rshankras/performance-profiling

Performance Profiling

Systematic guide for profiling Apple platform apps using Instruments, Xcode diagnostics, and MetricKit. Covers CPU, memory, launch time, and energy analysis with actionable fix patterns.

When This Skill Activates

Use this skill when the user:

• Reports app hangs, stutters, or dropped frames
• Needs to profile CPU usage or find hot code paths
• Has memory leaks, high memory usage, or OOM crashes
• Wants to optimize app launch time
• Needs to reduce battery/energy impact
• Asks about Instruments, Time Profiler, Allocations, or Leaks
• Wants to add os_signpost or performance measurement to code
• Is preparing for App Store review and needs performance validation

Decision Tree

What performance problem are you investigating?
│
├─ App hangs / stutters / dropped frames / slow UI
│  └─ Read time-profiler.md
│
├─ High memory / leaks / OOM crashes / growing footprint
│  └─ Read memory-profiling.md
│
├─ Slow app launch / time to first frame
│  └─ Read launch-optimization.md
│
├─ Battery drain / thermal throttling / background energy
│  └─ Read energy-diagnostics.md
│
├─ General "app feels slow" (unknown cause)
│  └─ Start with time-profiler.md, then memory-profiling.md
│
└─ Pre-release performance audit
   └─ Read ALL reference files, use Review Checklist below

Quick Reference

| Problem | Instrument / Tool | Key Metric | Reference | |---------|-------------------|------------|-----------| | UI hangs > 250ms | Time Profiler + Hangs | Hang duration, main thread stack | time-profiler.md | | High CPU usage | Time Profiler | CPU % by function, call tree weight | time-profiler.md | | Memory leak | Leaks + Memory Graph | Leaked bytes, retain cycle paths | memory-profiling.md | | Memory growth | Allocations | Live bytes, generation analysis | memory-profiling.md | | Slow launch | App Launch | Time to first frame (pre-main + post-main) | launch-optimization.md | | Battery drain | Energy Log | Energy Impact score, CPU/GPU/network | energy-diagnostics.md | | Thermal issues | Activity Monitor | Thermal state transitions | energy-diagnostics.md | | Network waste | Network profiler | Redundant fetches, large payloads | energy-diagnostics.md |

Process

1. Identify the Problem Category

Ask the user or inspect their description to classify the issue:

• Responsiveness: Hangs, stutters, animation drops
• Memory: Leaks, growth, OOM crashes
• Launch: Slow cold/warm start
• Energy: Battery drain, thermal throttling

2. Read the Appropriate Reference File

Each file contains:

• Which Instruments template to use
• Step-by-step profiling workflow
• How to interpret results
• Common fix patterns with code examples

3. Profile on Real Hardware

Always remind users:

• Profile on device, not Simulator (Simulator uses host CPU/memory)
• Use Release build configuration (optimizations change behavior)
• Profile with representative data (empty databases hide real perf)
• Close other apps to reduce noise

4. Apply Fixes and Verify

After identifying bottlenecks:

• Apply targeted fix from the reference file
• Re-profile to confirm improvement
• Add os_signpost markers for ongoing monitoring

Xcode Diagnostic Settings

Recommend enabling these in Scheme > Run > Diagnostics:

| Setting | What It Catches | |---------|-----------------| | Main Thread Checker | UI work off main thread | | Thread Sanitizer | Data races | | Address Sanitizer | Buffer overflows, use-after-free | | Malloc Stack Logging | Memory allocation call stacks | | Zombie Objects | Messages to deallocated objects |

MetricKit Integration

For production monitoring, recommend MetricKit:

import MetricKit

final class PerformanceReporter: NSObject, MXMetricManagerSubscriber {
    func startCollecting() {
        MXMetricManager.shared.add(self)
    }

    func didReceive(_ payloads: [MXMetricPayload]) {
        for payload in payloads {
            // Launch time
            if let launch = payload.applicationLaunchMetrics {
                log("Resume time: \(launch.histogrammedResumeTime)")
            }
            // Hang rate
            if let responsiveness = payload.applicationResponsivenessMetrics {
                log("Hang time: \(responsiveness.histogrammedApplicationHangTime)")
            }
            // Memory
            if let memory = payload.memoryMetrics {
                log("Peak memory: \(memory.peakMemoryUsage)")
            }
        }
    }

    func didReceive(_ payloads: [MXDiagnosticPayload]) {
        for payload in payloads {
            if let hangs = payload.hangDiagnostics {
                for hang in hangs {
                    log("Hang: \(hang.callStackTree)")
                }
            }
        }
    }
}

Review Checklist

Responsiveness

• [ ] No synchronous work on main thread > 100ms
• [ ] No file I/O or network calls on main thread
• [ ] Core Data / SwiftData fetches use background contexts for large queries
• [ ] Images decoded off main thread (use .preparingThumbnail or async decoding)
• [ ] @MainActor only on code that truly needs UI access

Memory

• [ ] No retain cycles (check delegate patterns, closures with self)
• [ ] Large resources freed when not visible (images, caches)
• [ ] Collections don't grow unbounded (capped caches, pagination)
• [ ] autoreleasepool used in tight loops creating ObjC objects

Launch Time

• [ ] No heavy work in init() of @main App struct
• [ ] Deferred non-essential initialization (analytics, prefetch)
• [ ] Minimal dynamic frameworks (prefer static linking)
• [ ] No synchronous network calls at launch

Energy

• [ ] Background tasks use BGProcessingTaskRequest appropriately
• [ ] Location accuracy matches actual need (not always .best)
• [ ] Timers use tolerance to allow coalescing
• [ ] Network requests batched where possible

References

• time-profiler.md — CPU profiling, hang detection, signpost API
• memory-profiling.md — Allocations, Leaks, memory graph debugger
• launch-optimization.md — App launch phases, cold/warm start optimization
• energy-diagnostics.md — Battery, thermal state, network efficiency
• WWDC: Ultimate Application Performance Survival Guide
• WWDC: Analyze Hangs with Instruments
• WWDC: Detect and Diagnose Memory Issues