接上篇

二 Framework OOM Adjustment 机制

Android 系统（以6.0 AOSP 代码为例），和 LMK 相关的源码主要有：

platform/frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java)
platform/frameworks/base/services/core/java/com/android/server/am/ProcessList.java
platform/system/core/lmkd/lmkd.c
kernel/common/drivers/staging/Android/lowmemorykiller.c

2.1 OOM Adjustment

ADJ 定义在 ProcessList

    // OOM adjustments for processes in various states:
    
    // Adjustment used in certain places where we don't know it yet.
    // (Generally this is something that is going to be cached, but we
    // don't know the exact value in the cached range to assign yet.)
    static final int UNKNOWN_ADJ = 16;
    
    // This is a process only hosting activities that are not visible,
    // so it can be killed without any disruption.
    static final int CACHED_APP_MAX_ADJ = 15;
    static final int CACHED_APP_MIN_ADJ = 9;
    
    // The B list of SERVICE_ADJ -- these are the old and decrepit
    // services that aren't as shiny and interesting as the ones in the A list.
    static final int SERVICE_B_ADJ = 8;
    
    // This is the process of the previous application that the user was in.
    // This process is kept above other things, because it is very common to
    // switch back to the previous app.  This is important both for recent
    // task switch (toggling between the two top recent apps) as well as normal
    // UI flow such as clicking on a URI in the e-mail app to view in the browser,
    // and then pressing back to return to e-mail.
    static final int PREVIOUS_APP_ADJ = 7;
    
    // This is a process holding the home application -- we want to try
    // avoiding killing it, even if it would normally be in the background,
    // because the user interacts with it so much.
    static final int HOME_APP_ADJ = 6;
    
    // This is a process holding an application service -- killing it will not
    // have much of an impact as far as the user is concerned.
    static final int SERVICE_ADJ = 5;
    
    // This is a process with a heavy-weight application.  It is in the
    // background, but we want to try to avoid killing it.  Value set in
    // system/rootdir/init.rc on startup.
    static final int HEAVY_WEIGHT_APP_ADJ = 4;
    
    // This is a process currently hosting a backup operation.  Killing it
    // is not entirely fatal but is generally a bad idea.
    static final int BACKUP_APP_ADJ = 3;
    
    // This is a process only hosting components that are perceptible to the
    // user, and we really want to avoid killing them, but they are not
    // immediately visible. An example is background music playback.
    static final int PERCEPTIBLE_APP_ADJ = 2;
    
    // This is a process only hosting activities that are visible to the
    // user, so we'd prefer they don't disappear.
    static final int VISIBLE_APP_ADJ = 1;
    
    // This is the process running the current foreground app.  We'd really
    // rather not kill it!
    static final int FOREGROUND_APP_ADJ = 0;
    
    // This is a process that the system or a persistent process has bound to,
    // and indicated it is important.
    static final int PERSISTENT_SERVICE_ADJ = -11;
    
    // This is a system persistent process, such as telephony.  Definitely
    // don't want to kill it, but doing so is not completely fatal.
    static final int PERSISTENT_PROC_ADJ = -12;
    
    // The system process runs at the default adjustment.
    static final int SYSTEM_ADJ = -16;
    
    // Special code for native processes that are not being managed by the system (so
    // don't have an oom adj assigned by the system).
    static final int NATIVE_ADJ = -17;

2.2 Process State

在 ActivityManager 中定义了从-1到16共18种进程状态:

