Merge pull request #152 from rust-lang/docs-rustc-perf-document-why-h…

…etzner

docs(rustc-perf): document why hetzner

service-catalog/rustc-perf/README.md

@@ -12,6 +12,8 @@ It uses an ECR repository to store the Docker image and an ECS service to run it
 It also uses the `shared` db of [rds-databases] and the `rustc-perf` S3 bucket.
 
 The collector (the machine that runs the benchmarks) is a dedicated physical server running at [Hetzner].
+Rustc-perf runs on a bare-metal server because we need predictable performance.
+Otherwise, test results might be spoiled by noisy neighbors in the cloud.
 
 ## Versions
 
@@ -21,6 +23,7 @@ These are the versions we need to keep up-to-date:
 - Dependencies of [Cargo.lock] and [package-lock.json]
 - Postgres version of the `shared` database
 - GitHub Actions versions in [workflows]
+- Operating system of the Hetzner server
 
 [Rustc perf]: https://perf.rust-lang.org/
 [rustc-perf]: https://github.com/rust-lang/rustc-perf