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    }