kunitoki Audio Performance Debug
Audio performance requires good worst-case complexity, not average-case — the audio thread fires on a hard deadline every buffer, regardless of what else is happening.
Step 1 — Identify the symptom
Measure before guessing. Different symptoms point to different root causes.
| Symptom | Measurement approach |
|---|---|
| Consistent high CPU average | CPU meter in DAW; perf stat / Instruments Time Profiler |
| Sporadic xruns / dropouts | Host xrun log; correlate with system events (GC, network, page fault) |
| CPU spikes on note-on | Profile with many simultaneous note-on events; watch for allocation spikes |
| Latency reporting wrong | Log getLatencySamples() before and after prepareToPlay at varying buffer sizes |
| Gets worse with more voices | Profile with 1 vs 8 vs 32 voices; O(N²) shows quadratic growth |
| Memory bandwidth pressure | perf mem / VTune memory bandwidth counter; cache miss rate |
Step 2 — Locate the hotspot
- Build a release (optimized) binary before profiling — debug builds are not representative.
- Use a reproducible test case: fixed buffer size, fixed voice count, looped audio.
- Attach a profiler to the audio thread specifically, not the whole process.
- Identify the top-3 hottest functions by exclusive CPU time, not inclusive.
- Check whether the spike is periodic (every N buffers → container rehash or GC) or random (OS jitter, page fault).
| Platform | Profiler | Notes |
|---|---|---|
| macOS | Instruments — Time Profiler | Filter to audio I/O thread; use "hide system libraries" to focus on your code |
| macOS | Instruments — Allocations | Catch allocations on the audio thread during a session |
| Linux | perf record -g + perf report |
--call-graph dwarf for C++ templates; perf stat for cache miss ratio |
| Windows | VTune Profiler — Hotspots | Use "Platform Profiler" preset; filter to realtime thread |
| Cross-platform | Tracy | Frame-level instrumentation; zero-cost when disabled; shows per-buffer timing |
| JUCE | juce::PerformanceCounter |
Inline timer around suspect blocks; logs to console |
Step 3 — Fix patterns
| Anti-pattern | Why it hurts | Optimization |
|---|---|---|
| O(N²) voice loop | Quadratic growth — 32 voices = 1024 iterations per sample | Restructure to O(N): batch per-voice work, use SIMD across voices |
| Wrong FFT size | Power-of-two FFT on non-power-of-two frames → zero-padding waste | Size FFT to next power-of-two; or use prime-factor FFT (FFTW FFTW_MEASURE) |
| Unnecessary buffer copy | memcpy of full buffer each callback |
Process in-place; pass pointer + length; avoid intermediate staging buffers |
std::map / unordered_map lookup on audio thread |
O(log N) / amortized O(1) but with cache misses; unordered_map rehash = alloc |
Replace with std::array + index, or a sorted std::array with lower_bound |
unordered_map insert |
Triggers rehash → heap allocation on audio thread | Pre-populate at prepareToPlay; never insert during playback |
| Heap allocation on audio thread | new/delete acquires global allocator lock |
Pre-allocate in prepareToPlay; use pool or ring buffer; see audio-dsp-review |
| First-touch page fault | OS maps physical pages on first write → stall | prepareToPlay: allocate AND write to every byte of every buffer |
| Scalar loop over samples | Compiler may not auto-vectorize complex loops | Use JUCE FloatVectorOperations, xsimd, or explicit SIMD intrinsics |
| Denormal-induced slowdown | Subnormal FP values cause 100× slowdown on x86 | Set FTZ+DAZ in prepareToPlay; see audio-numerics-review |
| Lock contention | Mutex held by UI thread blocks audio thread | Replace with atomics or SPSC queue; see audio-dsp-review |
For anti-pattern code examples and platform profiler workflows see
references/performance-patterns.md.