instruments-profiling

# Instruments Profiling (macOS/iOS) Use this skill when the user wants performance profiling or stack analysis for native apps. Focus: Time Profiler, `xctrace` CLI, and picking the correct binary/app instance. ## Quick Start (CLI) - List templates: `xcrun xctrace list templates` - Record Time Profiler (launch): - `xcrun xctrace record --template 'Time Profiler' --time-limit 60s --output /tmp/App.trace --launch -- /path/To/App.app` - Record Time Profiler (attach): - Launch app yourself, get PID, then: - `xcrun xctrace record --template 'Time Profiler' --time-limit 60s --output /tmp/App.trace --attach <pid>` - Open trace in Instruments: - `open -a Instruments /tmp/App.trace` Note: `xcrun xctrace --help` is not a valid subcommand. Use `xcrun xctrace help record`. ## Picking the Correct Binary (Critical) **Gotcha: Instruments may profile the wrong app** (e.g., one in `/Applications`) if LaunchServices resolves a different bundle. Use these rules: - Prefer direct binary path for deterministic launch: - `xcrun xctrace record ... --launch -- /path/App.app/Contents/MacOS/App` - If launching `.app`, ensure it’s the intended bundle: - `open -n /path/App.app` - Verify with `ps -p <pid> -o comm= -o command=` - If both `/Applications/App.app` and a local build exist, explicitly target the local build path. - After launch, confirm the process path before trusting the trace. ## Command Arguments (xctrace) - `--template 'Time Profiler'`: template name from `xctrace list templates`. - `--launch -- <cmd>`: everything after `--` is the target command (binary or app bundle). - `--attach <pid|name>`: attach to running process. - `--output <path>`: `.trace` output. If omitted, file saved in CWD. - `--time-limit 60s|5m`: set capture duration. - `--device <name|UDID>`: required for iOS device runs. - `--target-stdout -`: stream launched process stdout to terminal (useful for CLI tools). ## Exporting Stacks (CLI) - Inspect trace tables: - `xcrun xctrace export --input /tmp/App.trace --toc` - Export raw time-profile samples: - `xcrun xctrace export --input /tmp/App.trace --xpath '/trace-toc/run[@number="1"]/data/table[@schema="time-profile"]' --output /tmp/time-profile.xml` - Post-process in a script (Python/Rust) to aggregate stacks. ## Instruments UI Workflow - Template: Time Profiler - Use “Record” and capture the slow path (startup vs steady-state) - Call Tree tips: - Hide System Libraries - Invert Call Tree - Separate by Thread - Focus on hot frames and call counts ## Gotchas & Fixes - **Wrong app profiled**: LaunchServices resolves installed app instead of local build. - Fix: use direct binary path or `--attach` with known PID. - **No samples / empty trace**: App exits quickly or never hits work. - Fix: longer capture, trigger workload during recording. - **Privacy prompts**: `xctrace` may need Developer Tools permission. - Fix: System Settings → Privacy & Security → Developer Tools → allow Terminal/Xcode. - **Large XML exports**: `time-profile` exports are huge. - Fix: filter with XPath and aggregate offline; don’t print to terminal. ## iOS Specific Notes - Device: use `xcrun xctrace list devices` and `--device <UDID>`. - Launch via Xcode if needed; attach with `xctrace --attach`. - Ensure debug symbols for meaningful stacks. ## Verification Checklist - Confirm trace process path matches target build. - Confirm stacks show expected app frames. - Capture covers the slow operation (startup/refresh). - Export stacks for automated diffing if optimizing.