to > > > > 64 pages below swap ptes and 256 page young in 512 ptes > > > > > > I agree that the current 1 page threshold for collapsing is way too > > > optimistic. Same as the defaults we had for the page fault THP faulting > > > which has caused many issues. So I would be all for changing it. I do > > > > I don't know we should change all but if we change it for THP faulting, > > I believe THP swapin should be more conservative value rather than THP > > faulting because cost of THP swapin collapsing would be heavier. > > > > > not have good benchmarks to back any "good" number unfortunately. So > > > such a change would be quite arbitrary based on feeling... If you have > > > some workload where collapsing THP pages causes some real issues that > > > would be great justification though. > > > > Hope to have but our products never have turned on THP. :( > > I just wanted to say current problem and suggestion so a THP guy > > can have an interest on that. > > If it's not worth to do, simple igonore. > > I guess this is worth changing I am just not sure about the > justification I agree with you about THP faulting which have been there for several years so changing without justification, it has risk. Howerver, swapin collapsing is new feature so it might be worth for the feature. > > > > That being said khugepaged_max_ptes_none = HPAGE_PMD_NR/2 sounds like a > > > > max_ptes_none? > > Not sure I understand what you mean here. We are talking about max_ptes_swap and max_active_pages(i.e., pte_young) but suddenly you are saying max_ptes_none so I was curious it was just typo. > -- > 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>