optimization: cache ClassTag[Token] to avoid thread blocking

[fwbrasil]

Problem

I'm working on a Tapir module for Kyo and noticed that there's some thread blocking to materialize ClassTags in UriInterpolator. It seems a mild problem that should affect mostly benchmarks but there could be pathological scenarios with more complex routing that make it more problematic:

Solution

Cache the ClassTag in a local field to avoid the synchronized cache lookup. The change avoids the blocking in UriInterpolator but there are still other cases that arise from other collection operations:

Notes

• As an alternative users could disable the ClassTag cache via -Dscala.reflect.classtag.cache.disable=true but it could have a performance penalty.
• If there's an interest, I could explore ways to avoid the remaining sources of blocking.

[adamw]

If there's an interest, I could explore ways to avoid the remaining sources of blocking.

Of course, we're always open to PRs, and performance improvements in particular :)

[fwbrasil]

Of course, we're always open to PRs, and performance improvements in particular :)

Great! Do you have benchmarks you normally use to validate Tapir's performance?

[adamw]

@fwbrasil we do have some very basic tests, comparing "vanilla" akkahttp/http4s servers vs the same servers, but with tapir added on top, to check the overhead that tapir imposes. However, running them is not part of CI or the release process: https://github.com/softwaremill/tapir/tree/master/perf-tests