I'm working on a memory profiler for Python that is fast enough to run in production (see link below), so I've ended up with some similar problems re performanc

I'm working on a memory profiler for Python that is fast enough to run in production (see link below), so I've ended up with some similar problems re performance and importance of testing. A few things the article talks about where one can maybe do even better: 1. likely()/unlikely() not being in stable Rust. This is true, but the hashbrown project has some hacked-up variants it claims work on stable: https://github.com/rust-lang/hashbrown/blob/bbda6e0077bafb75c02d0ace665faa09b4514932/src/raw/mod.rs#L47 https://github.com/rust-lang/hashbrown/blob/bbda6e0077bafb75... 2. Rust not having fast thread locals. Same problem for me, so likewise did it in C with "initial-exec". But! If you use clang, you can get LTO across C and Rust, so you can get fast thread locals _and_ not have function call overhead. Basically need to use same version of Clang as Rust does (12 at the moment) and do a little song and dance in linker and compiler flags. See https://matklad.github.io/2020/10/03/fast-thread-locals-in-rust.html https://matklad.github.io/2020/10/03/fast-thread-locals-in-r... 3. For testing these sort of things, being able to assert "this test touched this code path" is extremely useful. In my case, for example, I have different code paths for sampled and unsampled allocations, but from perspective of code calling malloc() everything should be identical. So how to tell if correct code path was used? Coverage marks are a great solution for this: https://ferrous-systems.com/blog/coverage-marks/ https://ferrous-systems.com/blog/coverage-marks/ (The Python profiler, if anyone is interested: I've already released an open source memory profiler that tracks all allocations, https://pythonspeed.com/fil/ https://pythonspeed.com/fil/. Unlike most memory profilers it tells you source-of-allocations at peak memory, which is key for data processing applications. The commercial production-grade version I'm now working on uses sampling, and will be even more focused on data processing batch applications; the goal is to have essentially no performance overhead so it can always be on.)