While uncontended atomic operations are a lot faster than contended ones, it's hard to avoid the contended case. If you pass a single piece of read-only data th

While uncontended atomic operations are a lot faster than contended ones, it's hard to avoid the contended case. If you pass a single piece of read-only data throughout your program you will be adjusting its reference count frequently, and those contended atomics will dominate your performance profile. In a concurrent card-marking write barrier you would generally set bytes rather than individual bits so that you can avoid the use of atomic instructions. You still have the problem of false sharing. If this were really a problem, you could emit a conditional write barrier that checks the existing value (with a racy unsynchronized load) before setting the byte. The usual use of a write barrier for generational GC will produce fewer stores than automatic reference counting anyways, because a write barrier is only emitted for potential stores of a reference to a younger generation in an older generation. Automatic reference counting requires reference count adjustments for function parameters and local variables.