axiom-display-performance（公理显示性能）

Display Performance

Systematic diagnosis for frame rate issues on variable refresh rate displays (ProMotion, iPad Pro, future devices). Covers render loop configuration, frame pacing, hitch mechanics, and production telemetry.

Key insight: "ProMotion available" does NOT mean your app automatically runs at 120Hz. You must configure it correctly, account for system caps, and ensure proper frame pacing.

Part 1: Why You're Stuck at 60fps

Diagnostic Order

Check these in order when stuck at 60fps on ProMotion:

1. Info.plist key missing? (iPhone only) → Part 2
2. Render loop configured for 60? (MTKView defaults, CADisplayLink) → Part 3
3. System caps enabled? (Low Power Mode, Limit Frame Rate, Thermal) → Part 5
4. Frame time > 8.33ms? (Can't sustain 120fps) → Part 6
5. Frame pacing issues? (Micro-stuttering despite good FPS) → Part 7
6. Measuring wrong thing? (UIScreen vs actual presentation) → Part 9

Part 2: Enabling ProMotion on iPhone

Critical: Core Animation won't access frame rates above 60Hz on iPhone unless you add this key.

<!-- Info.plist -->
<key>CADisableMinimumFrameDurationOnPhone</key>
<true/>

Without this key:

• Your preferredFrameRateRange hints are ignored above 60Hz
• Other animations may affect your CADisplayLink callback rate
• iPad Pro does NOT require this key

When to add: Any iPhone app that needs >60Hz for games, animations, or smooth scrolling.

Part 3: Render Loop Configuration

MTKView Defaults to 60fps

This is the most common cause. MTKView's preferredFramesPerSecond defaults to 60.

// ❌ WRONG: Implicit 60fps (default)
let mtkView = MTKView(frame: frame, device: device)
mtkView.delegate = self
// Running at 60fps even on ProMotion!

// ✅ CORRECT: Explicit 120fps request
let mtkView = MTKView(frame: frame, device: device)
mtkView.preferredFramesPerSecond = 120
mtkView.isPaused = false
mtkView.enableSetNeedsDisplay = false  // Continuous, not on-demand
mtkView.delegate = self

Critical settings for continuous high-rate rendering:

CADisplayLink Configuration (iOS 15+)

Apple explicitly recommends CADisplayLink (not timers) for custom render loops.

// ❌ WRONG: Timer-based render loop (drifts, wastes frame time)
Timer.scheduledTimer(withTimeInterval: 1.0/120.0, repeats: true) { _ in
    self.render()
}

// ❌ WRONG: Default CADisplayLink (may hint 60)
let displayLink = CADisplayLink(target: self, selector: #selector(render))
displayLink.add(to: .main, forMode: .common)

// ✅ CORRECT: Explicit frame rate range
let displayLink = CADisplayLink(target: self, selector: #selector(render))
displayLink.preferredFrameRateRange = CAFrameRateRange(
    minimum: 80,      // Minimum acceptable
    maximum: 120,     // Preferred maximum
    preferred: 120    // What you want
)
displayLink.add(to: .main, forMode: .common)

Special priority for games: iOS 15+ gives 30Hz and 60Hz special priority. If targeting these rates:

// 30Hz and 60Hz get priority scheduling
let prioritizedRange = CAFrameRateRange(
    minimum: 30,
    maximum: 60,
    preferred: 60
)
displayLink.preferredFrameRateRange = prioritizedRange

Suggested Frame Rates by Content Type

Part 4: CAMetalDisplayLink (iOS 17+)

For Metal apps needing precise timing control, CAMetalDisplayLink provides more control than CADisplayLink.

class MetalRenderer: NSObject, CAMetalDisplayLinkDelegate {
    var displayLink: CAMetalDisplayLink?
    var metalLayer: CAMetalLayer!

    func setupDisplayLink() {
        displayLink = CAMetalDisplayLink(metalLayer: metalLayer)
        displayLink?.delegate = self
        displayLink?.preferredFrameRateRange = CAFrameRateRange(
            minimum: 60,
            maximum: 120,
            preferred: 120
        )
        // Control render latency (in frames)
        displayLink?.preferredFrameLatency = 2
        displayLink?.add(to: .main, forMode: .common)
    }

    func metalDisplayLink(_ link: CAMetalDisplayLink, needsUpdate update: CAMetalDisplayLink.Update) {
        // update.drawable - The drawable to render to
        // update.targetTimestamp - Deadline to finish rendering
        // update.targetPresentationTimestamp - When frame will display

        guard let drawable = update.drawable else { return }

        let workingTime = update.targetTimestamp - CACurrentMediaTime()
        // workingTime = seconds available before deadline

