Messages in this thread | | | Date | Mon, 21 Mar 2022 15:17:54 -0400 | Subject | Re: [PATCH v9 05/14] mm: multi-gen LRU: groundwork | From | Prarit Bhargava <> |
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On 3/21/22 14:58, Justin Forbes wrote: > On Mon, Mar 14, 2022 at 4:30 AM Yu Zhao <yuzhao@google.com> wrote: >> >> On Mon, Mar 14, 2022 at 2:09 AM Huang, Ying <ying.huang@intel.com> wrote: >>> >>> Hi, Yu, >>> >>> Yu Zhao <yuzhao@google.com> writes: >>>> diff --git a/mm/Kconfig b/mm/Kconfig >>>> index 3326ee3903f3..747ab1690bcf 100644 >>>> --- a/mm/Kconfig >>>> +++ b/mm/Kconfig >>>> @@ -892,6 +892,16 @@ config ANON_VMA_NAME >>>> area from being merged with adjacent virtual memory areas due to the >>>> difference in their name. >>>> >>>> +# the multi-gen LRU { >>>> +config LRU_GEN >>>> + bool "Multi-Gen LRU" >>>> + depends on MMU >>>> + # the following options can use up the spare bits in page flags >>>> + depends on !MAXSMP && (64BIT || !SPARSEMEM || SPARSEMEM_VMEMMAP) >>> >>> LRU_GEN depends on !MAXSMP. So, What is the maximum NR_CPUS supported >>> by LRU_GEN? >> >> LRU_GEN doesn't really care about NR_CPUS. IOW, it doesn't impose a >> max number. The dependency is with NODES_SHIFT selected by MAXSMP: >> default "10" if MAXSMP >> This combined with LAST_CPUPID_SHIFT can exhaust the spare bits in page flags. >> >> MAXSMP is meant for kernel developers to test their code, and it >> should not be used in production [1]. But some distros unfortunately >> ship kernels built with this option, e.g., Fedora and Ubuntu. And >> their users reported build errors to me after they applied MGLRU on >> those kernels ("Not enough bits in page flags"). Let me add Fedora and >> Ubuntu to this thread. >> >> Fedora and Ubuntu, >> >> Could you please clarify if there is a reason to ship kernels built >> with MAXSMP? Otherwise, please consider disabling this option. Thanks. >> >> As per above, MAXSMP enables ridiculously large numbers of CPUs and >> NUMA nodes for testing purposes. It is detrimental to performance, >> e.g., CPUMASK_OFFSTACK. > > It was enabled for Fedora, and RHEL because we did need more than 512 > CPUs, originally only in RHEL until SGI (years ago) complained that > they were testing very large machines with Fedora. The testing done > on RHEL showed that the performance impact was minimal. For a very > long time we had MAXSMP off and carried a patch which allowed us to > turn on CPUMASK_OFFSTACK without debugging because there was supposed > to be "something else" coming. In 2019 we gave up, dropped that patch > and just turned on MAXSMP. > > I do not have any metrics for how often someone runs Fedora on a > ridiculously large machine these days, but I would guess that number > is not 0.
It is not 0. I've seen data from large systems (1000+ logical threads) that are running Fedora albeit with a modified Fedora kernel.
Additionally the max limit for CPUS in RHEL is 1792, however, we have recently had a request to *double* that to 3584. You should just assume that number will continue to increase.
P.
> > Justin > >> [1] https://lore.kernel.org/lkml/20131106055634.GA24044@gmail.com/ >> > _______________________________________________ > kernel mailing list -- kernel@lists.fedoraproject.org > To unsubscribe send an email to kernel-leave@lists.fedoraproject.org > Fedora Code of Conduct: https://docs.fedoraproject.org/en-US/project/code-of-conduct/ > List Guidelines: https://fedoraproject.org/wiki/Mailing_list_guidelines > List Archives: https://lists.fedoraproject.org/archives/list/kernel@lists.fedoraproject.org > Do not reply to spam on the list, report it: https://pagure.io/fedora-infrastructure
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