Android进程优先级oom_adj

当Android运行了很多进程后，由于内存不足，系统会杀掉一下不重要的进程，提供内存给更重要的进程使用。那么系统是如何对进程的“重要性“进行判定的呢？

命令行中输入cat /proc/process_id/oom_adj 可以查看到某一进程的adj数值，系统就是根据这一数值来判定进程是否被杀或者保留。

ADJ等级定义在frameworks/base/services/core/java/com/android/server/am/ProcessList.java

// 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.
    // 只持有不可用activity的进程，可以随时被杀掉
    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.
    // B List中的老旧，不常用的服务
    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.
    // 上一个应用，因为用户很可能切换回上一个应用，所以显得格外重要
    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.
    // 桌面程序的进程，使用率高
    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.
    // 应用程序服务进程
    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.
    // 后台重量级应用进程
    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.
    // 托管备份操作的进程
    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.
    // 持有用户可感知组件的进程，他不会立即可见。比如后台音乐播放器
    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.
    // 用户可见的activity进程
    static final int VISIBLE_APP_ADJ = 1;

    // This is the process running the current foreground app.  We'd really
    // rather not kill it!
    // 前台应用进程，永远都会避免杀掉他
    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.
    // 由系统或持久性进程所绑定的进程
    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.
    // 持久性进程，例如电话
    static final int PERSISTENT_PROC_ADJ = -12;

    // The system process runs at the default adjustment.
    // 系统进程
    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).
    // native本地进程，不由系统管理，所以不会分配oom_adj
    static final int NATIVE_ADJ = -17;

来看一个实例：
我们经常会有这样的需求：希望我们程序的某一个服务尽可能的不被系统杀掉。我们常用的服务有如下几种，可以通过查看oom_adj来查看他们的优先级。

1.同一进程中的后台服务

2.同一进程中的前台服务

3.新的进程中的后台进程

4.新的进程中的前台进程

对比上面四种情况，2和4是比较稳定的，但是使用新的进程启动服务，可以在不需要主进程的时候，手动杀掉主进程而不会影响到服务进程，是应用内存使用进一步降低。因此，4应该是最好的方法。

前台进程：
一定要有一个notification与之相绑定。

    // 设置为前台服务
    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        Log.i("tag", "MusicService onStartCommand");
        Notification.Builder builder = new Notification.Builder(this.getApplicationContext());
        builder.setSmallIcon(R.drawable.ic_music_note_white_18dp);
        builder.setLargeIcon(BitmapFactory.decodeResource(this.getResources(), R.drawable.ic_music_note_white_36dp));
        builder.setContentTitle("下拉通知中的标题");
        builder.setContentText("下拉通知中要显示的内容");
        builder.setWhen(System.currentTimeMillis());
        Intent clickIntent = new Intent(this, MainActivity.class);
        builder.setContentIntent(PendingIntent.getActivity(this, 0, clickIntent, 0));
        Notification notification = builder.build();
        notification.defaults = Notification.DEFAULT_SOUND;

        startForeground(100, notification);
        return super.onStartCommand(intent, flags, startId);
    }

   // 结束前台服务
   @Override
    public void onDestroy() {
        super.onDestroy();
        stopForeground(true); //参数表示是否移除通知
    }