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Comment by wiseowise | Hacker News Reader

> Given that we now fully utilize 128 threads or 64 cores for pretty much the entire compile time, we can do a back of the envelope calculation for how long it should take: 25 min / 128 = 12 sec (or maybe 24 sec since hyper-threads aren't real cores). Yet it takes 170s to compile everything.

Amdahl’s Law would like to have a word.

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FWIW the article carefully worded it as a "back of the envelope" calculation, says we can't expect a linear speedup in practice and also gives the time it takes for linking (7 secs).

(Disclaimer I am the author of the article and I am quite familiar with the law)

This is an observed change of going from 1 core at 100% to 64 cores at 100%. This is establishing a lower bound, assuming there is no wasted work or contention for shared resources.

Amdahl's Law, like most 20th-century performance "wisdom" and metrics, focuses excessively on instruction count, neglecting memory and I/O pressure. 64 cores doesn't mean 64 independent cache pyramids and memory buses. In real life, the difference between CPU cycle frequency and memory latency is so great that memory pressure primarily determines performance, whereas core count really only matters to the extent that it contributes to that memory pressure.

Amdahls law is about coordination costs, so either you would expect cores to be starved or lots pf extra coordination-related compute to be happening, which, i guess is not totally crazy since there are that many crates, but as a first guess OP's back of the envelope is fine

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