Concurrency Profiling — Instruments Workflows

Purpose: Profile and optimize Swift async/await code using Instruments Xcode Version: Xcode 14+ (Swift Concurrency template) iOS Version: iOS 13+

When to Use This Skill

✅ Use this skill when:

• UI stutters during async operations
• Suspecting actor contention
• Tasks queued but not executing
• Main thread blocked during async work
• Need to visualize task execution flow

❌ Do NOT use this skill for:

• Pure CPU performance (use Time Profiler)
• Memory issues unrelated to concurrency (use Allocations)
• Haven't confirmed concurrency is the bottleneck

Swift Concurrency Template

What It Shows

Track	Information
Swift Tasks	Task lifetimes, parent-child relationships
Swift Actors	Actor access, contention visualization
Thread States	Blocked vs running vs suspended

Color Coding

• Blue: Task executing
• Red: Task waiting (contention)
• Gray: Task suspended (awaiting)

Key Workflows

Workflow 1: Main Thread Blocking

Symptom: UI freezes, main thread timeline full

1. Profile with Swift Concurrency template
2. Look at main thread → "Swift Tasks" lane
3. Find long blue bars (task executing on main)
4. Offload with Task.detached or nonisolated

Workflow 2: Actor Contention

Symptom: Tasks serializing unexpectedly

1. Enable "Swift Actors" instrument
2. High red:blue ratio = contention problem
3. Fix: Split actors, use nonisolated, or Mutex for hot paths

Workflow 3: Thread Pool Exhaustion

Symptom: Tasks queued but not executing

Cause: Blocking calls (semaphore.wait(), sync I/O) exhaust cooperative pool

Debug flag:

SWIFT_CONCURRENCY_COOPERATIVE_THREAD_BOUNDS=1

Related Skills

• Swift Concurrency — Concurrency patterns
• Synchronization — Mutex vs actor decisions
• Performance Profiling — General profiling

Resources

• WWDC: Visualize and optimize Swift concurrency (110350)
• WWDC: Swift concurrency: Behind the scenes (10254)