在 ActivityManager 中的 RunningAppProcessInfo 类中定义了进程的优先级 IMPORTANCE 值：

        /**
         * Constant for {@link #importance}: This process is running the
         * foreground UI; that is, it is the thing currently at the top of the screen
         * that the user is interacting with.
         */
        public static final int IMPORTANCE_FOREGROUND = 100;
        /**
         * Constant for {@link #importance}: This process is running a foreground
         * service, for example to perform music playback even while the user is
         * not immediately in the app.  This generally indicates that the process
         * is doing something the user actively cares about.
         */
        public static final int IMPORTANCE_FOREGROUND_SERVICE = 125;
        /**
         * Constant for {@link #importance}: This process is running the foreground
         * UI, but the device is asleep so it is not visible to the user.  This means
         * the user is not really aware of the process, because they can not see or
         * interact with it, but it is quite important because it what they expect to
         * return to once unlocking the device.
         */
        public static final int IMPORTANCE_TOP_SLEEPING = 150;
        /**
         * Constant for {@link #importance}: This process is running something
         * that is actively visible to the user, though not in the immediate
         * foreground.  This may be running a window that is behind the current
         * foreground (so paused and with its state saved, not interacting with
         * the user, but visible to them to some degree); it may also be running
         * other services under the system's control that it inconsiders important.
         */
        public static final int IMPORTANCE_VISIBLE = 200;
        /**
         * Constant for {@link #importance}: This process is not something the user
         * is directly aware of, but is otherwise perceptable to them to some degree.
         */
        public static final int IMPORTANCE_PERCEPTIBLE = 130;
        /**
         * Constant for {@link #importance}: This process is running an
         * application that can not save its state, and thus can't be killed
         * while in the background.
         * @hide
         */
        public static final int IMPORTANCE_CANT_SAVE_STATE = 170;
        /**
         * Constant for {@link #importance}: This process is contains services
         * that should remain running.  These are background services apps have
         * started, not something the user is aware of, so they may be killed by
         * the system relatively freely (though it is generally desired that they
         * stay running as long as they want to).
         */
        public static final int IMPORTANCE_SERVICE = 300;
        /**
         * Constant for {@link #importance}: This process process contains
         * background code that is expendable.
         */
        public static final int IMPORTANCE_BACKGROUND = 400;
        /**
         * Constant for {@link #importance}: This process is empty of any
         * actively running code.
         */
        public static final int IMPORTANCE_EMPTY = 500;
        /**
         * Constant for {@link #importance}: This process does not exist.
         */
        public static final int IMPORTANCE_GONE = 1000;
        /** @hide */
        public static int procStateToImportance(int procState) {
            if (procState == PROCESS_STATE_NONEXISTENT) {
                return IMPORTANCE_GONE;
            } else if (procState >= PROCESS_STATE_HOME) {
                return IMPORTANCE_BACKGROUND;
            } else if (procState >= PROCESS_STATE_SERVICE) {
                return IMPORTANCE_SERVICE;
            } else if (procState > PROCESS_STATE_HEAVY_WEIGHT) {
                return IMPORTANCE_CANT_SAVE_STATE;
            } else if (procState >= PROCESS_STATE_IMPORTANT_BACKGROUND) {
                return IMPORTANCE_PERCEPTIBLE;
            } else if (procState >= PROCESS_STATE_IMPORTANT_FOREGROUND) {
                return IMPORTANCE_VISIBLE;
            } else if (procState >= PROCESS_STATE_TOP_SLEEPING) {
                return IMPORTANCE_TOP_SLEEPING;
            } else if (procState >= PROCESS_STATE_FOREGROUND_SERVICE) {
                return IMPORTANCE_FOREGROUND_SERVICE;
            } else {
                return IMPORTANCE_FOREGROUND;
            }
        }

2.3 Framework ADJ 调整算法

ActivityManagerService 中三个核心方法：

• updateOomAdjLocked：更新adj，当目标进程为空，或者被杀则返回false；否则返回true;
• computeOomAdjLocked：计算adj，返回计算后RawAdj值;
• applyOomAdjLocked：应用adj，当需要杀掉目标进程则返回false；否则返回true。

updateOomAdjLocked 中会调用 computeOomAdjLocked 和 applyOomAdjLocked。

ADJ 的更新时机

1. Activity

   • ASS.realStartActivityLocked: 启动Activity
   • AS.resumeTopActivityInnerLocked: 恢复栈顶Activity
   • AS.finishCurrentActivityLocked: 结束当前Activity
   • AS.destroyActivityLocked: 摧毁当前Activity

