Parallel compilation support strategy

[pnkfelix]

Meeting proposal info

• Title: Parallel compilation support strategy
• Type: technical

Summary

Rust's parallel compilation support is very much experimental. It does have some CI dedicated to it, but recent investigations from @lqd indicate that that CI support may not be exercising it properly (it is only running atop one core). In any case ICEs in the current parallel compiler are very frequent, according to @lqd

Lets talk about what machine resources we want to dedicate to parallel compilation testing, and how we should try to ensure that they are testing what we think they are testing.

Edit (Mark): https://github.com/rust-lang/rust/blob/6d7684101a51f1c375ec84aef5d2fbdeb214bbc2/compiler/rustc_session/src/options.rs#L1460-L1465 suggests that unless explicitly set, -Zthreads (which is the parallel compiler thread count; it does not affect non-parallel builds) defaults to 1 currently. Nothing in CI sets this, so CI runs with just the one thread, just with all the Mutexes and such enabled.

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