On 09/06/2025 21:24, Zi Yan wrote: > On 9 Jun 2025, at 16:03, Usama Arif wrote: > >> On 09/06/2025 20:49, Zi Yan wrote: >>> On 9 Jun 2025, at 15:40, Lorenzo Stoakes wrote: >>> >>>> On Mon, Jun 09, 2025 at 11:20:04AM -0400, Zi Yan wrote: >>>>> On 9 Jun 2025, at 10:50, Lorenzo Stoakes wrote: >>>>> >>>>>> On Mon, Jun 09, 2025 at 10:37:26AM -0400, Zi Yan wrote: >>>>>>> On 9 Jun 2025, at 10:16, Lorenzo Stoakes wrote: >>>>>>> >>>>>>>> On Mon, Jun 09, 2025 at 03:11:27PM +0100, Usama Arif wrote: >>>>>> >>>>>> [snip] >>>>>> >>>>>>>>> So I guess the question is what should be the next step? The following has been discussed: >>>>>>>>> >>>>>>>>> - Changing pageblock_order at runtime: This seems unreasonable after Zi's explanation above >>>>>>>>> and might have unintended consequences if done at runtime, so a no go? >>>>>>>>> - Decouple only watermark calculation and defrag granularity from pageblock order (also from Zi). >>>>>>>>> The decoupling can be done separately. Watermark calculation can be decoupled using the >>>>>>>>> approach taken in this RFC. Although max order used by pagecache needs to be addressed. >>>>>>>>> >>>>>>>> >>>>>>>> I need to catch up with the thread (workload crazy atm), but why isn't it >>>>>>>> feasible to simply statically adjust the pageblock size? >>>>>>>> >>>>>>>> The whole point of 'defragmentation' is to _heuristically_ make it less >>>>>>>> likely there'll be fragmentation when requesting page blocks. >>>>>>>> >>>>>>>> And the watermark code is explicitly about providing reserves at a >>>>>>>> _pageblock granularity_. >>>>>>>> >>>>>>>> Why would we want to 'defragment' to 512MB physically contiguous chunks >>>>>>>> that we rarely use? >>>>>>>> >>>>>>>> Since it's all heuristic, it seems reasonable to me to cap it at a sensible >>>>>>>> level no? >>>>>>> >>>>>>> What is a sensible level? 2MB is a good starting point. If we cap pageblock >>>>>>> at 2MB, everyone should be happy at the moment. But if one user wants to >>>>>>> allocate 4MB mTHP, they will most likely fail miserably, because pageblock >>>>>>> is 2MB, kernel is OK to have a 2MB MIGRATE_MOVABLE pageblock next to a 2MB >>>>>>> MGIRATE_UNMOVABLE one, making defragmenting 4MB an impossible job. >>>>>>> >>>>>>> Defragmentation has two components: 1) pageblock, which has migratetypes >>>>>>> to prevent mixing movable and unmovable pages, as a single unmovable page >>>>>>> blocks large free pages from being created; 2) memory compaction granularity, >>>>>>> which is the actual work to move pages around and form a large free pages. >>>>>>> Currently, kernel assumes pageblock size = defragmentation granularity, >>>>>>> but in reality, as long as pageblock size >= defragmentation granularity, >>>>>>> memory compaction would still work, but not the other way around. So we >>>>>>> need to choose pageblock size carefully to not break memory compaction. >>>>>> >>>>>> OK I get it - the issue is that compaction itself operations at a pageblock >>>>>> granularity, and once you get so fragmented that compaction is critical to >>>>>> defragmentation, you are stuck if the pageblock is not big enough. >>>>> >>>>> Right. >>>>> >>>>>> >>>>>> Thing is, 512MB pageblock size for compaction seems insanely inefficient in >>>>>> itself, and if we're complaining about issues with unavailable reserved >>>>>> memory due to crazy PMD size, surely one will encounter the compaction >>>>>> process simply failing to succeed/taking forever/causing issues with >>>>>> reclaim/higher order folio allocation. >>>>> >>>>> Yep. Initially, we probably never thought PMD THP would be as large as >>>>> 512MB. >>>> >>>> Of course, such is the 'organic' nature of kernel development :) >>>> >>>>> >>>>>> >>>>>> I mean, I don't really know the compaction code _at all_ (ran out of time >>>>>> to cover in book ;), but is it all or-nothing? Does it grab a pageblock or >>>>>> gives up? >>>>> >>>>> compaction works on one pageblock at a time, trying to migrate in-use pages >>>>> within the pageblock away to create a free page for THP allocation. >>>>> It assumes PMD THP size is equal to pageblock size. It will keep working >>>>> until a PMD THP size free page is created. This is a very high level >>>>> description, omitting a lot of details like how to avoid excessive compaction >>>>> work, how to reduce compaction latency. >>>> >>>> Yeah this matches my assumptions. >>>> >>>>> >>>>>> >>>>>> Because it strikes me that a crazy pageblock size would cause really >>>>>> serious system issues on that basis alone if that's the case. >>>>>> >>>>>> And again this leads me back to thinking it should just be the page block >>>>>> size _as a whole_ that should be adjusted. >>>>>> >>>>>> Keep in mind a user can literally reduce the page block size already via >>>>>> CONFIG_PAGE_BLOCK_MAX_ORDER. >>>>>> >>>>>> To me it seems that we should cap it at the highest _reasonable_ mTHP size >>>>>> you can get on a 64KB (i.e. maximum right? RIGHT? :P) base page size >>>>>> system. >>>>>> >>>>>> That way, people _can still get_ super huge PMD sized huge folios up to the >>>>>> point of fragmentation. >>>>>> >>>>>> If we do reduce things this way we should give a config option to allow >>>>>> users who truly want collosal PMD sizes with associated >>>>>> watermarks/compaction to be able to