Android启动优化之精确测量启动各个阶段的耗时

1. 直观地观察应用启动时长

我们可以通过观察logcat日志查看Android应用启动耗时，过滤关键字"Displayed"：

ActivityTaskManager: Displayed com.peter.viewgrouptutorial/.activity.DashboardActivity: +797ms

启动时长(在这个例子中797ms)表示从启动App到系统认为App启动完成所花费的时间。

2. 启动时间包含哪几个阶段

从用户点击桌面图标，到Activity启动并将界面第一帧绘制出来大概会经过以下几个阶段。

system_server展示starting window
Zygote fork Android 进程
ActivityThread handleBindApplication(这个阶段又细分为)
• 加载程序代码和资源
• 初始化ContentProvider
• 执行Application.onCreate()
启动Activity(执行 onCreate、onStart、onResume等方法)
ViewRootImpl执行doFrame()绘制View，计算出首帧绘制时长。
流程图如下：

我们可以看出：阶段1和2都是由系统控制的。App开发者对这两个阶段的耗时能做的优化甚微。

3. 系统是如何测量启动时长的？

本文源码基于android-30
我们在cs.android.com源码阅读网站上全局搜索

1.在ActivityMetricsLogger.logAppDisplayed()方法中发现了打印日志语句

bash 复制代码

private void logAppDisplayed(
  TransitionInfoSnapshot info
) {
    if (info.type != TYPE_TRANSITION_WARM_LAUNCH && info.type != TYPE_TRANSITION_COLD_LAUNCH) {
        return;
    }
    EventLog.writeEvent(WM_ACTIVITY_LAUNCH_TIME,
            info.userId, info.activityRecordIdHashCode, info.launchedActivityShortComponentName,
            info.windowsDrawnDelayMs);
    StringBuilder sb = mStringBuilder;
    sb.setLength(0);
    sb.append("Displayed ");
    sb.append(info.launchedActivityShortComponentName);
    sb.append(": ");
    TimeUtils.formatDuration(info.windowsDrawnDelayMs, sb);
    Log.i(TAG, sb.toString());
}

TransitionInfoSnapshot.windowsDrawnDelayMs是启动的时长。它在以下方法中被赋值：
• ActivityMetricsLogger.notifyWindowsDrawn()
• ➡️ TransitionInfo.calculateDelay()

bash 复制代码

//ActivityMetricsLogger.java
TransitionInfoSnapshot notifyWindowsDrawn(
  ActivityRecord r, 
  long timestampNs
) {  
  TransitionInfo info = getActiveTransitionInfo(r);
  info.mWindowsDrawnDelayMs = info.calculateDelay(timestampNs);
  return new TransitionInfoSnapshot(info);
}
private static final class TransitionInfo {
 int calculateDelay(long timestampNs) {
   long delayNanos = timestampNs - mTransitionStartTimeNs;
   return (int) TimeUnit.NANOSECONDS.toMillis(delayNanos);
 }
}

timestampNs表示启动结束时间，mTransitionStartTimeNs表示启动开始时间。它们分别是在哪赋值的呢?
mTransitionStartTimeNs启动开始时间在notifyActivityLaunching方法中被赋值。调用堆栈如下：
• ActivityManagerService.startActivity()
• ➡️ActivityManagerService.startActivityAsUser()
• ➡️ActivityStarter.execute()
• ➡️ActivityMetricsLogger.notifyActivityLaunching()

ActivityMetricsLogger.notifyActivityLaunching(...)

bash 复制代码

//ActivityMetricsLogger.java
private LaunchingState notifyActivityLaunching(
  Intent intent,
  ActivityRecord caller,
  int callingUid
) {
  ...
  long transitionStartNs = SystemClock.elapsedRealtimeNanos();
  LaunchingState launchingState = new LaunchingState();
  launchingState.mCurrentTransitionStartTimeNs = transitionStartNs;
  ...
  return launchingState; 
}

