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Summary Of The Forms Of Multithreading And Conclusion
Finally, a recap of all the forms of multithreading.
Here, a) is a traditional superscalar, b) a 2-way CMP (On-chip multiprocessor), c) a 4-way CMT (Coarse-grained Multithreading), d) a 4-way FMT (Fine-grained Multithreading), and e) a 4-way SMT (Simultaneous Multithreading). CMP uses two (or more) smaller cores to increase functional unit efficiency (removing horizontal waste), and has shown itself in new processors such as the Sun MAJC architecture, and the IBM POWER4; and the SledgeHammer from AMD should too. CMT and FMT both use the ability to switch rapidly between threads to hide memory latencies, and decrease vertical waste. CMT can be found in the MAJC architecture, and FMT in the Terra Supercomputing architecture. SMT operates by running any thread, in any functional unit, on any clock, thus removing both horizontal, and vertical waste, and will be found in the Alpha 21464, and possibly in a later incarnation of the P4 architecture. DMT and Slipstreaming processors both aim at increasing single-thread performance.
Though the many varied forms of multithreading take very different approaches, the goal is the same: higher real-world throughput. All of these techniques allow additional functional units to be added to a processor, and show something more akin to return to scales than to diminishing returns. While previous processors tended not to go far beyond 4 functional units per processor, due to diminishing returns, there are now techniques available which allow more units to be added with increased efficiency. We shall see some strange architectures in the future….
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