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They are different. HyperThreading is where one core acts as two by rapidly switching contexts. It isn't actually two hardware threads, and the one core never

They are different. HyperThreading is where one core acts as two by rapidly switching contexts. It isn't actually two hardware threads, and the one core never does two things at once. SIMD is one thread doing one instruction on multiple pieces of data at the same time. SIMD can give higher throughout from the CPU, and you must organize your data types to use SIMD. Symmetric multithreading is where software takes advantage of multiple logical hardware threads to do multiple pieces of work per clock cycle. SMT can use HyperThreads and/or multiple physical cores, and/or multiple physical CPUs with one or more logical hardware threads each.

nkurz 8y ago
It isn't actually two hardware threads, and the one core never does two things at once. I don't think that's quite true. Rather, the two "virtual cores" are mostly independent, but share some backend resources. For example, modern Intel CPU's can sustain execution of 4 instructions per cycle, and with hyperthreading, two of these instructions can be from one thread and two from the other. Or one and three, or four and zero. Both threads truly do execute at the same time, it's just that the competition for resources means it sometimes takes longer for them to execute on a shared core than on separate physical cores.
fulafel 8y ago
> They are different. HyperThreading is where one core acts as two by rapidly switching contexts. This would be quite the opposite of Intel's actual SMT implementation, which aims to keep all the parallel execution resources of an OoO core fed and busy. There have been some machines that do what you describe too (eg Tera/Cray MTA, many GPUs, Sun's Niagara), to combat memory latency and reduce the need for cache. Those machines have a big thread count since they want to have a lot of outstanding memory operations in flight. You will notice that these machines are not called SMT, since the S stands for "Simultaneous".

nkurz 8y ago

It isn't actually two hardware threads, and the one core never does two things at once. I don't think that's quite true. Rather, the two "virtual cores" are mostly independent, but share some backend resources. For example, modern Intel CPU's can sustain execution of 4 instructions per cycle, and with hyperthreading, two of these instructions can be from one thread and two from the other. Or one and three, or four and zero. Both threads truly do execute at the same time, it's just that the competition for resources means it sometimes takes longer for them to execute on a shared core than on separate physical cores.

fulafel 8y ago

> They are different. HyperThreading is where one core acts as two by rapidly switching contexts. This would be quite the opposite of Intel's actual SMT implementation, which aims to keep all the parallel execution resources of an OoO core fed and busy. There have been some machines that do what you describe too (eg Tera/Cray MTA, many GPUs, Sun's Niagara), to combat memory latency and reduce the need for cache. Those machines have a big thread count since they want to have a lot of outstanding memory operations in flight. You will notice that these machines are not called SMT, since the S stands for "Simultaneous".