On 25/04/2024 07:20, Baolin Wang wrote: > > > On 2024/4/24 22:20, Ryan Roberts wrote: >> On 24/04/2024 14:49, Baolin Wang wrote: >>> >>> >>> On 2024/4/24 18:01, Ryan Roberts wrote: >>>> On 24/04/2024 10:55, Baolin Wang wrote: >>>>> >>>>> >>>>> On 2024/4/24 16:26, Ryan Roberts wrote: >>>>>> On 24/04/2024 07:55, Baolin Wang wrote: >>>>>>> >>>>>>> >>>>>>> On 2024/4/23 18:41, Ryan Roberts wrote: >>>>>>>> On 22/04/2024 08:02, Baolin Wang wrote: >>>>>>>>> Anonymous pages have already been supported for multi-size (mTHP) >>>>>>>>> allocation >>>>>>>>> through commit 19eaf44954df, that can allow THP to be configured >>>>>>>>> through the >>>>>>>>> sysfs interface located at >>>>>>>>> '/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled'. >>>>>>>>> >>>>>>>>> However, the anonymous shared pages will ignore the anonymous mTHP rule >>>>>>>>> configured through the sysfs interface, and can only use the PMD-mapped >>>>>>>>> THP, that is not reasonable. Many implement anonymous page sharing through >>>>>>>>> mmap(MAP_SHARED | MAP_ANONYMOUS), especially in database usage scenarios, >>>>>>>>> therefore, users expect to apply an unified mTHP strategy for anonymous >>>>>>>>> pages, >>>>>>>>> also including the anonymous shared pages, in order to enjoy the >>>>>>>>> benefits of >>>>>>>>> mTHP. For example, lower latency than PMD-mapped THP, smaller memory bloat >>>>>>>>> than PMD-mapped THP, contiguous PTEs on ARM architecture to reduce TLB >>>>>>>>> miss >>>>>>>>> etc. >>>>>>>> >>>>>>>> This sounds like a very useful addition! >>>>>>>> >>>>>>>> Out of interest, can you point me at any workloads (and off-the-shelf >>>>>>>> benchmarks >>>>>>>> for those workloads) that predominantly use shared anon memory? >>>>>>> >>>>>>> As far as I know, some database related workloads make extensive use of >>>>>>> shared >>>>>>> anonymous page, such as PolarDB[1] in our Alibaba fleet, or MySQL likely >>>>>>> also >>>>>>> uses shared anonymous memory. And I still need to do some investigation to >>>>>>> measure the performance. >>>>>>> >>>>>>> [1] https://github.com/ApsaraDB/PolarDB-for-PostgreSQL >>>>>> >>>>>> Thanks for the pointer! >>>>>> >>>>>>> >>>>>>>>> The primary strategy is that, the use of huge pages for anonymous shared >>>>>>>>> pages >>>>>>>>> still follows the global control determined by the mount option "huge=" >>>>>>>>> parameter >>>>>>>>> or the sysfs interface at >>>>>>>>> '/sys/kernel/mm/transparent_hugepage/shmem_enabled'. >>>>>>>>> The utilization of mTHP is allowed only when the global 'huge' switch is >>>>>>>>> enabled. >>>>>>>>> Subsequently, the mTHP sysfs interface >>>>>>>>> (/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled) >>>>>>>>> is checked to determine the mTHP size that can be used for large folio >>>>>>>>> allocation >>>>>>>>> for these anonymous shared pages. >>>>>>>> >>>>>>>> I'm not sure about this proposed control mechanism; won't it break >>>>>>>> compatibility? I could be wrong, but I don't think shmem's use of THP >>>>>>>> used to >>>>>>>> depend upon the value of /sys/kernel/mm/transparent_hugepage/enabled? So it >>>>>>> >>>>>>> Yes, I realized this after more testing. >>>>>>> >>>>>>>> doesn't make sense to me that we now depend upon the >>>>>>>> /sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled values (which by >>>>>>>> default disables all sizes except 2M, which is set to "inherit" from >>>>>>>> /sys/kernel/mm/transparent_hugepage/enabled). >>>>>>>> >>>>>>>> The other problem is that shmem_enabled has a different set of options >>>>>>>> (always/never/within_size/advise/deny/force) to enabled >>>>>>>> (always/madvise/never) >>>>>>>> >>>>>>>> Perhaps it would be cleaner to do the same trick we did for enabled; >>>>>>>> Introduce >>>>>>>> /mm/transparent_hugepage/hugepage-XXkb/shmem_enabled, which can have all >>>>>>>> the >>>>>>>> same values as the top-level >>>>>>>> /sys/kernel/mm/transparent_hugepage/shmem_enabled, >>>>>>>> plus the additional "inherit" option. By default all sizes will be set to >>>>>>>> "never" except 2M, which is set to "inherit". >>>>>>> >>>>>>> Sounds good to me. But I do not want to copy all same values from top-level >>>>>>> '/sys/kernel/mm/transparent_hugepage/shmem_enabled': >>>>>>> always