Audio performance debug

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.