[6.0] Use an AtomicInt32 to count pendingUnitCount instead of using AsyncQueue by ahoppen · Pull Request #1744 · swiftlang/sourcekit-lsp (original) (raw)

…yncQueue`

Adding an item to AsyncQueue<Serial> is linear in the number of pending queue items, thus adding n items to an AsyncQueue before any can execute is in O(n^2). This decision was made intentionally because the primary use case for AsyncQueue was to track pending LSP requests, of which we don’t expect to have too many pending requests at any given time.

SourceKitIndexDelegate was also using AsyncQueue to track the number of pending units to be processed and eg. after indexing SourceKit-LSP, I have seen this grow up to ~20,000. With the quadratic behavior, this explodes time-wise.

Turns out that we don’t actually need to use a queue here at all, an atomic is sufficient and much faster.

Independently, we should consider mitigating the quadratic behavior of AsyncQueue<Serial> or AsyncQueue in general.

Fixes swiftlang#1541 rdar://130844901