iOS Instruments Performance CLI

Instructions

When helping with iOS app performance optimization using Instruments command line tools:

1. Setup Analysis Environment

CRITICAL: Always use device UUID, never device names

• Get device UUID: xcrun simctl list devices available | grep "iPhone"
• Device names like "iPhone 17 Pro" are ambiguous and will fail
• Use UUID format: F464E766-555C-4B95-B8CC-763702A70791

Clean installation state (REQUIRED)

• Always uninstall app before profiling: xcrun simctl uninstall $DEVICE_UUID <bundle id>
• Multiple app installations cause "process is ambiguous" errors
• Alternative: Completely erase simulator for cleanest state

Build configuration

• Build in Release mode for accurate performance measurements

2. Choose Appropriate Instrument Template

Use xcrun xctrace list templates to see available templates:

• App Launch - Essential for launch time optimization (most common)
• Time Profiler - CPU performance analysis
• Allocations - Memory usage tracking
• Leaks - Memory leak detection
• Network - API calls and network activity
• Animation Hitches - UI performance issues

3. Run Performance Analysis

Standard workflow:

DEVICE_UUID="F464E766-555C-4B95-B8CC-763702A70791" # this is sample uuid, run command line to get exist uuid
xcrun simctl uninstall $DEVICE_UUID <bundle id>
xcrun simctl install $DEVICE_UUID /path/to/<app name>.app
sleep 2
xcrun xctrace record --template "App Launch" --device $DEVICE_UUID \
  --launch -- /path/to/<app name>.app 2>&1

Important notes:

• Trace files auto-generate names like Launch_<app name>.app_2025-10-30_3.55.40 PM_39E6A410.trace
• --output parameter may be ignored; accept auto-generated names
• Wait 2 seconds after installation before profiling
• Use 2>&1 to capture all output
• Recording duration: 10-30s for launch, 2-5m for other analyses

4. Analyze Results

Finding trace files:

ls -lt *.trace | head -1  # Most recent trace
find . -name "*.trace" -type d -mmin -5  # Files from last 5 minutes

Analysis approaches:

• CLI export often fails - "trace is malformed" errors are common

• Recommended: Open in Instruments.app GUI for reliable analysis

  open -a Instruments.app Launch_<app name>*.trace
  

• Parse signpost markers for performance metrics

• Identify bottlenecks in launch sequence, memory allocations, CPU hotspots

5. Common Optimization Patterns

Based on successful optimizations:

• Lazy initialization - Defer expensive dependency resolution
• Background operations - Move non-critical setup off main thread
• Deferred bindings - Set up subscriptions after UI appears
• Font loading - Register fonts asynchronously
• Method swizzling - Only essential swizzles during launch

Critical Pitfalls to Avoid

❌ Using device names instead of UUIDs ❌ Skipping clean installation step ❌ Building in Debug mode ❌ Trying to export trace immediately after recording ❌ Assuming --output path will be used ❌ Not waiting between installation and profiling

Reference Documentation

See examples in this skill directory for:

• Detailed command examples with correct syntax
• Complete troubleshooting guide with solutions
• Real-world lessons learned
• Production-ready workflow scripts