Message-ID: <CAA1CXcCMPFqiiTi7hfVhhEvHs4Mddiktvpmb7dMe4coLDF0bgg@mail.gmail.com>
Date: Wed, 3 Sep 2025 20:54:39 -0600
From: Nico Pache <npache@...hat.com>
To: Usama Arif <usamaarif642@...il.com>
Cc: David Hildenbrand <david@...hat.com>, Baolin Wang <baolin.wang@...ux.alibaba.com>, 
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Subject: Re: [PATCH v10 00/13] khugepaged: mTHP support

On Tue, Sep 2, 2025 at 2:23 PM Usama Arif <usamaarif642@...il.com> wrote:
>
>
>
> On 02/09/2025 12:03, David Hildenbrand wrote:
> > On 02.09.25 12:34, Usama Arif wrote:
> >>
> >>
> >> On 02/09/2025 10:03, David Hildenbrand wrote:
> >>> On 02.09.25 04:28, Baolin Wang wrote:
> >>>>
> >>>>
> >>>> On 2025/9/2 00:46, David Hildenbrand wrote:
> >>>>> On 29.08.25 03:55, Baolin Wang wrote:
> >>>>>>
> >>>>>>
> >>>>>> On 2025/8/28 18:48, Dev Jain wrote:
> >>>>>>>
> >>>>>>> On 28/08/25 3:16 pm, Baolin Wang wrote:
> >>>>>>>> (Sorry for chiming in late)
> >>>>>>>>
> >>>>>>>> On 2025/8/22 22:10, David Hildenbrand wrote:
> >>>>>>>>>>> Once could also easily support the value 255 (HPAGE_PMD_NR / 2- 1),
> >>>>>>>>>>> but not sure
> >>>>>>>>>>> if we have to add that for now.
> >>>>>>>>>>
> >>>>>>>>>> Yeah not so sure about this, this is a 'just have to know' too, and
> >>>>>>>>>> yes you
> >>>>>>>>>> might add it to the docs, but people are going to be mightily
> >>>>>>>>>> confused, esp if
> >>>>>>>>>> it's a calculated value.
> >>>>>>>>>>
> >>>>>>>>>> I don't see any other way around having a separate tunable if we
> >>>>>>>>>> don't just have
> >>>>>>>>>> something VERY simple like on/off.
> >>>>>>>>>
> >>>>>>>>> Yeah, not advocating that we add support for other values than 0/511,
> >>>>>>>>> really.
> >>>>>>>>>
> >>>>>>>>>>
> >>>>>>>>>> Also the mentioned issue sounds like something that needs to be
> >>>>>>>>>> fixed elsewhere
> >>>>>>>>>> honestly in the algorithm used to figure out mTHP ranges (I may be
> >>>>>>>>>> wrong - and
> >>>>>>>>>> happy to stand corrected if this is somehow inherent, but reallly
> >>>>>>>>>> feels that
> >>>>>>>>>> way).
> >>>>>>>>>
> >>>>>>>>> I think the creep is unavoidable for certain values.
> >>>>>>>>>
> >>>>>>>>> If you have the first two pages of a PMD area populated, and you
> >>>>>>>>> allow for at least half of the #PTEs to be non/zero, you'd collapse
> >>>>>>>>> first a
> >>>>>>>>> order-2 folio, then and order-3 ... until you reached PMD order.
> >>>>>>>>>
> >>>>>>>>> So for now we really should just support 0 / 511 to say "don't
> >>>>>>>>> collapse if there are holes" vs. "always collapse if there is at
> >>>>>>>>> least one pte used".
> >>>>>>>>
> >>>>>>>> If we only allow setting 0 or 511, as Nico mentioned before, "At 511,
> >>>>>>>> no mTHP collapses would ever occur anyway, unless you have 2MB
> >>>>>>>> disabled and other mTHP sizes enabled. Technically, at 511, only the
> >>>>>>>> highest enabled order would ever be collapsed."
> >>>>>>> I didn't understand this statement. At 511, mTHP collapses will occur if
> >>>>>>> khugepaged cannot get a PMD folio. Our goal is to collapse to the
> >>>>>>> highest order folio.
> >>>>>>
> >>>>>> Yes, I’m not saying that it’s incorrect behavior when set to 511. What I
> >>>>>> mean is, as in the example I gave below, users may only want to allow a
> >>>>>> large order collapse when the number of present PTEs reaches half of the
> >>>>>> large folio, in order to avoid RSS bloat.
> >>>>>
> >>>>> How do these users control allocation at fault time where this parameter
> >>>>> is completely ignored?