2. Service

   • realStartServiceLocked: 启动服务
   • bindServiceLocked: 绑定服务(只更新当前app)
   • unbindServiceLocked: 解绑服务 (只更新当前app)
   • bringDownServiceLocked: 结束服务 (只更新当前app)
   • sendServiceArgsLocked: 在bringup或则cleanup服务过程调用 (只更新当前app)

3. Broadcast

   • BQ.processNextBroadcast: 处理下一个广播
   • BQ.processCurBroadcastLocked: 处理当前广播
   • BQ.deliverToRegisteredReceiverLocked: 分发已注册的广播 (只更新当前app)

4. ContentProvider

   • AMS.removeContentProvider: 移除provider
   • AMS.publishContentProviders: 发布provider (只更新当前app)
   • AMS.getContentProviderImpl: 获取provider (只更新当前app)

5. Process

   • AMS.setSystemProcess: 创建并设置系统进程
   • AMS.addAppLocked: 创建persistent进程
   • AMS.attachApplicationLocked: 进程创建后attach到system_server的过程;
   • AMS.trimApplications: 清除没有使用app
   • AMS.appDiedLocked: 进程死亡
   • AMS.killAllBackgroundProcesses: 杀死所有后台进程.即(ADJ>9或removed=true的普通进程)
   • AMS.killPackageProcessesLocked: 以包名的形式 杀掉相关进程;

updateOomAdjLocked

ActivityManagerService 会在每次需要更新 app oom_adj 的时候调用updateOomAdjLocked 方法更新每个进程的 oom_adj。

final void updateOomAdjLocked() {
        // 获取栈顶 Activity
        final ActivityRecord TOP_ACT = resumedAppLocked();
        final ProcessRecord TOP_APP = TOP_ACT != null ? TOP_ACT.app : null;
        final long now = SystemClock.uptimeMillis();
        final long nowElapsed = SystemClock.elapsedRealtime();
        final long oldTime = now - ProcessList.MAX_EMPTY_TIME;
        final int N = mLruProcesses.size();
        if (false) {
            RuntimeException e = new RuntimeException();
            e.fillInStackTrace();
            Slog.i(TAG, "updateOomAdj: top=" + TOP_ACT, e);
        }
        // 重置所有uid记录，把curProcState设置成16(空进程)
        // Reset state in all uid records.
        for (int i=mActiveUids.size()-1; i>=0; i--) {
            final UidRecord uidRec = mActiveUids.valueAt(i);
            if (false && DEBUG_UID_OBSERVERS) Slog.i(TAG_UID_OBSERVERS,
                    "Starting update of " + uidRec);
            uidRec.reset();
        }
        mAdjSeq++;
        mNewNumServiceProcs = 0;
        mNewNumAServiceProcs = 0;
        final int emptyProcessLimit;
        final int cachedProcessLimit;
        
