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

[Peter Zijlstra]

On Wed, Mar 26, 2025 at 06:27:20PM +0530, Shrikanth Hegde wrote:
> 
> 
> 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?

I was confused; I should've just gone sleep :-)

The futex_queues[] array is sized MAX_NUMNODES, such that every possible
node_id has a spot. I thought we did dynamic sizing, but not so.

Anyway, using online here would lead to having to deal with hotplug,
which in turn either leads to more over-all hash buckets in the system,
or having to resize and rehash everything.

Neither are really attractive options.