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This PR is an off-by-default revision v2 of the (since-reverted) PR #6555 / commit `3220f830b7fbb785d6db8a93775f46314f10a99b`. See that PR for details on why running with a single runtime is desirable and why we should be ready. We reverted #6555 because it showed regressions in prodlike cloudbench, see the revert commit message `ad072de4209193fd21314cf7f03f14df4fa55eb1` for more context. This PR makes it an opt-in choice via an env var. The default is to use the 4 separate runtimes that we have today, there shouldn't be any performance change. I tested manually that the env var & added metric works. ``` # undefined env var => no change to before this PR, uses 4 runtimes ./target/debug/neon_local start # defining the env var enables one-runtime mode, value defines that one runtime's configuration NEON_PAGESERVER_USE_ONE_RUNTIME=current_thread ./target/debug/neon_local start NEON_PAGESERVER_USE_ONE_RUNTIME=multi_thread:1 ./target/debug/neon_local start NEON_PAGESERVER_USE_ONE_RUNTIME=multi_thread:2 ./target/debug/neon_local start NEON_PAGESERVER_USE_ONE_RUNTIME=multi_thread:default ./target/debug/neon_local start ``` I want to use this change to do more manualy testing and potentially testing in staging. Future Work ----------- Testing / deployment ergonomics would be better if this were a variable in `pageserver.toml`. It can be done, but, I don't need it right now, so let's stick with the env var.