        // mProcessLimit默认值等于32，通过开发者选择可设置，或者厂商会自行调整
        if (mProcessLimit <= 0) {
            emptyProcessLimit = cachedProcessLimit = 0;
        } else if (mProcessLimit == 1) {
            emptyProcessLimit = 1;
            cachedProcessLimit = 0;
        } else {
            // emptyProcessLimit = 16， cachedProcessLimit = 16
            emptyProcessLimit = ProcessList.computeEmptyProcessLimit(mProcessLimit);
            cachedProcessLimit = mProcessLimit - emptyProcessLimit;
        }
        // Let's determine how many processes we have running vs.
        // how many slots we have for background processes; we may want
        // to put multiple processes in a slot of there are enough of
        // them.
        // numSlots = 3
        int numSlots = (ProcessList.CACHED_APP_MAX_ADJ
                - ProcessList.CACHED_APP_MIN_ADJ + 1) / 2;
        int numEmptyProcs = N - mNumNonCachedProcs - mNumCachedHiddenProcs;
        if (numEmptyProcs > cachedProcessLimit) {
            // If there are more empty processes than our limit on cached
            // processes, then use the cached process limit for the factor.
            // This ensures that the really old empty processes get pushed
            // down to the bottom, so if we are running low on memory we will
            // have a better chance at keeping around more cached processes
            // instead of a gazillion empty processes.
            numEmptyProcs = cachedProcessLimit;
        }
        int emptyFactor = numEmptyProcs/numSlots;
        if (emptyFactor < 1) emptyFactor = 1;
        int cachedFactor = (mNumCachedHiddenProcs > 0 ? mNumCachedHiddenProcs : 1)/numSlots;
        if (cachedFactor < 1) cachedFactor = 1;
        int stepCached = 0;
        int stepEmpty = 0;
        int numCached = 0;
        int numEmpty = 0;
        int numTrimming = 0;
        mNumNonCachedProcs = 0;
        mNumCachedHiddenProcs = 0;
        // First update the OOM adjustment for each of the
        // application processes based on their current state.
        int curCachedAdj = ProcessList.CACHED_APP_MIN_ADJ;
        int nextCachedAdj = curCachedAdj+1;
        int curEmptyAdj = ProcessList.CACHED_APP_MIN_ADJ;
        int nextEmptyAdj = curEmptyAdj+2;
        for (int i=N-1; i>=0; i--) {
            ProcessRecord app = mLruProcesses.get(i);
            if (!app.killedByAm && app.thread != null) {
                app.procStateChanged = false;
                computeOomAdjLocked(app, ProcessList.UNKNOWN_ADJ, TOP_APP, true, now);
                // If we haven't yet assigned the final cached adj
                // to the process, do that now.
                if (app.curAdj >= ProcessList.UNKNOWN_ADJ) {
                    switch (app.curProcState) {
                        case ActivityManager.PROCESS_STATE_CACHED_ACTIVITY:
                        case ActivityManager.PROCESS_STATE_CACHED_ACTIVITY_CLIENT:
                            // This process is a cached process holding activities...
                            // assign it the next cached value for that type, and then
                            // step that cached level.
                            app.curRawAdj = curCachedAdj;
                            app.curAdj = app.modifyRawOomAdj(curCachedAdj);
                            if (DEBUG_LRU && false) Slog.d(TAG_LRU, "Assigning activity LRU #" + i
                                    + " adj: " + app.curAdj + " (curCachedAdj=" + curCachedAdj
                                    + ")");
                            if (curCachedAdj != nextCachedAdj) {
                                stepCached++;
                                if (stepCached >= cachedFactor) {
                                    stepCached = 0;
                                    curCachedAdj = nextCachedAdj;
                                    nextCachedAdj += 2;
                                    if (nextCachedAdj > ProcessList.CACHED_APP_MAX_ADJ) {
                                        nextCachedAdj = ProcessList.CACHED_APP_MAX_ADJ;
                                    }
                                }
                            }
                            break;
                        default:
                            // For everything else, assign next empty cached process
                            // level and bump that up.  Note that this means that
                            // long-running services that have dropped down to the
                            // cached level will be treated as empty (since their process
                            // state is still as a service), which is what we want.
                            app.curRawAdj = curEmptyAdj;
                            app.curAdj = app.modifyRawOomAdj(curEmptyAdj);
                            if (DEBUG_LRU && false) Slog.d(TAG_LRU, "Assigning empty LRU #" + i
