BEAM’s :atomics and :counters are off-heap, shared integer arrays that outperform ETS for high-concurrency counting workloads.
Key Takeaways
:atomics provides CAS, exchange, and atomic add-get mapped to CPU instructions with sequential-consistency guarantees across array cells.
:counters in [:write_concurrency] mode shards one logical integer across per-scheduler slots, eliminating write contention at the cost of eventually-consistent reads.
Benchmarks on Apple M4 Pro (14 cores, OTP 28, JIT) show :counters throughput scales linearly up to core count; :atomics holds steady; ETS degrades past core count.
Broadway’s rate-limiter uses :atomics in production, a concrete real-world anchor for the atomic primitives.
:counters.put/3 is expensive: it resets every scheduler’s slot to 0 then writes the new value to one, making bulk sets rare-use operations.
