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"Barriers are slow" in what way? Azul was hitting 10-20ms pauses while successfully garbage collecting 500GB+ heaps using hardware memory barriers on their Vega architecture, which had up to 768 CPUs.

Cliff Click, Azul's principal JVM architect, seemed somewhat ambivalent about how much hardware transactional memory actually helped them hit those targets (i.e. it's not a silver bullet), but he chided Sun for not including it on their Niagra chips when they were providing both the language runtime and the hardware platform.

I'll say it again: Getting them on a commodity architecture is a great thing for concurrent garbage collected languages.