[PATCH v9 05/14] mm: multi-gen LRU: groundwork

Prarit Bhargava prarit at redhat.com
Mon Mar 21 19:17:54 UTC 2022

On 3/21/22 14:58, Justin Forbes wrote:
> On Mon, Mar 14, 2022 at 4:30 AM Yu Zhao <yuzhao at google.com> wrote:
>> On Mon, Mar 14, 2022 at 2:09 AM Huang, Ying <ying.huang at intel.com> wrote:
>>> Hi, Yu,
>>> Yu Zhao <yuzhao at 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,
> 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.


> Justin
>> [1] https://lore.kernel.org/lkml/20131106055634.GA24044@gmail.com/
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