x/tools/gopls: optimize didChange handling

[findleyr]

Recent profiling has shown that, particularly on large codebases, snapshot cloning can significantly impact gopls' responsiveness.

Snapshot cloning is not optimized: there are lots of opportunities for performance improvement. For example:

• URI.Filename() is a huge source of unnecessary cost; almost all URI manipulations could be done directly with the URI string rather than converting to file paths (which involves URI parsing).
• Most of the maps in the snapshot could be replaced with an alternative data structure, optimized for fast cloning. For example, with an overlay that is occasionally compactified.
• The import graph doesn't need to be rebuilt from scratch.
• Known directories could be memoized.
• The explicit inheritance of cache keys could probably instead be achieved with an asynchronous sweep.

Not sure which of these we'll do. Filing this as an umbrella issue to track improving performance.

CC @heschi @stamblerre

[gopherbot]

Change https://golang.org/cl/312689 mentions this issue: internal/lsp/cache: preallocate internal maps when cloning snapshots

[anton-kuklin]

Working on unnecessary URI.Filename() usage in snapshot reduction

[gopherbot]

Change https://golang.org/cl/312809 mentions this issue: internal/lsp/cache: improve snapshot clone perfomance

[findleyr]

Extending this more generally to didChange handling, as snapshot.clone is only one of the problems. There is some other stuff in e.g. didModifyFiles that's quite slow.

@justplesh thanks very much for your contribution, and interest. Right now we're still trying to figure out the best path forward for some of these optimizations. Hopefully we'll have a better sense of what needs to be prioritized next week. If you'd like, we can keep you in mind for anything that is relatively self-contained.

[anton-kuklin]

@findleyr Sure, please mention me or assign any lsp issue that I may work on. I'll be happy to work on it on some +- regular basis.

[anton-kuklin]

@findleyr will we merge my PR then or will we wait for some more fundamental approach?