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He's completely right. Hyperthreading is all about putting idle functional units to work when you can't extract instruction level parallelism from single threa

He's completely right. Hyperthreading is all about putting idle functional units to work when you can't extract instruction level parallelism from single threaded code. His claim is that Intel processors did better and thus only got a small relative boost from HT (IIRC it used to be 10%, maybe it's 20% now). If your processor does poorer at ILP extraction, you get relatively more out of HT. This all agree with the benchmarks in which Intel still generally is ahead on single thread performance. For a more extreme case look to older POWER processors which were 4-way threaded, presumably because they were even worse on single threaded code.

aPoCoMiLogin 6y ago
> This all agree with the benchmarks in which Intel still generally is ahead on single thread performance. citation needed. benchmarks that I've seen [0][1] there is not that big of difference between the two. [0] https://www.anandtech.com/show/15578/cloud-clash-amazon-graviton2-arm-against-intel-and-amd/5 https://www.anandtech.com/show/15578/cloud-clash-amazon-grav... ZEN1 [1] https://www.anandtech.com/show/14694/amd-rome-epyc-2nd-gen/9 https://www.anandtech.com/show/14694/amd-rome-epyc-2nd-gen/9 ZEN2
- FullyFunctional 6y ago
  https://browser.geekbench.com/processor-benchmarks https://browser.geekbench.com/processor-benchmarks Even normalizing for the frequency isn't enough to explain the difference
  - kasabali 6y ago
    You need to look at turbo frequencies. Geekbench runs at turbo speed almost all the time.
    - FullyFunctional 6y ago
      Like I said, I did account for that
nullc 6y ago
HT is also about memory latency. No matter how amazing your ILP ninja logic is, if you're waiting on a read-- you're waiting on a read.
zozbot234 6y ago
"Extracting ILP from single-threaded code" is energy intensive and comes with potential security issues (Spectre). SMT makes sense given these conditions.

aPoCoMiLogin 6y ago

> This all agree with the benchmarks in which Intel still generally is ahead on single thread performance. citation needed. benchmarks that I've seen [0][1] there is not that big of difference between the two. [0] https://www.anandtech.com/show/15578/cloud-clash-amazon-graviton2-arm-against-intel-and-amd/5 https://www.anandtech.com/show/15578/cloud-clash-amazon-grav... ZEN1 [1] https://www.anandtech.com/show/14694/amd-rome-epyc-2nd-gen/9 https://www.anandtech.com/show/14694/amd-rome-epyc-2nd-gen/9 ZEN2

FullyFunctional 6y ago
https://browser.geekbench.com/processor-benchmarks https://browser.geekbench.com/processor-benchmarks Even normalizing for the frequency isn't enough to explain the difference
- kasabali 6y ago
  You need to look at turbo frequencies. Geekbench runs at turbo speed almost all the time.
  - FullyFunctional 6y ago
    Like I said, I did account for that

nullc 6y ago

HT is also about memory latency. No matter how amazing your ILP ninja logic is, if you're waiting on a read-- you're waiting on a read.

zozbot234 6y ago

"Extracting ILP from single-threaded code" is energy intensive and comes with potential security issues (Spectre). SMT makes sense given these conditions.