                                    + " adj: " + app.curAdj + " (curEmptyAdj=" + curEmptyAdj
                                    + ")");
                            if (curEmptyAdj != nextEmptyAdj) {
                                stepEmpty++;
                                if (stepEmpty >= emptyFactor) {
                                    stepEmpty = 0;
                                    curEmptyAdj = nextEmptyAdj;
                                    nextEmptyAdj += 2;
                                    if (nextEmptyAdj > ProcessList.CACHED_APP_MAX_ADJ) {
                                        nextEmptyAdj = ProcessList.CACHED_APP_MAX_ADJ;
                                    }
                                }
                            }
                            break;
                    }
                }
                applyOomAdjLocked(app, true, now, nowElapsed);
                // Count the number of process types.
                switch (app.curProcState) {
                    case ActivityManager.PROCESS_STATE_CACHED_ACTIVITY:
                    case ActivityManager.PROCESS_STATE_CACHED_ACTIVITY_CLIENT:
                        mNumCachedHiddenProcs++;
                        numCached++;
                        if (numCached > cachedProcessLimit) {
                            app.kill("cached #" + numCached, true);
                        }
                        break;
                    case ActivityManager.PROCESS_STATE_CACHED_EMPTY:
                        if (numEmpty > ProcessList.TRIM_EMPTY_APPS
                                && app.lastActivityTime < oldTime) {
                            app.kill("empty for "
                                    + ((oldTime + ProcessList.MAX_EMPTY_TIME - app.lastActivityTime)
                                    / 1000) + "s", true);
                        } else {
                            numEmpty++;
                            if (numEmpty > emptyProcessLimit) {
                                app.kill("empty #" + numEmpty, true);
                            }
                        }
                        break;
                    default:
                        mNumNonCachedProcs++;
                        break;
                }
                if (app.isolated && app.services.size() <= 0) {
                    // If this is an isolated process, and there are no
                    // services running in it, then the process is no longer
                    // needed.  We agressively kill these because we can by
                    // definition not re-use the same process again, and it is
                    // good to avoid having whatever code was running in them
                    // left sitting around after no longer needed.
                    app.kill("isolated not needed", true);
                } else {
                    // Keeping this process, update its uid.
                    final UidRecord uidRec = app.uidRecord;
                    if (uidRec != null && uidRec.curProcState > app.curProcState) {
                        uidRec.curProcState = app.curProcState;
                    }
                }
                if (app.curProcState >= ActivityManager.PROCESS_STATE_HOME
                        && !app.killedByAm) {
                    numTrimming++;
                }
            }
        }
        mNumServiceProcs = mNewNumServiceProcs;
        // Now determine the memory trimming level of background processes.
        // Unfortunately we need to start at the back of the list to do this
        // properly.  We only do this if the number of background apps we
        // are managing to keep around is less than half the maximum we desire;
        // if we are keeping a good number around, we'll let them use whatever
        // memory they want.
        final int numCachedAndEmpty = numCached + numEmpty;
        int memFactor;
        if (numCached <= ProcessList.TRIM_CACHED_APPS
                && numEmpty <= ProcessList.TRIM_EMPTY_APPS) {
            if (numCachedAndEmpty <= ProcessList.TRIM_CRITICAL_THRESHOLD) {
                memFactor = ProcessStats.ADJ_MEM_FACTOR_CRITICAL;
            } else if (numCachedAndEmpty <= ProcessList.TRIM_LOW_THRESHOLD) {
                memFactor = ProcessStats.ADJ_MEM_FACTOR_LOW;
            } else {
                memFactor = ProcessStats.ADJ_MEM_FACTOR_MODERATE;
            }
        } else {
            memFactor = ProcessStats.ADJ_MEM_FACTOR_NORMAL;
        }
        // We always allow the memory level to go up (better).  We only allow it to go
        // down if we are in a state where that is allowed, *and* the total number of processes
        // has gone down since last time.
        if (DEBUG_OOM_ADJ) Slog.d(TAG_OOM_ADJ, "oom: memFactor=" + memFactor
                + " last=" + mLastMemoryLevel + " allowLow=" + mAllowLowerMemLevel
                + " numProcs=" + mLruProcesses.size() + " last=" + mLastNumProcesses);
