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

[Prarit Bhargava]

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