alt Hacker NewsHyperthreading/SMT is a significant boon for heavily threaded workloads. What makes that such a win while Bulldozer's implementation of "two integer units sharing a front-end, cache and FPU" is supposedly so bad? Because that description makes the 8 core Bulldozers sound exactly like a 4 core with SMT.
>Hyperthreading/SMT is a significant boon for heavily threaded workloads.
That's hugely debatable and depends on SW workloads and the SMT implementation + CPU pipeline design.
In SMT the execution engines, ALUs, FPUs, and caches are completely shared. When one thread stalls waiting for RAM, the second thread sneaks into the idle execution units. At best, SMT yields a ~10% to 20% throughput boost over a single thread.
>Because that description makes the 8 core Bulldozers sound exactly like a 4 core with SMT.
It's not the same thing. Bulldozer arch sits between a true 8-core and 4-core + SMT implementation.