within_size advise never deny force >>>>>>> >>>>>>> For mTHP's shmem_enabled interface, we can just keep below values: >>>>>>> always within_size advise never >>>>>>> >>>>>>> Cause when checking if mTHP can be used for anon shmem, 'deny' is equal to >>>>>>> 'never', and 'force' is equal to 'always'. >>>>>> >>>>>> I'll admit it wasn't completely clear to me after reading the docs, but my >>>>>> rough >>>>>> understanding is: >>>>>> >>>>>> - /sys/kernel/mm/transparent_hugepage/shmem_enabled controls >>>>>> mmap(SHARED|ANON) allocations (mostly; see rule 3) >>>>>> - huge=... controls tmpfs allocations >>>>>> - deny and force in shmem_enabled are equivalent to never and always for >>>>>> mmap(SHARED|ANON) but additionally override all tmpfs mounts so they >>>>>> act as >>>>>> if they were mounted with huge=never or huge=always >>>>>> >>>>>> Is that correct? If so, then I think it still makes sense to support per-size >>>>> >>>>> Correct. >>>>> >>>>>> deny/force. Certainly if a per-size control is set to "inherit" and the >>>>>> top-level control is set to deny or force, you would need that to mean >>>>>> something. >>>>> >>>>> IMHO, the '/mm/transparent_hugepage/hugepage-XXkb/shmem_enabled' interface >>>>> should only control the anonymous shmem. And 'huge=' controls tmpfs >>>>> allocation, >>>>> so we should not use anonymous control to override tmpfs control, which >>>>> seems a >>>>> little mess? >>>> >>>> I agree it would be cleaner to only handle mmap(SHARED|ANON) here, and leave >>>> the >>>> tmpfs stuff for another time. But my point is that >>>> /mm/transparent_hugepage/shmem_enabled already interferes with tmpfs if the >>>> value is deny or force. So if you have: >>>> >>>> echo deny > /mm/transparent_hugepage/shmem_enabled >>> >>> IIUC, this global control will cause shmem_is_huge() to always return false, so >>> no matter how '/mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled' is set, >>> anonymous shmem will not use mTHP. No? >> >> No, that's not how '/mm/transparent_hugepage/hugepage-xxxkB/enabled' works, and >> I think '/mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled' should follow >> the established pattern. >> >> For anon-private, each size is controlled by its >> /mm/transparent_hugepage/hugepage-xxxkB/enabled value. Unless that value is >> "inherit", in which case the value in /mm/transparent_hugepage/enabled is used >> for that size. >> >> That approach enables us to 1) maintain back-compat and 2) control each size >> independently >> >> 1) is met because the default is that all sizes are initially set to "never", >> except the PMD-size (e.g. /mm/transparent_hugepage/hugepage-2048kB/enabled) >> which is initially set to inherit. So any mTHP unaware SW can still modify >> /mm/transparent_hugepage/enabled and it will still only apply to PMD size. >> >> 2) is met because mTHP aware SW can come along and e.g. enable the 64K size >> (echo always > /mm/transparent_hugepage/hugepage-64kB/enabled) without having to >> modify the value in /mm/transparent_hugepage/enabled. > > Thanks for explanation. Initially, I want to make > ‘/mm/transparent_hugepage/shmem_enabled’ be a global control for huge page, but > I think it should follow the same strategy as anon mTHP as you said. > >>>> echo inherit > /mm/transparent_hugepage/hugepage-64kB/shmem_enabled >>>> >>>> What does that mean? >> >> So I think /mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled will need to >> support the deny and force values. When applied to non-PMD sizes, "deny" can >> just be a noop for now, because there was no way to configure a tmpfs mount for >> non-PMD size THP in the first place. But I'm not sure what to do with "force"? > > OK. And I also prefer that "force" should be a noop too, since anon shmem > control should not configure tmpfs huge page allocation. I guess technically they won't be noops, but (for the non-PMD-sizes) "force" will be an alias for "always" and "deny" will be an alias for "never"? I was just a bit concerned about later changing that behavior to also impact tmpfs once tmpfs supports mTHP; could that cause breaks? But thinking about it, I don't see that as a problem.
