Re: [RFC PATCH] futex: Dynamically allocate futex_queues depending on nr_node_ids

[Sebastian Andrzej Siewior]

On 2026-02-25 09:06:08 [+0530], K Prateek Nayak wrote:
> Hello Sebastian,
Hi Prateek,

> On 2/24/2026 4:43 PM, Sebastian Andrzej Siewior wrote:
> > My question was initially is 1024 for max-nodes something that people
> > really use. It was introduced as of
> > 	https://lore.kernel.org/all/alpine.DEB.2.00.1003101537330.30724@xxxxxxxxxxxxxxxxxxxxxxxxx/
> > 
> > but it looks odd. It might just one or two machines which are left :)
> 
> I have it on good faith that some EPYC user on distro kernels turn on
> "L3 as NUMA" option which currently results into 32 NUMA nodes on our
> largest configuration.
> 
> Adding a little bit more margin for CXL nodes should make even
> CONFIG_NODES_SHIFT=6 pretty sane default for most real-word configs.
> I don't think we can go more than 10 or so CXL nodes considering the
> number of PCIe lanes unless there are more creative ways to attach
> tiered memory that appear as a NUMA node.
> 
> I'm not sure if Intel has similar crazy combination but NODES_SHIFT=6
> can accommodate (16 socket * SNC-3) + up to 16 CXL nodes so it should
> be fine for most distro users too?

Okay. According to Kconfig, this is the default for X86_64. The 10 gets
set by MAXSMP. This option raises the NR_CPUS_DEFAULT to 8192. That
might the overkill. What would be a sane value for NR_CPUS_DEFAULT? I
don't have anything that exceeds 3 digits but I also don't have anything
with more than 4 nodes ;)

Sebastian