> >>>>
> >>>> Sorry, I did not get your point. Why does the 'max_pte_none' need to
> >>>> control allocation at fault time? Could you be more specific? Thanks.
> >>>
> >>> The comment over khugepaged_max_ptes_none gives a hint:
> >>>
> >>> /*
> >>>   * default collapse hugepages if there is at least one pte mapped like
> >>>   * it would have happened if the vma was large enough during page
> >>>   * fault.
> >>>   *
> >>>   * Note that these are only respected if collapse was initiated by khugepaged.
> >>>   */
> >>>
> >>> In the common case (for anything that really cares about RSS bloat) you will just a
> >>> get a THP during page fault and consequently RSS bloat.
> >>>
> >>> As raised in my other reply, the only documented reason to set max_ptes_none=0 seems
> >>> to be when an application later (after once possibly getting a THP already during
> >>> page faults) did some MADV_DONTNEED and wants to control the usage of THPs itself using
> >>> MADV_COLLAPSE.
> >>>
> >>> It's a questionable use case, that already got more problematic with mTHP and page
> >>> table reclaim.
> >>>
> >>> Let me explain:
> >>>
> >>> Before mTHP, if someone would MADV_DONTNEED (resulting in
> >>> a page table with at least one pte_none entry), there would have been no way we would
> >>> get memory over-allocated afterwards with max_ptes_none=0.
> >>>
> >>> (1) Page faults would spot "there is a page table" and just fallback to order-0 pages.
> >>> (2) khugepaged was told to not collapse through max_ptes_none=0.
> >>>
> >>> But now:
> >>>
> >>> (A) With mTHP during page-faults, we can just end up over-allocating memory in such
> >>>      an area again: page faults will simply spot a bunch of pte_nones around the fault area
> >>>      and install an mTHP.
> >>>
> >>> (B) With page table reclaim (when zapping all PTEs in a table at once), we will reclaim the
> >>>      page table. The next page fault will just try installing a PMD THP again, because there is
> >>>      no PTE table anymore.
> >>>
> >>> So I question the utility of max_ptes_none. If you can't tame page faults, then there is only
> >>> limited sense in taming khugepaged. I think there is vale in setting max_ptes_none=0 for some
> >>> corner cases, but I am yet to learn why max_ptes_none=123 would make any sense.
> >>>
> >>>
> >>
> >> For PMD mapped THPs with THP shrinker, this has changed. You can basically tame pagefaults, as when you encounter
> >> memory pressure, the shrinker kicks in if the value is less than HPAGE_PMD_NR -1 (i.e. 511 for x86), and
> >> will break down those hugepages and free up zero-filled memory.
> >
> > You are not really taming page faults, though, you are undoing what page faults might have messed up :)
> >
> > I have seen in our prod workloads where
> >> the memory usage and THP usage can spike (usually when the workload starts), but with memory pressure,
> >> the memory usage is lower compared to with max_ptes_none = 511, while still still keeping the benefits
> >> of THPs like lower TLB misses.
> >
> > Thanks for raising that: I think the current behavior is in place such that you don't bounce back-and-forth between khugepaged collapse and shrinker-split.
> >
>
> Yes, both collapse and shrinker split hinge on max_ptes_none to prevent one of these things thrashing the effect of the other.
I believe with mTHP support in khugepaged, the max_ptes_none value in
the shrinker must also leverage the 'order' scaling to properly
prevent thrashing.
I've been testing a patch for this that I might include in the V11.
>
> > There are likely other ways to achieve that, when we have in mind that the thp shrinker will install zero pages and max_ptes_none includes
> > zero pages.
> >
> >>
> >> I do agree that the value of max_ptes_none is magical and different workloads can react very differently
> >> to it. The relationship is definitely not linear. i.e. if I use max_ptes_none = 256, it does not mean
> >> that the memory regression of using THP=always vs THP=madvise is halved.
> >
> > To which value would you set it? Just 510? 0?
> >
>
> There are some very large workloads in the meta fleet that I experimented with and found that having
> a small value works out. I experimented with 0, 51 (10%) and 256 (50%). 51 was found to be an optimal
> comprimise in terms of application metrics improving, having an acceptable amount of memory regression and
> improved system level metrics (lower TLB misses, lower page faults). I am sure there was a better value out
> there for these workloads, but not possible to experiment with every value.
>
> In terms of wider rollout across the fleet, we are going to target 0 (or a very very small value)
> when moving from THP=madvise to always. Mainly because it is the least likely to cause a memory regression as
> THP shrinker will deal with page faults faulting in mostly zero-filled pages and khugepaged wont collapse
> pages that are dominated by 4K zero-filled chunks.
>

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