Re: [PATCH v10 00/13] khugepaged: mTHP support

[David Hildenbrand <david@redhat.com>]

On 05.09.25 13:48, Lorenzo Stoakes wrote:
> On Wed, Sep 03, 2025 at 08:54:39PM -0600, Nico Pache wrote:
>> On Tue, Sep 2, 2025 at 2:23 PM Usama Arif <usamaarif642@gmail.com> wrote:
>>>>>> 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.
> 
> No please do not extend this 'scalling' stuff somewhere else, it's really horrid.
> 
> We have to find an alternative to that, it's extremely confusing in what is
> already extremely confusing THP code.
> 
> As I said before, if we can't have a boolean we need another interface, which
> makes most sense to be a ratio or in practice, a percentage sysctl.
> 
> Speaking with David off-list, maybe the answer - if we must have this - is to
> add a new percentage interface and have this in lock-step with the existing
> max_ptes_none interface. One updates the other, but internally we're just using
> the percentage value.

Yes, I'll try hacking something up and sending it as an RFC.

> 
>> 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?

Sorry, I missed Usama's reply. Thanks Usama!

>>>>
>>>
>>> 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.
> 
> (->Usama) It's a pity that such workloads exist. But then the percentage solution should work.

Good. So if there is no strong case for > 255, that's already valuable 
for mTHP.

-- 
Cheers

David / dhildenb