        if (memFactor > mLastMemoryLevel) {
            if (!mAllowLowerMemLevel || mLruProcesses.size() >= mLastNumProcesses) {
                memFactor = mLastMemoryLevel;
                if (DEBUG_OOM_ADJ) Slog.d(TAG_OOM_ADJ, "Keeping last mem factor!");
            }
        }
        mLastMemoryLevel = memFactor;
        mLastNumProcesses = mLruProcesses.size();
        boolean allChanged = mProcessStats.setMemFactorLocked(memFactor, !isSleeping(), now);
        final int trackerMemFactor = mProcessStats.getMemFactorLocked();
        if (memFactor != ProcessStats.ADJ_MEM_FACTOR_NORMAL) {
            if (mLowRamStartTime == 0) {
                mLowRamStartTime = now;
            }
            int step = 0;
            int fgTrimLevel;
            switch (memFactor) {
                case ProcessStats.ADJ_MEM_FACTOR_CRITICAL:
                    fgTrimLevel = ComponentCallbacks2.TRIM_MEMORY_RUNNING_CRITICAL;
                    break;
                case ProcessStats.ADJ_MEM_FACTOR_LOW:
                    fgTrimLevel = ComponentCallbacks2.TRIM_MEMORY_RUNNING_LOW;
                    break;
                default:
                    fgTrimLevel = ComponentCallbacks2.TRIM_MEMORY_RUNNING_MODERATE;
                    break;
            }
            int factor = numTrimming/3;
            int minFactor = 2;
            if (mHomeProcess != null) minFactor++;
            if (mPreviousProcess != null) minFactor++;
            if (factor < minFactor) factor = minFactor;
            int curLevel = ComponentCallbacks2.TRIM_MEMORY_COMPLETE;
            for (int i=N-1; i>=0; i--) {
                ProcessRecord app = mLruProcesses.get(i);
                if (allChanged || app.procStateChanged) {
                    setProcessTrackerStateLocked(app, trackerMemFactor, now);
                    app.procStateChanged = false;
                }
                if (app.curProcState >= ActivityManager.PROCESS_STATE_HOME
                        && !app.killedByAm) {
                    if (app.trimMemoryLevel < curLevel && app.thread != null) {
                        try {
                            if (DEBUG_SWITCH || DEBUG_OOM_ADJ) Slog.v(TAG_OOM_ADJ,
                                    "Trimming memory of " + app.processName + " to " + curLevel);
                            app.thread.scheduleTrimMemory(curLevel);
                        } catch (RemoteException e) {
                        }
                        if (false) {
                            // For now we won't do this; our memory trimming seems
                            // to be good enough at this point that destroying
                            // activities causes more harm than good.
                            if (curLevel >= ComponentCallbacks2.TRIM_MEMORY_COMPLETE
                                    && app != mHomeProcess && app != mPreviousProcess) {
                                // Need to do this on its own message because the stack may not
                                // be in a consistent state at this point.
                                // For these apps we will also finish their activities
                                // to help them free memory.
                                mStackSupervisor.scheduleDestroyAllActivities(app, "trim");
                            }
                        }
                    }
                    app.trimMemoryLevel = curLevel;
                    step++;
                    if (step >= factor) {
                        step = 0;
                        switch (curLevel) {
                            case ComponentCallbacks2.TRIM_MEMORY_COMPLETE:
                                curLevel = ComponentCallbacks2.TRIM_MEMORY_MODERATE;
                                break;
                            case ComponentCallbacks2.TRIM_MEMORY_MODERATE:
                                curLevel = ComponentCallbacks2.TRIM_MEMORY_BACKGROUND;
                                break;
                        }
                    }
                } else if (app.curProcState == ActivityManager.PROCESS_STATE_HEAVY_WEIGHT) {
                    if (app.trimMemoryLevel < ComponentCallbacks2.TRIM_MEMORY_BACKGROUND
                            && app.thread != null) {
                        try {
                            if (DEBUG_SWITCH || DEBUG_OOM_ADJ) Slog.v(TAG_OOM_ADJ,
                                    "Trimming memory of heavy-weight " + app.processName
                                    + " to " + ComponentCallbacks2.TRIM_MEMORY_BACKGROUND);
                            app.thread.scheduleTrimMemory(
                                    ComponentCallbacks2.TRIM_MEMORY_BACKGROUND);
                        } catch (RemoteException e) {
                        }
                    }
                    app.trimMemoryLevel = ComponentCallbacks2.TRIM_MEMORY_BACKGROUND;
