Ryan Roberts <ryan.roberts@xxxxxxx> writes: > On 10/07/2023 04:03, Huang, Ying wrote: >> Ryan Roberts <ryan.roberts@xxxxxxx> writes: >> >>> On 07/07/2023 15:07, David Hildenbrand wrote: >>>> On 07.07.23 15:57, Matthew Wilcox wrote: >>>>> On Fri, Jul 07, 2023 at 01:29:02PM +0200, David Hildenbrand wrote: >>>>>> On 07.07.23 11:52, Ryan Roberts wrote: >>>>>>> On 07/07/2023 09:01, Huang, Ying wrote: >>>>>>>> Although we can use smaller page order for FLEXIBLE_THP, it's hard to >>>>>>>> avoid internal fragmentation completely. So, I think that finally we >>>>>>>> will need to provide a mechanism for the users to opt out, e.g., >>>>>>>> something like "always madvise never" via >>>>>>>> /sys/kernel/mm/transparent_hugepage/enabled. I'm not sure whether it's >>>>>>>> a good idea to reuse the existing interface of THP. >>>>>>> >>>>>>> I wouldn't want to tie this to the existing interface, simply because that >>>>>>> implies that we would want to follow the "always" and "madvise" advice too; >>>>>>> That >>>>>>> means that on a thp=madvise system (which is certainly the case for android and >>>>>>> other client systems) we would have to disable large anon folios for VMAs that >>>>>>> haven't explicitly opted in. That breaks the intention that this should be an >>>>>>> invisible performance boost. I think it's important to set the policy for >>>>>>> use of >>>>>> >>>>>> It will never ever be a completely invisible performance boost, just like >>>>>> ordinary THP. >>>>>> >>>>>> Using the exact same existing toggle is the right thing to do. If someone >>>>>> specify "never" or "madvise", then do exactly that. >>>>>> >>>>>> It might make sense to have more modes or additional toggles, but >>>>>> "madvise=never" means no memory waste. >>>>> >>>>> I hate the existing mechanisms. They are an abdication of our >>>>> responsibility, and an attempt to blame the user (be it the sysadmin >>>>> or the programmer) of our code for using it wrongly. We should not >>>>> replicate this mistake. >>>> >>>> I don't agree regarding the programmer responsibility. In some cases the >>>> programmer really doesn't want to get more memory populated than requested -- >>>> and knows exactly why setting MADV_NOHUGEPAGE is the right thing to do. >>>> >>>> Regarding the madvise=never/madvise/always (sys admin decision), memory waste >>>> (and nailing down bugs or working around them in customer setups) have been very >>>> good reasons to let the admin have a word. >>>> >>>>> >>>>> Our code should be auto-tuning. I posted a long, detailed outline here: >>>>> https://lore.kernel.org/linux-mm/Y%2FU8bQd15aUO97vS@xxxxxxxxxxxxxxxxxxxx/ >>>>> >>>> >>>> Well, "auto-tuning" also should be perfect for everybody, but once reality >>>> strikes you know it isn't. >>>> >>>> If people don't feel like using THP, let them have a word. The "madvise" config >>>> option is probably more controversial. But the "always vs. never" absolutely >>>> makes sense to me. >>>> >>>>>> I remember I raised it already in the past, but you *absolutely* have to >>>>>> respect the MADV_NOHUGEPAGE flag. There is user space out there (for >>>>>> example, userfaultfd) that doesn't want the kernel to populate any >>>>>> additional page tables. So if you have to respect that already, then also >>>>>> respect MADV_HUGEPAGE, simple. >>>>> >>>>> Possibly having uffd enabled on a VMA should disable using large folios, >>>> >>>> There are cases where we enable uffd *after* already touching memory (postcopy >>>> live migration in QEMU being the famous example). That doesn't fly. >>>> >>>>> I can get behind that. But the notion that userspace knows what it's >>>>> doing ... hahaha. Just ignore the madvise flags. Userspace doesn't >>>>> know what it's doing. >>>> >>>> If user space sets MADV_NOHUGEPAGE, it exactly knows what it is doing ... in >>>> some cases. And these include cases I care about messing with sparse VM memory :) >>>> >>>> I have strong opinions against populating more than required when user space set >>>> MADV_NOHUGEPAGE. >>> >>> I can see your point about honouring MADV_NOHUGEPAGE, so think that it is >>> reasonable to fallback to allocating an order-0 page in a VMA that has it set. >>> The app has gone out of its way to explicitly set it, after all. >>> >>> I think the correct behaviour for the global thp controls (cmdline and sysfs) >>> are less obvious though. I could get on board with disabling large anon folios >>> globally when thp="never". But for other situations, I would prefer to keep >>> large anon folios enabled (treat "madvise" as "always"), >> >> If we have some mechanism to auto-tune the large folios usage, for >> example, detect the internal fragmentation and split the large folio, >> then we can use thp="always" as default configuration. If my memory >> were correct, this is what Johannes and Alexander is working on. > > Could you point me to that work? I'd like to understand what the mechanism is. > The other half of my work aims to use arm64's pte "contiguous bit" to tell the > HW that a span of PTEs share the same mapping and is therefore coalesced into a > single TLB entry. The side effect of this, however, is that we only have a > single access and dirty bit for the whole contpte extent. So I'd like to avoid > any mechanism that relies on getting access/dirty at the base page granularity > for a large folio. Please take a look at the THP shrinker patchset, https://lore.kernel.org/linux-mm/cover.1667454613.git.alexlzhu@xxxxxx/ >> >>> with the argument that >>> their order is much smaller than traditional THP and therefore the internal >>> fragmentation is significantly reduced. >> >> Do you have any data for this? > > Some; its partly based on intuition that the smaller the allocation unit, the > smaller the internal fragmentation. And partly on peak memory usage data I've > collected for the benchmarks I'm running, comparing baseline-4k kernel with > baseline-16k and baseline-64 kernels along with a 4k kernel that supports large > anon folios (I appreciate that's not exactly what we are talking about here, and > it's not exactly an extensive set of results!): > > > Kernel Compliation with 8 Jobs: > | kernel | peak | > |:--------------|-------:| > | baseline-4k | 0.0% | > | anonfolio | 0.1% | > | baseline-16k | 6.3% | > | baseline-64k | 28.1% | > > > Kernel Compliation with 80 Jobs: > | kernel | peak | > |:--------------|-------:| > | baseline-4k | 0.0% | > | anonfolio | 1.7% | > | baseline-16k | 2.6% | > | baseline-64k | 12.3% | > Why is anonfolio better than baseline-64k if you always allocate 64k anonymous folio? Because page cache uses 64k in baseline-64k? We may need to test some workloads with sparse access patterns too. Best Regards, Huang, Ying >> >>> I really don't want to end up with user >>> space ever having to opt-in (with MADV_HUGEPAGE) to see the benefits of large >>> anon folios. >>> >>> I still feel that it would be better for the thp and large anon folio controls >>> to be independent though - what's the argument for tying them together? >>>