Re: [PATCH 1/1] mm/khugepaged: reduce process visible downtime by pre-zeroing hugepage

[Lance Yang]

Hey David,

Thanks for taking time to review!

On Tue, Mar 12, 2024 at 12:19 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 08.03.24 08:49, Lance Yang wrote:
> > The patch reduces the process visible downtime during hugepage
> > collapse. This is achieved by pre-zeroing the hugepage before
> > acquiring mmap_lock(write mode) if nr_pte_none >= 256, without
> > affecting the efficiency of khugepaged.
> >
> > On an Intel Core i5 CPU, the process visible downtime during
> > hugepage collapse is as follows:
> >
> > | nr_ptes_none  | w/o __GFP_ZERO | w/ __GFP_ZERO  |  Change |
> > --------------------------------------------------—----------
> > |      511      |     233us      |      95us      |  -59.21%|
> > |      384      |     376us      |     219us      |  -41.20%|
> > |      256      |     421us      |     323us      |  -23.28%|
> > |      128      |     523us      |     507us      |   -3.06%|
> >
> > Of course, alloc_charge_hpage() will take longer to run with
> > the __GFP_ZERO flag.
> >
> > |       Func           | w/o __GFP_ZERO | w/ __GFP_ZERO |
> > |----------------------|----------------|---------------|
> > | alloc_charge_hpage   |      198us     |      295us    |
> >
> > But it's not a big deal because it doesn't impact the total
> > time spent by khugepaged in collapsing a hugepage. In fact,
> > it would decrease.
>
> It does look sane to me and not overly complicated.
>
> But, it's an optimization really only when we have quite a bunch of
> pte_none(), possibly repeatedly so that it really makes a difference.
>
> Usually, when we repeatedly collapse that many pte_none() we're just
> wasting a lot of memory and should re-evaluate life choices :)

Agreed! It seems that the default value of max_pte_none may be set too
high, which could result in the memory wastage issue we're discussing.

>
> So my question is: do we really care about it that much that we care to
> optimize?

IMO, although it may not be our main concern, reducing the impact of
khugepaged on the process remains crucial. I think that users also prefer
minimal interference from khugepaged.

>
> But again, LGTM.

Thanks again for your time!

Best,
Lance
>
> --
> Cheers,
>
> David / dhildenb
>