                } else {
                    if ((app.curProcState >= ActivityManager.PROCESS_STATE_IMPORTANT_BACKGROUND
                            || app.systemNoUi) && app.pendingUiClean) {
                        // If this application is now in the background and it
                        // had done UI, then give it the special trim level to
                        // have it free UI resources.
                        final int level = ComponentCallbacks2.TRIM_MEMORY_UI_HIDDEN;
                        if (app.trimMemoryLevel < level && app.thread != null) {
                            try {
                                if (DEBUG_SWITCH || DEBUG_OOM_ADJ) Slog.v(TAG_OOM_ADJ,
                                        "Trimming memory of bg-ui " + app.processName
                                        + " to " + level);
                                app.thread.scheduleTrimMemory(level);
                            } catch (RemoteException e) {
                            }
                        }
                        app.pendingUiClean = false;
                    }
                    if (app.trimMemoryLevel < fgTrimLevel && app.thread != null) {
                        try {
                            if (DEBUG_SWITCH || DEBUG_OOM_ADJ) Slog.v(TAG_OOM_ADJ,
                                    "Trimming memory of fg " + app.processName
                                    + " to " + fgTrimLevel);
                            app.thread.scheduleTrimMemory(fgTrimLevel);
                        } catch (RemoteException e) {
                        }
                    }
                    app.trimMemoryLevel = fgTrimLevel;
                }
            }
        } else {
            if (mLowRamStartTime != 0) {
                mLowRamTimeSinceLastIdle += now - mLowRamStartTime;
                mLowRamStartTime = 0;
            }
            for (int i=N-1; i>=0; i--) {
                ProcessRecord app = mLruProcesses.get(i);
                if (allChanged || app.procStateChanged) {
                    setProcessTrackerStateLocked(app, trackerMemFactor, now);
                    app.procStateChanged = false;
                }
                if ((app.curProcState >= ActivityManager.PROCESS_STATE_IMPORTANT_BACKGROUND
                        || app.systemNoUi) && app.pendingUiClean) {
                    if (app.trimMemoryLevel < ComponentCallbacks2.TRIM_MEMORY_UI_HIDDEN
                            && app.thread != null) {
                        try {
                            if (DEBUG_SWITCH || DEBUG_OOM_ADJ) Slog.v(TAG_OOM_ADJ,
                                    "Trimming memory of ui hidden " + app.processName
                                    + " to " + ComponentCallbacks2.TRIM_MEMORY_UI_HIDDEN);
                            app.thread.scheduleTrimMemory(
                                    ComponentCallbacks2.TRIM_MEMORY_UI_HIDDEN);
                        } catch (RemoteException e) {
                        }
                    }
                    app.pendingUiClean = false;
                }
                app.trimMemoryLevel = 0;
            }
        }
        if (mAlwaysFinishActivities) {
            // Need to do this on its own message because the stack may not
            // be in a consistent state at this point.
            mStackSupervisor.scheduleDestroyAllActivities(null, "always-finish");
        }
        if (allChanged) {
            requestPssAllProcsLocked(now, false, mProcessStats.isMemFactorLowered());
        }
        // Update from any uid changes.
        for (int i=mActiveUids.size()-1; i>=0; i--) {
            final UidRecord uidRec = mActiveUids.valueAt(i);
            if (uidRec.setProcState != uidRec.curProcState) {
                if (DEBUG_UID_OBSERVERS) Slog.i(TAG_UID_OBSERVERS,
                        "Changes in " + uidRec + ": proc state from " + uidRec.setProcState
                        + " to " + uidRec.curProcState);
                uidRec.setProcState = uidRec.curProcState;
                enqueueUidChangeLocked(uidRec, false);
            }
        }
        if (mProcessStats.shouldWriteNowLocked(now)) {
            mHandler.post(new Runnable() {
                @Override public void run() {
                    synchronized (ActivityManagerService.this) {
                        mProcessStats.writeStateAsyncLocked();
                    }
                }
            });
        }
        if (DEBUG_OOM_ADJ) {
            final long duration = SystemClock.uptimeMillis() - now;
            if (false) {
                Slog.d(TAG_OOM_ADJ, "Did OOM ADJ in " + duration + "ms",
                        new RuntimeException("here").fillInStackTrace());
            } else {
                Slog.d(TAG_OOM_ADJ, "Did OOM ADJ in " + duration + "ms");
            }
        }
    }

NEXT:http://www.jianshu.com/p/04901127fd18