Android ActivityManagerService根据oom_adj数值内存回收机制

当系统的内存不足时, Android系统将根据进程优先级选择杀死一些不太重要的进程。

那么进程的优先级是怎样判别的呢？对，就是这个根据进程的oom_adj值。oom_adj的值越小，进程的优先级越高。

如何查看某个应用的oom_adj数值？

首先adb shell#ps查看应用的PID

然后#cat  /proc/PID/oom_adj的结果就是。

ProcessList中对oom_adj的定义，里面加了一些我个人的翻译，可能不是很精确。

frameworks/base/services/java/com/android/server/am/ProcessList.java:

    // The minimum time we allow between crashes, for us to consider this
    // application to be bad and stop and its services and reject broadcasts.
    static final int MIN_CRASH_INTERVAL = 60*1000;

    // 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.)
    // ZMS:用作对一些特定的我们未知的地方进行调整。
    //（通常是一些缓存，但是我们并不知道确切的缓存划分）
    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.
    // ZMS:这个是一个仅仅拥有后台不可见Activity的进程，可以被无破坏杀掉。
    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.
    // ZMS:B列表Service ADJ--B列表Service是一些老旧的Service，没有A列表里的Service新。
    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.
    // ZMS:这是用户前一个使用应用的进程。此进程优先级之所以排到其他缓存进程之上，
    // 是因为切回前一个应用的场景很常见……
    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.
    // ZMS:主界面进程--尽管它经常在后台，我们同样想避免杀掉它，毕竟用户和主界面交互很多。
    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.
    // ZMS:服务进程--由于用户关切，杀掉它会有不小影响。
    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.
    // ZMS:重量级应用进程。它在后台，但是我们想避免杀掉它。
    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.
    // ZMS:执行备份操作的进程。杀掉它不完全致命，但通常是个坏想法。
    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.
    // ZMS:拥有用户可感知组件的进程，所以我们尽量要避免杀掉它，尽管它不是立即可见。
    // 例如：后台音乐播放。
    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.
    // ZMS:此进程仅仅拥有用户可见的Activity，所以我们不希望它消失。
    static final int VISIBLE_APP_ADJ = 1;

    // This is the process running the current foreground app.  We'd really
    // rather not kill it!
    // ZMS:前台应用进程。最好不要杀掉它！
    static final int FOREGROUND_APP_ADJ = 0;

    // This is a system persistent process, such as telephony.  Definitely
    // don't want to kill it, but doing so is not completely fatal.
    // ZMS:系统常驻进程，比如电话。绝对不想要杀死它，但是即便杀死也不是很致命。
    static final int PERSISTENT_PROC_ADJ = -12;

    // The system process runs at the default adjustment.
    // ZMS: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).
    // ZMS:为native进程保留，他们不被系统管理。
    static final int NATIVE_ADJ = -17;

    // Memory pages are 4K.
    static final int PAGE_SIZE = 4*1024;

    // The minimum number of cached apps we want to be able to keep around,
    // without empty apps being able to push them out of memory.
    static final int MIN_CACHED_APPS = 2;

    // The maximum number of cached processes we will keep around before killing them.
    // NOTE: this constant is *only* a control to not let us go too crazy with
    // keeping around processes on devices with large amounts of RAM.  For devices that
    // are tighter on RAM, the out of memory killer is responsible for killing background
    // processes as RAM is needed, and we should *never* be relying on this limit to
    // kill them.  Also note that this limit only applies to cached background processes;
    // we have no limit on the number of service, visible, foreground, or other such
    // processes and the number of those processes does not count against the cached
    // process limit.
    static final int MAX_CACHED_APPS = 24;

    // We allow empty processes to stick around for at most 30 minutes.
    static final long MAX_EMPTY_TIME = 30*60*1000;

    // The maximum number of empty app processes we will let sit around.
    private static final int MAX_EMPTY_APPS = computeEmptyProcessLimit(MAX_CACHED_APPS);

    // The number of empty apps at which we don't consider it necessary to do
    // memory trimming.
    static final int TRIM_EMPTY_APPS = MAX_EMPTY_APPS/2;

    // The number of cached at which we don't consider it necessary to do
    // memory trimming.
    static final int TRIM_CACHED_APPS = ((MAX_CACHED_APPS-MAX_EMPTY_APPS)*2)/3;

    // Threshold of number of cached+empty where we consider memory critical.
    static final int TRIM_CRITICAL_THRESHOLD = 3;

    // Threshold of number of cached+empty where we consider memory critical.
    static final int TRIM_LOW_THRESHOLD = 5;

    // These are the various interesting memory levels that we will give to
    // the OOM killer.  Note that the OOM killer only supports 6 slots, so we
    // can't give it a different value for every possible kind of process.
    private final int[] mOomAdj = new int[] {
            FOREGROUND_APP_ADJ, VISIBLE_APP_ADJ, PERCEPTIBLE_APP_ADJ,
            BACKUP_APP_ADJ, CACHED_APP_MIN_ADJ, CACHED_APP_MAX_ADJ
    };
    // These are the low-end OOM level limits.  This is appropriate for an
    // HVGA or smaller phone with less than 512MB.  Values are in KB.
    private final long[] mOomMinFreeLow = new long[] {
            8192, 12288, 16384,
            24576, 28672, 32768
    };
    // These are the high-end OOM level limits.  This is appropriate for a
    // 1280x800 or larger screen with around 1GB RAM.  Values are in KB.
    private final long[] mOomMinFreeHigh = new long[] {
            49152, 61440, 73728,
            86016, 98304, 122880
    };

以上代码的后面又两个数组，定义了开始回收的阈值（单位KB）。

adb shell#cat /sys/module/lowmemorykiller/parameters/minfree

查看机器当前的设定，结果乘以PAGE_SIZE 即是对应的内存阈值。

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修改指定包名应用的oom_adj，避免被系统回收：

Android 4.4:frameworks/base/services/java/com/android/server/am/ActivityManagerService.java

Android 5.1:frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java

@@ -8903,6 +8903,11 @@ public final class ActivityManagerService extends ActivityManagerNative
             app.persistent = true;
             app.maxAdj = ProcessList.PERSISTENT_PROC_ADJ;
         }
+       if("com.zms.test".equals(info.packageName)){
+           app.persistent = true;
+           app.maxAdj = ProcessList.SYSTEM_ADJ;
+       }
         if (app.thread == null && mPersistentStartingProcesses.indexOf(app) < 0) {
             mPersistentStartingProcesses.add(app);
             startProcessLocked(app, "added application", app.processName);