Re: [RFC PATCH 2/3] mm, thp: convert from optimistic to conservative

[Ebru Akagunduz]

On Wed, Jun 15, 2016 at 03:40:53PM +0900, Minchan Kim wrote:
> Hello,
> 
> On Sat, Jun 11, 2016 at 10:16:00PM +0300, Ebru Akagunduz wrote:
> > Currently, khugepaged collapses pages saying only
> > a referenced page enough to create a THP.
> > 
> > This patch changes the design from optimistic to conservative.
> > It gives a default threshold which is half of HPAGE_PMD_NR
> > for referenced pages, also introduces a new sysfs knob.
> 
> Strictly speaking, It's not what I suggested.
> 
> I didn't mean that let's change threshold for deciding whether we should
> collapse or not(although just *a* reference page seems be too
> optimistic) and export the knob to the user. In fact, I cannot judge
> whether it's worth or not because I never have an experience with THP
> workload in practice although I believe it does make sense.
> 
> What I suggested is that a swapin operation would be much heavier than
> a THP cost to collapse populated anon page so it should be more
> conservative than THP collasping decision, at least. Given that thought,
> decision point for collasping a THP is *a* reference page now so *half*
> reference of populated pages for reading swapped-out page is more
> conservative.
>
Then passing referenced parameter from khugepaged_scan_pmd to
collapse_huge_page_swapin seems okay. A referenced is enough to
create THP, if needs to swapin, we check the value that should
be higher than 256 and so that, we don't need a new sysfs knob.
 
> > 
> > Signed-off-by: Ebru Akagunduz <ebru.akagunduz@xxxxxxxxx>
> > ---

> > +static unsigned int khugepaged_min_ptes_young __read_mostly;
> 
> We should set it to 1 to preserve old behavior.