Re: futex2 numa stuff

Davidlohr Bueso
2023-09-28T13:41:11Z

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.