On 10/07/2023 10:18, Huang, Ying wrote: > 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/ Thanks! > >>> >>>> 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? No, because the VMA boundaries are aligned to 4K and not 64K. Large Anon Folios only allocates a 64K folio if it does not breach the bounds of the VMA (and if it doesn't overlap other allocated PTEs). > > We may need to test some workloads with sparse access patterns too. Yes, I agree if you have a workload with a pathalogical memory access pattern where it writes to addresses with a stride of 64K, all contained in a single VMA, then you will end up allocating 16x the memory. This is obviously an unrealistic extreme though. > > 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? >>>> >
