Performance Analysis for Elixir/Phoenix

Overview

NO OPTIMIZATION WITHOUT PROFILING DATA. Code review cannot tell you where bottlenecks are — only measurement can. Refuse to suggest optimizations without profiling results.

Profiler Selection

Start with eprof (lower overhead). Use fprof only when you need call trees.

Escalation Ladder

Do you have a number for "how slow"?
  NO  → L0: Measure first (Benchee baseline)
  YES → Know WHERE time is spent?
          NO  → L1: Profile (eprof/fprof)
          YES → Algorithmic problem (O(n²)+)?
                  YES → L2: Algorithm/data structure fix
                  NO  → CPU-bound?
                          YES → L3: BEAM opts (Task.async_stream, ETS)
                          NO  → Database/I/O?
                                  YES → L4: DB optimization (preload, indexes)
                                  NO  → L5: System tuning (last resort)

Common Mistakes

• Optimizing without profiling — code review ≠ profiling. Measure first.
• No baseline benchmark — measure current state before changing anything
• Assuming optimization worked — benchmark after changes to verify
• Server-side metrics only — client-observed latency can be 15x higher
• Wrong profiler — fprof measures time not memory; cprof counts calls not time

Reference Files

Read the file that matches your current problem:

• profiling.md — When: About to profile code or choosing a profiler. Iron Law enforcement, profiler usage, escalation ladder L0-L5, common patterns
• benchmarking.md — When: Comparing implementation alternatives with Benchee. Benchee templates, complexity analysis, validation workflow
• latency.md — When: Investigating tail latency or fan-out amplification. Tail latency reduction (hedged requests), fan-out amplification, measurement pitfalls, pool sizing
• beam-gc.md — When: Suspecting garbage collection pressure or large heaps. Per-process GC, ETS for heap reduction, 4 mitigation techniques
• beam-efficiency.md — When: Hitting BEAM-specific performance pitfalls (binaries, maps, lists). BEAM performance pitfalls: Seven Myths, binary handling (IO lists, append vs prepend), map efficiency (32-key threshold, key sharing), list O(n) traps, process data copying, atom table, NIFs

Commands

• /benchmark — Create and run Benchee benchmarks for comparison
• /review [file] — Review code including performance analysis

Related Skills

• algorithms: Better data structures when profiling reveals O(n²)+
• production-quality: Observability and telemetry setup
• elixir-patterns: BEAM-specific patterns (ETS, Task, GenServer)