Is Your Elixir Code Actually Fast? Tobias Pfeiffer on Benchmarking
Notes de l’épisode
Everyone has an opinion about whether the BEAM is fast or slow. Almost nobody has actually measured it properly.
Tobias Pfeiffer -PragTob on the internet, benchmarking nerd, real-time engineer at Supabase -came to Elixir from Ruby because Ruby couldn't handle WebSockets at scale. Then he learned the language the way he learns every language: by building a benchmarking library. Ten years later, Benchee is what most of the Erlang and Elixir world reaches for when it wants a number instead of an argument.
Topics include:
why Tobias writes a benchmarking library every time he learns a new language -and what that discipline actually teaches about how a runtime works
the production scar: a benchmark that showed one query was faster, deployed it, and discovered the benchmark had been measuring the wrong input entirely
the single most important insight about benchmarking: most people quietly fool themselves and walk away more confident and more wrong than when they started
inputs as a first-class feature -the lesson José Valim taught Tobias within 17 minutes of an open GitHub issue
why body-recursive map is faster than tail-recursive map for small lists on the BEAM -and what that tells you about the runtime
what reductions are, why they're a more stable measurement than wall time, and when they diverge
how Benchee measures memory: spawning a monitored process, watching GC events, reporting exact bytes -and why deterministic code gives deterministic memory
the JSON benchmarking arms race -built-in OTP JSON, jason, Jiffy, and why comparison benchmarks usually miss the point
why Benchee has no macros, no DSL, and deliberately only uses functions -and why that design decision made it composable in ways nobody predicted
the hardest benchmarking mistake to stop making: confusing latency and throughput
what Toby wishes more Elixir developers understood about BEAM performance before they opened a PR claiming something was faster
Recorded August 21, 2026.
Resources mentioned:
Benchee on Hex: hex.pm/packages/benchee
Benchee HTML formatter -self-contained HTML output with downloadable graphs
Benchee markdown formatter
PragTob on GitHub: github.com/bencheeorg