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);