启动时间记录到LaunchingState.mCurrentTransitionStartTimeNs中
ActivityStarter.execute()

bash 复制代码

//ActivityStarter.java
int execute() {
    try {
        final LaunchingState launchingState;
        synchronized (mService.mGlobalLock) {
            final ActivityRecord caller = ActivityRecord.forTokenLocked(mRequest.resultTo);
            launchingState = mSupervisor.getActivityMetricsLogger().notifyActivityLaunching(
                    mRequest.intent, caller);
        }
        if (mRequest.activityInfo == null) {
            mRequest.resolveActivity(mSupervisor);
        }
        int res;
        synchronized (mService.mGlobalLock) {
            
            mSupervisor.getActivityMetricsLogger().notifyActivityLaunched(launchingState, res,
                    mLastStartActivityRecord);
            return getExternalResult(mRequest.waitResult == null ? res
                    : waitForResult(res, mLastStartActivityRecord));
        }
    } finally {
        onExecutionComplete();
    }
}

该方法作用如下：

调用ActivityMetricsLogger().notifyActivityLaunching()生成LaunchingState。将启动时间记录其中
执行StartActivity逻辑
调用ActivityMetricsLogger().notifyActivityLaunched()把launchingState和ActivityRecord映射保存起来
ActivityMetricsLogger.notifyActivityLaunched(...)

bash 复制代码

//ActivityMetricsLogger.java
void notifyActivityLaunched(
  	LaunchingState launchingState,
  	int resultCode,
  	ActivityRecord launchedActivity) {
    ...
    final TransitionInfo newInfo = TransitionInfo.create(launchedActivity, launchingState,
            processRunning, processSwitch, resultCode);
    if (newInfo == null) {
        abort(info, "unrecognized launch");
        return;
    }
    if (DEBUG_METRICS) Slog.i(TAG, "notifyActivityLaunched successful");
    // A new launch sequence has begun. Start tracking it.
    mTransitionInfoList.add(newInfo);
    mLastTransitionInfo.put(launchedActivity, newInfo);
    startLaunchTrace(newInfo);
    if (newInfo.isInterestingToLoggerAndObserver()) {
        launchObserverNotifyActivityLaunched(newInfo);
    } else {
        // As abort for no process switch.
        launchObserverNotifyIntentFailed();
    }
}

该方法将根据LaunchingState和ActivityRecord生成TransitionInfo保存到mTransitionInfoList中。这样就将启动开始时间保存起来了。
ActivityMetricsLogger.notifyWindowsDrawn(...)

bash 复制代码

//ActivityMetricsLogger.java
TransitionInfoSnapshot notifyWindowsDrawn(
  ActivityRecord r, 
  long timestampNs
) {  
  TransitionInfo info = getActiveTransitionInfo(r);
  info.mWindowsDrawnDelayMs = info.calculateDelay(timestampNs);
  return new TransitionInfoSnapshot(info);
}

notifyWindowsDraw方法正是通过查找mTransitionInfoList中对应的TransitionInfo获取到Activity的启动开始时间。
启动完成调用堆栈如下

• ActivityRecord.onFirstWindowDrawn()

• ➡️ActivityRecord.updateReportedVisibilityLocked()

• ➡️ActivityRecord.onWindowsDrawn()

• ➡️ActivityMetricsLogger.notifyWindowsDrawn()

ActivityRecord.updateReportedVisibilityLocked()

bash 复制代码

//ActivityRecord.java
void updateReportedVisibilityLocked() {
	...
    boolean nowDrawn = numInteresting > 0 && numDrawn >= numInteresting;
    boolean nowVisible = numInteresting > 0 && numVisible >= numInteresting && isVisible();
		
    if (nowDrawn != reportedDrawn) {
        onWindowsDrawn(nowDrawn, SystemClock.elapsedRealtimeNanos());
        reportedDrawn = nowDrawn;
    }
	...
}
void onWindowsDrawn(boolean drawn, long timestampNs) {
    mDrawn = drawn;
    if (!drawn) {
        return;
    }
    final TransitionInfoSnapshot info = mStackSupervisor
            .getActivityMetricsLogger().notifyWindowsDrawn(this, timestampNs);
    ...
}

我们看到在updateReportedVisibilityLocked()方法中把SystemClock.elapsedRealtimeNanos()传递给onWindowsDrawn(nowDrawn, SystemClock.elapsedRealtimeNanos())

