Re: futex2 numa stuff

[Davidlohr Bueso]

On Fri, 22 Sep 2023, Peter Zijlstra wrote:

> Hi!
>
> Updated version of patch 15/15 and a few extra patches for testing the
> FUTEX2_NUMA bits. The last patch (17/15) should never be applied for anything
> you care about and exists purely because I'm too lazy to generate actual
> hash-bucket contention.
>
> On my 2 node IVB-EP:
>
> $ echo FUTEX_SQUASH > /debug/sched/features
>
> Effectively reducing each node to 1 bucket.
>
> $ numactl -m0 -N0 ./futex_numa -c10 -t2 -n0 -N0 &
>   numactl -m1 -N1 ./futex_numa -c10 -t2 -n0 -N0
>
> ...
> contenders: 16154935
> contenders: 16202472
>
> $ numactl -m0 -N0 ./futex_numa -c10 -t2 -n0 -N0 &
>   numactl -m1 -N1 ./futex_numa -c10 -t2 -n0 -N1
>
> contenders: 48584991
> contenders: 48680560
>
> (loop counts, higher is better)
>
> Clearly showing how separating the hashes works.
>
> The first one runs 10 contenders on each node but forces the (numa) futex to
> hash to node 0 for both. This ensures all 20 contenders hash to the same
> bucket and *ouch*.
>
> The second one does the same, except now fully separates the nodes. Performance
> is much improved.
>
> Proving the per-node hashing actually works as advertised.

Very nice.