        // Render to drawable...
        renderFrame(to: drawable)
    }
}

Key differences from CADisplayLink:

When to use CAMetalDisplayLink:

• Need precise control over render timing window
• Want to minimize input latency
• Building games or intensive Metal apps
• iOS 17+ only deployment

Part 5: System Caps

System states can force 60fps even when your code requests 120:

Low Power Mode

Caps ProMotion devices to 60fps.

// Check programmatically
if ProcessInfo.processInfo.isLowPowerModeEnabled {
    // System caps display to 60Hz
}

// Observe changes
NotificationCenter.default.addObserver(
    forName: .NSProcessInfoPowerStateDidChange,
    object: nil,
    queue: .main
) { _ in
    let isLowPower = ProcessInfo.processInfo.isLowPowerModeEnabled
    self.adjustRenderingForPowerState(isLowPower)
}

Limit Frame Rate (Accessibility)

Settings → Accessibility → Motion → Limit Frame Rate caps to 60fps.

No API to detect. If user reports 60fps despite configuration, have them check this setting.

Thermal Throttling

System restricts 120Hz when device overheats.

// Check thermal state
switch ProcessInfo.processInfo.thermalState {
case .nominal, .fair:
    preferredFramesPerSecond = 120
case .serious, .critical:
    preferredFramesPerSecond = 60  // Reduce proactively
@unknown default:
    break
}

// Observe thermal changes
NotificationCenter.default.addObserver(
    forName: ProcessInfo.thermalStateDidChangeNotification,
    object: nil,
    queue: .main
) { _ in
    self.adjustForThermalState()
}

Adaptive Power (iOS 26+, iPhone 17)

New in iOS 26: Adaptive Power is ON by default on iPhone 17/17 Pro. Can throttle even at 60% battery.

User action for testing: Settings → Battery → Power Mode → disable Adaptive Power.

No public API to detect Adaptive Power state.

Part 6: Performance Budget

Frame Time Budgets

If you consistently exceed budget, system drops to next sustainable rate.

Measuring GPU Frame Time

func draw(in view: MTKView) {
    guard let commandBuffer = commandQueue.makeCommandBuffer() else { return }

    // Your rendering code...

    commandBuffer.addCompletedHandler { buffer in
        let gpuTime = buffer.gpuEndTime - buffer.gpuStartTime
        let gpuMs = gpuTime * 1000

        if gpuMs > 8.33 {
            print("⚠️ GPU: \(String(format: "%.2f", gpuMs))ms exceeds 120Hz budget")
        }
    }

    commandBuffer.commit()
}

Can't Sustain 120? Target Lower Rate Evenly

Critical: Uneven frame pacing looks worse than consistent lower rate.

// If you can't sustain 8.33ms, explicitly target 60 for smooth cadence
if averageGpuTime > 8.33 && averageGpuTime <= 16.67 {
    mtkView.preferredFramesPerSecond = 60
}

Part 7: Frame Pacing

The Micro-Stuttering Problem

Even with good average FPS, inconsistent frame timing causes visible jitter.

// BAD: Inconsistent intervals despite ~40 FPS average
Frame 1: 25ms
Frame 2: 40ms  ← stutter
Frame 3: 25ms
Frame 4: 40ms  ← stutter

// GOOD: Consistent intervals at 30 FPS
Frame 1: 33ms
Frame 2: 33ms
Frame 3: 33ms
Frame 4: 33ms

Presenting immediately after rendering causes this. Use explicit timing control.

Frame Pacing APIs

present(afterMinimumDuration:) — Recommended

Ensures consistent spacing between frames:

func draw(in view: MTKView) {
    guard let commandBuffer = commandQueue.makeCommandBuffer(),
          let drawable = view.currentDrawable else { return }

    // Render to drawable...

