Re: [PATCH v10 20/21] futex: Implement FUTEX2_NUMA

[Shrikanth Hegde]

On 3/26/25 04:26, Peter Zijlstra wrote:
> On Wed, Mar 26, 2025 at 01:22:19AM +0530, Shrikanth Hegde wrote:
>
> > > +	return &futex_queues[node][hash & futex_hashmask];
>
>                              ^^^^^^^
>
> > > +	hashsize = 256 * num_possible_cpus();
> > > +	hashsize /= num_possible_nodes();
> >
> > Wouldn't it be better to use num_online_nodes? each node may get a bigger
> > hash bucket which means less collision no?
>
> No. There are two problems with num_online_nodes, and both are evident
> above.
>
> Consider the case of a sparse set.

I am sorry, i didn't understand. Could you please explain?