On 05/09/2025 12:55, David Hildenbrand wrote: > 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@xxxxxxxxx> 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. > tbh the default value of 511 is horrible. I have thought about sending a patch to change it to 0 as default in upstream for sometime, but it might mean that people who upgrade their kernel might suddenly see their memory not getting hugified and it could be confusing for them?