    // Present with minimum 33ms between frames (30 FPS target)
    commandBuffer.present(drawable, afterMinimumDuration: 0.033)
    commandBuffer.commit()
}

present(at:) — Precise Timing

Schedule presentation at specific time:

// Present at specific Mach absolute time
let presentTime = CACurrentMediaTime() + 0.033
commandBuffer.present(drawable, atTime: presentTime)

presentedTime — Verify Actual Presentation

Check when frames actually appeared:

drawable.addPresentedHandler { drawable in
    let actualTime = drawable.presentedTime
    if actualTime == 0.0 {
        // Frame was dropped!
        print("⚠️ Frame dropped")
    } else {
        print("Frame presented at: \(actualTime)")
    }
}

Frame Pacing Pattern

class SmoothRenderer: NSObject, MTKViewDelegate {
    private var targetFrameDuration: CFTimeInterval = 1.0 / 60.0  // 60 FPS target

    func draw(in view: MTKView) {
        guard let commandBuffer = commandQueue.makeCommandBuffer(),
              let drawable = view.currentDrawable else { return }

        renderScene(to: drawable)

        // Use frame pacing to ensure consistent intervals
        commandBuffer.present(drawable, afterMinimumDuration: targetFrameDuration)
        commandBuffer.commit()
    }

    func adjustTargetFrameRate(canSustain fps: Int) {
        switch fps {
        case 90...:
            targetFrameDuration = 1.0 / 120.0
        case 50...:
            targetFrameDuration = 1.0 / 60.0
        default:
            targetFrameDuration = 1.0 / 30.0
        }
    }
}

Part 8: Understanding Hitches

Render Loop Phases

Frame lifecycle: Begin Time → Commit Deadline → Presentation Time

1. App Process (CPU): Handle events, compute UI updates, Core Animation commit
2. Render Server (CPU+GPU): Transform UI to bitmap, render to buffer
3. Display Driver: Swap buffer to screen at vsync

At 120Hz, each phase has ~8.33ms. Miss any deadline = hitch.

Commit Hitch vs Render Hitch

Commit Hitch: App process misses commit deadline

• Cause: Main thread work takes too long
• Fix: Move work off main thread, reduce view complexity

Render Hitch: Render server misses presentation deadline

• Cause: GPU work too complex (blur, shadows, layers)
• Fix: Simplify visual effects, reduce overdraw

Double vs Triple Buffering

Double Buffer (default):

• Frame lifetime: 2 vsync intervals
• Tighter deadlines
• Lower latency

Triple Buffer (system may enable):

• Frame lifetime: 3 vsync intervals
• Render server gets 2 vsync intervals
• Higher latency but more headroom

The system automatically switches to triple buffering to recover from render hitches.

Hitch Duration

Expected Frame Lifetime = Begin Time → Presentation Time
Actual Frame Lifetime = Begin Time → Actual Vsync

Hitch Duration = Actual - Expected

If hitch duration > 0, the frame was late and previous frame stayed onscreen longer.

Part 9: Measurement

UIScreen Lies, Actual Presentation Tells Truth

// ❌ This says 120 even when system caps you to 60
let maxFPS = UIScreen.main.maximumFramesPerSecond
// Reports capability, not actual rate!

// ✅ Measure from CADisplayLink timing
@objc func displayLinkCallback(_ link: CADisplayLink) {
    // Time available to prepare next frame
    let workingTime = link.targetTimestamp - CACurrentMediaTime()

    // Actual interval since last callback
    if lastTimestamp > 0 {
        let interval = link.timestamp - lastTimestamp
        let actualFPS = 1.0 / interval
    }
    lastTimestamp = link.timestamp
}

Metal Performance HUD

Enable on-device real-time performance overlay:

Via Xcode scheme:

1. Edit Scheme → Run → Diagnostics
2. Enable "Show Graphics Overview"
3. Optionally enable "Log Graphics Overview"

Via environment variable:

MTL_HUD_ENABLED=1

Via device settings: Settings → Developer → Graphics HUD → Show Graphics HUD

HUD shows:

• FPS (average)
• GPU time per frame
• Frame interval chart (last 120 frames)
• Memory usage

