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.
>
> That being said khugepaged_max_ptes_none = HPAGE_PMD_NR/2 sounds like a
max_ptes_none?
> good start to me. Whether all of the present pages have to be young I am
> not so sure.
>
> --
> Michal Hocko
> SUSE Labs
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