Author: Vincent Diepeveen
Date: 07:01:15 11/03/02
Go up one level in this thread
On November 03, 2002 at 03:29:09, Gian-Carlo Pascutto wrote: >On November 03, 2002 at 02:00:41, Anthony Cozzie wrote: > >>If I understand SMT correctly, its even more than that. A modern processor has >>a lot of functional units lying around. An Athlon can in theory execute 3 >>branches, 3 integer instructions, and 3 floating point instructions every >>cycle. In reality, most of the time those units are just sitting around. One >>of the ideas behind SMT is that you can run 2 threads, and split the >>functional units between them. > >The problem with this (and the reason I was surprised SMT works) is that >it only has 3 decoders and a single cache that is used by both processes. > >It is somewhat irrelevant that you have 9 function units if your >processor is decoder-limited (true for most modern cpus). I guess >all improvement from SMT is because of memory waiting as Robert >describes. 1 decoder for P4 even. So if a program is not insane small and if a program doesn't fit in the 12K trace cache then you are history in advance. also there is other problems with registers. Just 40 registers to use for renaming. Too little. > >-- >GCP
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