Production Telemetry with MetricKit

Monitor hitches in production:

import MetricKit

class MetricsManager: NSObject, MXMetricManagerSubscriber {
    func didReceive(_ payloads: [MXMetricPayload]) {
        for payload in payloads {
            if let animationMetrics = payload.animationMetrics {
                // Ratio of time spent hitching during scroll
                let scrollHitchRatio = animationMetrics.scrollHitchTimeRatio

                // Ratio of time spent hitching in all animations
                if #available(iOS 17.0, *) {
                    let hitchRatio = animationMetrics.hitchTimeRatio
                }

                analyzeHitchMetrics(scrollHitchRatio: scrollHitchRatio)
            }
        }
    }
}

// Register for metrics
MXMetricManager.shared.add(metricsManager)

What to track:

• scrollHitchTimeRatio: Time spent hitching while scrolling (UIScrollView only)
• hitchTimeRatio (iOS 17+): Time spent hitching in all tracked animations

Part 10: Quick Diagnostic Checklist

When debugging frame rate issues:

Part 11: Common Patterns

Pattern: Adaptive Frame Rate with Thermal Awareness

class AdaptiveRenderer: NSObject, MTKViewDelegate {
    private var recentFrameTimes: [Double] = []
    private let sampleCount = 30
    private var targetFrameDuration: CFTimeInterval = 1.0 / 60.0

    func draw(in view: MTKView) {
        guard let commandBuffer = commandQueue.makeCommandBuffer(),
              let drawable = view.currentDrawable else { return }

        let startTime = CACurrentMediaTime()
        renderScene(to: drawable)
        let frameTime = (CACurrentMediaTime() - startTime) * 1000

        updateTargetRate(frameTime: frameTime, view: view)

        commandBuffer.present(drawable, afterMinimumDuration: targetFrameDuration)
        commandBuffer.commit()
    }

    private func updateTargetRate(frameTime: Double, view: MTKView) {
        recentFrameTimes.append(frameTime)
        if recentFrameTimes.count > sampleCount {
            recentFrameTimes.removeFirst()
        }

        let avgFrameTime = recentFrameTimes.reduce(0, +) / Double(recentFrameTimes.count)
        let thermal = ProcessInfo.processInfo.thermalState
        let lowPower = ProcessInfo.processInfo.isLowPowerModeEnabled

        // Constrain based on what we can sustain AND system state
        if lowPower || thermal >= .serious {
            view.preferredFramesPerSecond = 30
            targetFrameDuration = 1.0 / 30.0
        } else if avgFrameTime < 7.0 && thermal == .nominal {
            view.preferredFramesPerSecond = 120
            targetFrameDuration = 1.0 / 120.0
        } else if avgFrameTime < 14.0 {
            view.preferredFramesPerSecond = 60
            targetFrameDuration = 1.0 / 60.0
        } else {
            view.preferredFramesPerSecond = 30
            targetFrameDuration = 1.0 / 30.0
        }
    }
}

Pattern: Frame Drop Detection

class FrameDropMonitor {
    private var expectedPresentTime: CFTimeInterval = 0
    private var dropCount = 0

    func trackFrame(drawable: MTLDrawable, expectedInterval: CFTimeInterval) {
        drawable.addPresentedHandler { [weak self] drawable in
            guard let self = self else { return }

            if drawable.presentedTime == 0.0 {
                self.dropCount += 1
                print("⚠️ Frame dropped (total: \(self.dropCount))")
            } else if self.expectedPresentTime > 0 {
                let actualInterval = drawable.presentedTime - self.expectedPresentTime
                let variance = abs(actualInterval - expectedInterval)

                if variance > expectedInterval * 0.5 {
                    print("⚠️ Frame timing variance: \(variance * 1000)ms")
                }
            }

            self.expectedPresentTime = drawable.presentedTime
        }
    }
}

Resources

WWDC: 2021-10147, 2018-612, 2022-10083, 2023-10123

Tech Talks: 10855, 10856, 10857 (Hitch deep dives)

Docs: /quartzcore/cadisplaylink, /quartzcore/cametaldisplaylink, /quartzcore/optimizing-iphone-and-ipad-apps-to-support-promotion-displays, /xcode/understanding-hitches-in-your-app, /metal/mtldrawable/present(afterminimumduration:), /metrickit/mxanimationmetric

Skills: axiom-energy, axiom-ios-graphics, axiom-metal-migration-ref, axiom-performance-profiling