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Likely true in some cases but are these the bottlenecks on a laptop? I've never experienced HT on a multicore CPU being more than maybe a 40% increase in throug

Likely true in some cases but are these the bottlenecks on a laptop? I've never experienced HT on a multicore CPU being more than maybe a 40% increase in throughput and no real help on latency or efficiency. Implementing HT isn't free and doubling the number of active processes halves the per-process cache while doubling the concurrency in the memory controller (or more because less cache). Then there's the scheduler complexity... On the other hand there's the dual efficiency win of adding more E cores with shorter/simpler pipeline and lower clock speeds. Back in the P4 days HT made desktop interactivity much smoother, these days with 8+ core laptops the win is less clear and at least some of the top design teams think it's not worth it.

adrian_b 2y ago
As a software developer, by far the most imortant advantage of SMT is the reduction in compilation time for big software projects. I have not encountered any other application that benefits so much from SMT as a compilation that uses all available threads. On older CPUs, such as Skylake derivatives, a 25% compilation speed improvement from SMT was frequent. On newer CPUs, e.g. Zen 3 (presumably also on Zen 4, which has a very similar microarchitecture), the benefits of SMT are smaller (due to improved out-of-order execution), but a 20% compilation speed improvement is still frequent. However, doing compilation on a device with Lunar Lake, a CPU with only 4+4 cores, would be a waste of time, so it clearly does not need SMT. On the other hand, doing compilation on a bigger laptop, with an AMD Strix Point 12-core/24-thread CPU or with whatever Arrow Lake CPU will be launched by Intel to compete with it (presumably having much more smaller Skymont cores than Lunar Lake, to compensate for the lack of SMT) would certainly be useful.
- hedgehog 2y ago
  I've moved to almost entirely remote dev workflow for similar reasons. Out of curiosity how much RAM do you need per build thread?
  - adrian_b 2y ago
    When compiling an entire Linux distribution, there are a few outliers, all of which are derivatives of the Google Chrome code base, which may fail when less than 2 GB is available per build thread (e.g. for a 16C/32T CPU with make -j32, 64 GB of DRAM are required). For most other software projects 1 GB per build thread is enough.

adrian_b 2y ago

As a software developer, by far the most imortant advantage of SMT is the reduction in compilation time for big software projects. I have not encountered any other application that benefits so much from SMT as a compilation that uses all available threads. On older CPUs, such as Skylake derivatives, a 25% compilation speed improvement from SMT was frequent. On newer CPUs, e.g. Zen 3 (presumably also on Zen 4, which has a very similar microarchitecture), the benefits of SMT are smaller (due to improved out-of-order execution), but a 20% compilation speed improvement is still frequent. However, doing compilation on a device with Lunar Lake, a CPU with only 4+4 cores, would be a waste of time, so it clearly does not need SMT. On the other hand, doing compilation on a bigger laptop, with an AMD Strix Point 12-core/24-thread CPU or with whatever Arrow Lake CPU will be launched by Intel to compete with it (presumably having much more smaller Skymont cores than Lunar Lake, to compensate for the lack of SMT) would certainly be useful.

hedgehog 2y ago
I've moved to almost entirely remote dev workflow for similar reasons. Out of curiosity how much RAM do you need per build thread?
- adrian_b 2y ago
  When compiling an entire Linux distribution, there are a few outliers, all of which are derivatives of the Google Chrome code base, which may fail when less than 2 GB is available per build thread (e.g. for a 16C/32T CPU with make -j32, 64 GB of DRAM are required). For most other software projects 1 GB per build thread is enough.