Concurrency Profiling — Instruments Workflows

Profile and optimize Swift async/await code using Instruments.

When to Use

✅ Use 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

❌ Don't use when: - Issue is 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

Statistics

• Running Tasks: Tasks currently executing
• Alive Tasks: Tasks present at a point in time
• Total Tasks: Cumulative count created

Color Coding

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

Workflow 1: Diagnose 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. Check if work could be offloaded

Solution patterns:

// ❌ Heavy work on MainActor
@MainActor
class ViewModel: ObservableObject {
    func process() {
        let result = heavyComputation()  // Blocks UI
        self.data = result
    }
}

// ✅ Offload heavy work
@MainActor
class ViewModel: ObservableObject {
    func process() async {
        let result = await Task.detached {
            heavyComputation()
        }.value
        self.data = result
    }
}

Workflow 2: Find Actor Contention

Symptom: Tasks serializing unexpectedly, parallel work running sequentially

1. Enable "Swift Actors" instrument
2. Look for serialized access patterns
3. Red = waiting, Blue = executing
4. High red:blue ratio = contention problem

Solution patterns:

// ❌ All work serialized through actor
actor DataProcessor {
    func process(_ data: Data) -> Result {
        heavyProcessing(data)  // All callers wait
    }
}

// ✅ Mark heavy work as nonisolated
actor DataProcessor {
    nonisolated func process(_ data: Data) -> Result {
        heavyProcessing(data)  // Runs in parallel
    }

    func storeResult(_ result: Result) {
        // Only actor state access serialized
    }
}

More fixes: - Split actor into multiple (domain separation) - Use Mutex for hot paths (faster than actor hop) - Reduce actor scope (fewer isolated properties)

Workflow 3: Thread Pool Exhaustion

Symptom: Tasks queued but not executing, gaps in task execution

Cause: Blocking calls exhaust cooperative pool

1. Look for gaps in task execution across all threads
2. Check for blocking primitives
3. Replace with async equivalents

Common culprits:

// ❌ Blocks cooperative thread
Task {
    semaphore.wait()  // NEVER do this
    // ...
    semaphore.signal()
}

// ❌ Synchronous file I/O in async context
Task {
    let data = Data(contentsOf: fileURL)  // Blocks
}

// ✅ Use async APIs
Task {
    let (data, _) = try await URLSession.shared.data(from: fileURL)
}

Debug flag:

SWIFT_CONCURRENCY_COOPERATIVE_THREAD_BOUNDS=1

Detects unsafe blocking in async context.

Workflow 4: Priority Inversion

Symptom: High-priority task waits for low-priority

1. Inspect task priorities in Instruments
2. Follow wait chains
3. Ensure critical paths use appropriate priority

// ✅ Explicit priority for critical work
Task(priority: .userInitiated) {
    await criticalUIUpdate()
}

Thread Pool Model

Swift uses a cooperative thread pool matching CPU core count:

Why blocking is catastrophic: - Each blocked thread holds memory + kernel structures - Limited threads means blocked = no progress - Pool exhaustion deadlocks the app

Quick Checks (Before Profiling)

Run these checks first:

1. Is work actually async? - Look for suspension points (await) - Sync code in async function still blocks

2. Holding locks across await? swift // ❌ Deadlock risk mutex.withLock { await something() // Never! }

3. Tasks in tight loops? ```swift // ❌ Overhead may exceed benefit for item in items { Task { process(item) } }

// ✅ Structured concurrency await withTaskGroup(of: Void.self) { group in for item in items { group.addTask { process(item) } } } ```

4. DispatchSemaphore in async context? - Always unsafe — use withCheckedContinuation instead

Common Issues Summary

Safe vs Unsafe Primitives

Safe with cooperative pool: - await, actors, task groups - os_unfair_lock, NSLock (short critical sections) - Mutex (iOS 18+)

Unsafe (violate forward progress): - DispatchSemaphore.wait() - pthread_cond_wait - Sync file/network I/O - Thread.sleep() in Task

Resources

WWDC: 2022-110350, 2021-10254

Docs: /xcode/improving-app-responsiveness

Skills: axiom-swift-concurrency, axiom-performance-profiling, axiom-synchronization