4. 调试技巧

通过断点调试记录应用冷启动记录耗时调用栈

准备一台root的手机(或者非Google Play版本模拟器)
compileSdkVersion、targetSdkVersion与模拟器版本一致(本文30)
notifyActivityLaunching和notifyWindowsDrawn中增加断点
调试勾选Show all processes选择system_process

几个重要的时间节点
ActivityManagerService接收到startActivity信号时间，等价于launchingState.mCurrentTransitionStartTimeNs。时间单位纳秒。
进程Fork的时间，时间单位毫秒。可以通过以下方式获取：

bash 复制代码

object Processes {
    @JvmStatic
    fun readProcessForkRealtimeMillis(): Long {
        val myPid = android.os.Process.myPid()
        val ticksAtProcessStart = readProcessStartTicks(myPid)
        // Min API 21, use reflection before API 21.
        // See https://stackoverflow.com/a/42195623/703646
        val ticksPerSecond = Os.sysconf(OsConstants._SC_CLK_TCK)
        return ticksAtProcessStart * 1000 / ticksPerSecond
    }
    // Benchmarked (with Jetpack Benchmark) on Pixel 3 running
    // Android 10. Median time: 0.13ms
    fun readProcessStartTicks(pid: Int): Long {
        val path = "/proc/$pid/stat"
        val stat = BufferedReader(FileReader(path)).use { reader ->
            reader.readLine()
        }
        val fields = stat.substringAfter(") ")
            .split(' ')
        return fields[19].toLong()
    }
}

ActivityThread.handleBindApplication时设置的进程启动时间，单位毫秒。Process.getStartElapsedRealtime()。

bash 复制代码

//ActivityThread.java
private void handleBindApplication(AppBindData data) {
    ...
    // Note when this process has started.
    Process.setStartTimes(SystemClock.elapsedRealtime(), SystemClock.uptimeMillis());
	  ...
}

4.程序代码和资源加载的时间，时间单位毫秒。Application类初始化时的时间与handleBindApplication的时间差

bash 复制代码

class MyApp extends Application {
    static {
          if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
            long loadApkAndResourceDuration = SystemClock.elapsedRealtime() - Process.getStartElapsedRealtime();
          }
    }
}

5.ContentProvider初始化时间，时间单位毫秒。 Application.onCreate() 与Application.attachBaseContext(Context context) 之间的时间差

bash 复制代码

class MyApp extends Application {
   long mAttachBaseContextTime = 0L;
   long mContentProviderDuration = 0L;
   @Override
    protected void attachBaseContext(Context base) {
        super.attachBaseContext(base);
        mAttachBaseContextTime = SystemClock.elapsedRealtime();
    }
    @Override
    public void onCreate() {
        super.onCreate();
        mContentProviderDuration = SystemClock.elapsedRealtime() - mAttachBaseContextTime;
    }

Application.onCreate()花费时间，时间单位毫秒。很简单方法开始和结束时间差。
首帧绘制时间，比较复杂，使用到了com.squareup.curtains:curtains:1.0.1代码如下，firstDrawTime就是首帧的绘制时间。从ActivityThread.handleBindApplication()到首帧绘制所花费的时间：

bash 复制代码

class MyApp extends Application {
    @Override
    public void onCreate() {
        super.onCreate();
        registerActivityLifecycleCallbacks(new ActivityLifecycleCallbacks() {
            @Override
            public void onActivityCreated(Activity activity, Bundle savedInstanceState) {
                Window window = activity.getWindow();
                WindowsKt.onNextDraw(window, () -> {
                    if (firstDraw) return null;
                    firstDraw = true;
                    handler.postAtFrontOfQueue(() -> {
                        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
                            long firstDrawTime =  (SystemClock.elapsedRealtime() - Process.getStartElapsedRealtime()));
                        }
                    });
                    return null;
                });
            }
        }
    }
}

调试launchingState.mCurrentTransitionStartTimeNs

由于ActivityMetricsLogger是运行在system_process进程中。我们无法在应用进程中获取到transitionStartTimeNs，我们可以用过Debug打印日志。我们需要将断点设置成non-suspending。如图将Suspend反勾选。选中Evaluate and log，并写入日志语句。

Android启动优化之精确测量启动各个阶段的耗时

1. 直观地观察应用启动时长

2. 启动时间包含哪几个阶段

3. 系统是如何测量启动时长的？

4. 调试技巧