still have it. >>>>>> >>>>>> CONFIG_PAGE_BLOCK_HARD_LIMIT_MB or something? >>>>> >>>>> I agree with capping pageblock size at a highest reasonable mTHP size. >>>>> In case there is some user relying on this huge PMD THP, making >>>>> pageblock a boot time variable might be a little better, since >>>>> they do not need to recompile the kernel for their need, assuming >>>>> distros will pick something like 2MB as the default pageblock size. >>>> >>>> Right, this seems sensible, as long as we set a _default_ that limits to >>>> whatever it would be, 2MB or such. >>>> >>>> I don't think it's unreasonable to make that change since this 512 MB thing >>>> is so entirely unexpected and unusual. >>>> >>>> I think Usama said it would be a pain it working this way if it had to be >>>> explicitly set as a boot time variable without defaulting like this. >>>> >>>>> >>>>>> >>>>>> I also question this de-coupling in general (I may be missing somethig >>>>>> however!) - the watermark code _very explicitly_ refers to providing >>>>>> _pageblocks_ in order to ensure _defragmentation_ right? >>>>> >>>>> Yes. Since without enough free memory (bigger than a PMD THP), >>>>> memory compaction will just do useless work. >>>> >>>> Yeah right, so this is a key thing and why we need to rework the current >>>> state of the patch. >>>> >>>>> >>>>>> >>>>>> We would need to absolutely justify why it's suddenly ok to not provide >>>>>> page blocks here. >>>>>> >>>>>> This is very very delicate code we have to be SO careful about. >>>>>> >>>>>> This is why I am being cautious here :) >>>>> >>>>> Understood. In theory, we can associate watermarks with THP allowed orders >>>>> the other way around too, meaning if user lowers vm.min_free_kbytes, >>>>> all THP/mTHP sizes bigger than the watermark threshold are disabled >>>>> automatically. This could fix the memory compaction issues, but >>>>> that might also drive user crazy as they cannot use the THP sizes >>>>> they want. >>>> >>>> Yeah that's interesting but I think that's just far too subtle and people will >>>> have no idea what's going on. >>>> >>>> I really think a hard cap, expressed in KB/MB, on pageblock size is the way to >>>> go (but overrideable for people crazy enough to truly want 512 MB pages - and >>>> who cannot then complain about watermarks). >>> >>> I agree. Basically, I am thinking: >>> 1) use something like 2MB as default pageblock size for all arch (the value can >>> be set differently if some arch wants a different pageblock size due to other reasons), this can be done by modifying PAGE_BLOCK_MAX_ORDER’s default >>> value; >>> >>> 2) make pageblock_order a boot time parameter, so that user who wants >>> 512MB pages can still get it by changing pageblock order at boot time. >>> >>> WDYT? >>> >> >> I was really hoping we would come up with a dynamic way of doing this, >> especially one that doesn't require any more input from the user apart >> from just setting the mTHP size via sysfs.. > > Then we will need to get rid of pageblock size from both watermark calculation > and memory compaction and think about a new anti-fragmentation mechanism > to handle unmovable pages as current pageblock based mechanism no longer > fit the need. > > What you are expecting is: > 1) watermarks should change as the largest enabled THP/mTHP size changes; > 2) memory compaction targets the largest enabled THP/mTHP size (next step > would improve memory compaction to optimize for all enabled sizes); > 3) partitions of movable and unmovable pages can change dynamically > based on the largest enabled THP/mTHP size; > 4) pageblock size becomes irrelevant. > I think both 1 and 2 can be achieved in a similar way? i.e. changing pageblock_order to be min(largest_enabled_thp_order(), PAGE_BLOCK_MAX_ORDER). But there a lot of instances of pageblock_order and pageblock_nr_pages and all of them would need to be audited very carefully. For 3, we need to do the dynamic array resizing that you mentioned for pageblock_flags? Yeah overall it sounds like quite a big change and would need a lot of testing to make sure nothing breaks. >> >> 1) in a way is already done. We can set it to 2M by setting >> ARCH_FORCE_MAX_ORDER to 5: >> >> In arch/arm64/Kconfig we already have: >> >> config ARCH_FORCE_MAX_ORDER >> int >> default "13" if ARM64_64K_PAGES >> default "11" if ARM64_16K_PAGES >> default "10" > > Nah, that means user no longer can allocate pages larger than 2MB, > because the cap is in the buddy allocator. > >> >> Doing 2) would require reboot and doing this just for changing mTHP size >> will probably be a nightmare for workload orchestration. > > No. That is not what I mean. pageblock_order set at boot time only limits > the largest mTHP size. By default, user can get up to 2MB THP/mTHP, > but if they want to get 512MB THP, they can reboot with a larger pageblock > order and they can still use 2MB mTHP. The downside is that with > larger pageblock order, user cannot get the optimal THP/mTHP performance > kernel is designed to achieve. > Yes, I mean this as well. If the largest mTHP size enabled goes from 2M to 512M than we need a reboot to actually obtain 512M THPs. If we switch from 512M to 2M, we again need a reboot to get the best performance out of the server. > Best Regards, > Yan, Zi
