Rust for Rustaceans by Jon Gjengset

Author:Jon Gjengset [Gjengset, Jon]
Language: eng
Format: epub, mobi, pdf
ISBN: 9781718501867
Published: 2021-09-28T00:00:00+00:00

The Size of Generators

The data structure used to back a generator’s state must be able to hold the combined state at any one yield point. If your async fn contains, say, a [u8; 8192], those 8KiB must be stored in the generator itself. Even if your async fn contains only smaller local variables, it must also contain any future that it awaits, since it needs to be able to poll such a future later, when poll is invoked.

This nesting means that generators, and thus futures based on async functions and blocks, can get quite large without any visible indicator of that increased size in your code. This can in turn impact your program’s runtime performance, since those giant generators may have to be copied across function calls and in and out of data structures, which amounts to a fair amount of memory copying. In fact, you can usually identify when the size of your generator-based futures is affecting performance by looking for excessive amounts of time spent in the memcpy function in your application’s performance profiles!

Finding these large futures isn’t always easy, however, and often requires manually identifying long or complex chains of async functions. Clippy may be able to help with this in the future, but at the time of writing, you’re on your own. When you do find a particularly large future, you have two options: you can try to reduce the amount of local state the async functions need, or you can move the future to the heap (with Box::pin) so that moving the future just requires moving the pointer to it. The latter is by far the easiest way to go, but it also introduces an extra allocation and a pointer indirection. Your best bet is usually to put the problematic future on the heap, measure your performance, and then use your performance benchmarks to guide you from there.

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