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 / unresponsive to taps / slow UI
│  └─ Read time-profiler.md
│
├─ Scroll or animation stutter / dropped frames / hitches
│  └─ Read hitches.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 | | Scroll/animation hitches | Animation Hitches template | Hitch time ratio (< 5 ms/s good, > 10 critical) | hitches.md | | High CPU usage | Time Profiler / CPU 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 / Power Profiler | 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 |

The 8 Key Metrics (WWDC21 10181)

Apple enumerates exactly eight things to track for app performance, all coverable with five tools (Xcode Organizer, MetricKit, Instruments, XCTest, App Store Connect API):

| # | Metric | Threshold / signal | Tool | |---|--------|--------------------|------| | 1 | Battery usage | Energy Gauge flags CPU use > 20% as High CPU, plus CPU Wake Overhead regions; top subsystems to watch: CPU, Networking, Location | Energy Gauge → Time Profiler | | 2 | Launch time | Time between icon tap and first frame rendered | App Launch template, XCTApplicationLaunchMetric | | 3 | Hang rate | Unresponsive to input for ≥ 250ms | Hangs instrument, Organizer | | 4 | Memory | Organizer charts peak memory and memory at suspension; a spike with no termination spike yet is your early warning | Allocations, Leaks, VM Tracker | | 5 | Disk writes | Exception report when the app writes > 1 GB in 24 hours (with stack trace + Insights annotations) | File Activity template, XCTStorageMetric | | 6 | Scrolling | Red bars in the Organizer scrolling chart = poor scroll experience, fix immediately | Animation Hitches, XCTOSSignpostMetric.scrollDecelerationMetric | | 7 | Terminations | Every termination forces a slow cold launch next time and loses user state | MXAppExitMetric, Organizer | | 8 | MXSignposts | Custom marked intervals for your critical code sections | MetricKit |

The Organizer aggregates all of these from consented user devices across the last 16 app versions; the Regressions pane (Xcode 13) isolates every metric that increased significantly in the most recent version, in one place. The same data is available as JSON via the App Store Connect API (WWDC21 10181).

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 (WWDC20 10081, WWDC21 10181):

import MetricKit

final class PerformanceReporter: NSObject, MXMetricManagerSubscriber {
    func startCollecting() {
        MXMetricManager.shared.add(self)   // subscribe once, early in launch
    }
    // best practice: remove(self) in deinit (WWDC21 10181)

    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)")
            }
            // Scroll hitches (ratio of time hitching to time scrolling)
            if let animation = payload.animationMetrics {
                log("Scroll hitch ratio: \(animation.scrollHitchTimeRatio)")
            }
            // Exit reasons — daily counts per termination cause, fg + bg
            if let exits = payload.applicationExitMetrics {
                log("Exits: \(exits.backgroundExitData)")
            }
        }
    }

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

Key facts (WWDC20 10081, WWDC21 10181):

• ❌ Subscribing lazily (e.g. in a settings screen) silently misses delivered payloads — ✅ subscribe in App/AppDelegate init; the previous day's payloads are handed to you on launch.
• Metrics arrive as 24-hour payloads, at most once per day, in three aggregation forms: cumulative, averaged, and bucketized (MXHistogram). On iOS 15+/macOS 12+, all diagnostics are delivered immediately after the issue occurs instead of daily.
• Termination and memory telemetry arrive in the daily metric payload by default — no extra instrumentation.
• Diagnostic types: MXHangDiagnostic (main-thread unresponsive time + backtraces), MXCrashDiagnostic (exception info, termination reason, VM region info), MXCPUExceptionDiagnostic (threads burning CPU — the programmatic form of Organizer energy logs), MXDiskWriteExceptionDiagnostic (fires on the 1 GB/day write threshold).
• MXCallStackTree backtraces are unsymbolicated by design — symbolicate off-device with atos-class tools + your dSYMs; ❌ don't try on-device.
• MXSignpost marks critical code sections for field telemetry; the animation variant captures hitch-rate telemetry for the interval:

let handle = MXMetricManager.makeLogHandle(category: "animation_telemetry")
mxSignpostAnimationIntervalBegin(log: handle, name: "custom_animation")
// ... animation ...
mxSignpost(OSSignpostType.end, log: handle, name: "custom_animation")

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 ladder, signpost API, Instruments 27 tools
• hitches.md — Render loop, hitch time ratio, commit/render-phase fix catalogs
• memory-profiling.md — Allocations, Leaks, memory graph debugger, memgraph CLI, termination reasons
• launch-optimization.md — App launch phases, cold/warm start optimization, linker levers
• energy-diagnostics.md — Battery, thermal state, network efficiency, Power Profiler
• WWDC: Ultimate Application Performance Survival Guide
• WWDC: Understand and Eliminate Hangs from Your App
• WWDC: Track Down Hangs with Xcode and On-Device Detection
• WWDC: Analyze Hangs with Instruments
• WWDC: Detect and Diagnose Memory Issues
• WWDC: Analyze Heap Memory
• WWDC: Why Is My App Getting Killed?
• WWDC: What's New in MetricKit
• WWDC: Optimizing App Launch
• WWDC: Optimize CPU Performance with Instruments
• WWDC: Profile and Optimize Power Usage in Your App
• WWDC: Profile, Fix, and Verify — Improve App Responsiveness with Instruments