Re: + mm-thp-avoid-unnecessary-swapin-in-khugepaged.patch added to -mm tree

[Minchan Kim <minchan@kernel.org>]

On Fri, May 20, 2016 at 10:02:18AM +0200, Michal Hocko wrote:
> On Fri 20-05-16 16:44:50, Minchan Kim wrote:
> > > > > 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.
> 
> Because the default for pte_none resp. zero pages collapsing into THP is
> khugepaged_max_ptes_none and the current default means that a single
> present page is sufficient. That is way too optimistic. So I consider
> this to be a good start. I am not so sure about minimum young pages
> because that would probably require yet another tunable and we have more
> than enough of them. Anyway I guess we are getting off-topic here...

Optimistic swapin collapsing

1. it could be too optimisitic to lose the gain due to evicting workingset
2. let's detect memory pressure
3. current allocstall magic is not a good idea.
4. let's change the design from optimistic to conservative
5. how we can be conservative
6. two things - detect hot pages and threshold of swap pte
7. threhsold of swap pte is already done so remained thing is detect hot page
8. how to detect hot page - let's use young bit
9. Now, we are conservatie so we will swap in when it's worth
10. let's remove allocstall magic

I think it's not off-topic.
Anyway, it's just my thought and don't have any real workload and objection.
Feel free to ignore.

> -- 
> Michal Hocko
> SUSE Labs
> 
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