instruments-profiling

name	instruments-profiling
description	Use when profiling native macOS or iOS apps with Instruments/xctrace. Covers correct binary selection, CLI arguments, exports, and common gotchas.
metadata	{"short-description":"Instruments profiling for macOS/iOS apps"}

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.