axiom-concurrency-profiling skill

---
name: axiom-concurrency-profiling
description: Use when profiling async/await performance, diagnosing actor contention, or investigating thread pool exhaustion. Covers Swift Concurrency Instruments template, task visualization, actor contention analysis, thread pool debugging.
license: MIT
metadata:
  version: "1.0.0"
---

# 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

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

### 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**:

```swift
// ❌ 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**:

```swift
// ❌ 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**:

```swift
// ❌ 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

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

## Thread Pool Model

Swift uses a **cooperative thread pool** matching CPU core count:

| Aspect | GCD | Swift Concurrency |
|--------|-----|-------------------|
| Threads | Grows unbounded | Fixed to core count |
| Blocking | Creates new threads | Suspends, frees thread |
| Dependencies | Hidden | Runtime-tracked |
| Context switch | Full kernel switch | Lightweight continuation |

**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

| Issue | Symptom in Instruments | Fix |
|-------|------------------------|-----|
| MainActor overload | Long blue bars on main | `Task.detached`, `nonisolated` |
| Actor contention | High red:blue ratio | Split actors, use `nonisolated` |
| Thread exhaustion | Gaps in all threads | Remove blocking calls |
| Priority inversion | High-pri waits for low-pri | Check task priorities |
| Too many tasks | Task creation overhead | Use task groups |

## 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