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Comment by FeepingCreature | Hacker News Reader

No, if your CPU is one of the worst in single-threaded performance, then hyper-threading will look like the best thing since sliced bread on it, relative to its baseline performance. Hyperthreading will seem to double your performance, because so much of your CPU is otherwise idle. But the more you improve your scheduling, the worse hyperthreading will look, again relative to your singlethread performance.

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That makes a lot of sense in theory, but Pentium 4 HT was a real evidence disagreeing with it.

Netburst have had horrible IPC, so in theory HT should've brought big improvements, yet in practice it didn't bring significantly more relative performance improvement than HT implementations of later Intel cores with much better IPC.

Adding to that SMT in Zen cores have brings slightly better relative performance compared to Intel cores (and Zen doesn't have as terrible IPC as Netburst), I think SMT implementation is not as trivial as we thought and before and its implementation indeed matters.

But poor single-threaded performance doesn't imply that the core is sitting idle. It may well be running as fast as it possibly can and using all the shared components of SMT leaving nothing for the other thread to use.

In the same way the single-core performance may be really good yet everything shared by SMT is still only utilized to half its potential so the 2nd thread runs almost as fast as the first.

Yes.

I think the point of the article was more that people don't understand where HT/virtual SMT gets its speedup from.

From the very start I've always heard HT being good for memory-bound applications. That's how it was explained and marketed.

Some years ago I worked on such an application, and we got a ~80% speed increase going from 4 threads to 8 threads on a 4-core with HT enabled. I wouldn't expect compute-bound applications to get anywhere near such gains.

Makes me wonder where those with a misconception got it from.

From most of the test that I've seen [0][1], there is not that big of difference in single-threaded performance between AMD & Intel. IMO this is rather related to difference in SMT implementation, than idication of weak ST.

[0] https://www.anandtech.com/show/15578/cloud-clash-amazon-grav... ZEN1

[1] https://www.anandtech.com/show/14694/amd-rome-epyc-2nd-gen/9 ZEN2

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