在AOSP中,Low Memory Killer(LMK)是一种内存管理机制,它根据进程的优先级来决定哪些进程应该被终止以释放内存。这个机制通过内核任务(kswapd)来实现,该任务不断扫描内存使用情况,并在内存不足时触发LMK机制。在LMK机制中,每个进程都被分配了一个优先级,当内存不足时,系统会根据进程的优先级选择性地终止某些进程,以释放更多的内存。
/**
* Set the out-of-memory badness adjustment for a process.
* If {@code pid <= 0}, this method will be a no-op.
*
* @param pid The process identifier to set.
* @param uid The uid of the app
* @param amt Adjustment value -- lmkd allows -1000 to +1000
*
* {@hide}
*/
public static void setOomAdj(int pid, int uid, int amt) {
// This indicates that the process is not started yet and so no need to proceed further.
if (pid <= 0) {
return;
}
if (amt == UNKNOWN_ADJ)
return;
long start = SystemClock.elapsedRealtime();
ByteBuffer buf = ByteBuffer.allocate(4 * 4);
buf.putInt(LMK_PROCPRIO);
buf.putInt(pid);
buf.putInt(uid);
buf.putInt(amt);
writeLmkd(buf, null);
long now = SystemClock.elapsedRealtime();
if ((now-start) > 250) {
Slog.w("ActivityManager", "SLOW OOM ADJ: " + (now-start) + "ms for pid " + pid
+ " = " + amt);
}
}
可以通过ProcessList.java
中的setOomAdj
方法去调整进程的优先级, OOM_ADJ (Out-Of-Memory Adjustment),进程优先级影响系统对进程的内存回收策略。OOM_ADJ 值越低,表示该进程的优先级越高,系统回收内存时越不会杀死该进程。OOM_ADJ 值越高,则表示该进程的优先级越低,系统回收内存时越容易杀死该进程。
在 Android 中,OOM_ADJ 值的范围为 -1000 到 1000,其中 -1000 表示最高优先级(最不容易被杀死),而 1000 表示最低优先级(最容易被杀死)。具体来说,-1000 到 -900 被认为是“前台进程”;-800 到 0 被认为是“可见进程”;而 1000 则被认为是“空进程”。
关于 -1000 和 1000 优先级的区别,主要体现在它们所代表的进程类型不同。具体来说:
Line 2813: S000AFC 07-25 09:35 ActivityManager: Killing process com.example.app (adj 975):**empty** for 2011s
Line 2813: S000AFC 07-25 09:35 ActivityManager: Killing process com.example.app (adj 900):**remove task**
Line 2813: S000AFC 07-25 09:35 ActivityManager: Killing process com.example.app (adj 910):**stop xx due to from pid 1xx**
Line 2813: S000AFC 07-25 09:35 ActivityManager: Killing process com.example.app (adj 965):**rampolicy available memory:1001**
可以从日志上看到进程被kill的原因(am kill),有进程优先级,900~1000都属于要被kill的对象,例如empty原因,也就是empty对象达到上限,好像是10来个,就会触发empty kill。
再例如rampolicy原因,由内存策略进行的一个kill,应该是低内存原因
ProcessRecord newProcessRecordLocked(ApplicationInfo info, String customProcess,
boolean isolated, int isolatedUid, HostingRecord hostingRecord) {
String proc = customProcess != null ? customProcess : info.processName;
final int userId = UserHandle.getUserId(info.uid);
int uid = info.uid;
if (isolated) {
...
**final ProcessRecord r = new ProcessRecord(mService, info, proc, uid);**
final ProcessStateRecord state = r.mState;
**if (!mService.mBooted && !mService.mBooting
&& userId == UserHandle.USER_SYSTEM
&& (info.flags & PERSISTENT_MASK) == PERSISTENT_MASK) {
// The system process is initialized to SCHED_GROUP_DEFAULT in init.rc.
state.setCurrentSchedulingGroup(ProcessList.SCHED_GROUP_DEFAULT);
state.setSetSchedGroup(ProcessList.SCHED_GROUP_DEFAULT);
r.setPersistent(true);
state.setMaxAdj(ProcessList.PERSISTENT_PROC_ADJ);
}**
...
addProcessNameLocked(r);
return r;
}
以上代码是应用的启动流程,也是系统进程的启动流程,如果发现是系统服务,也就是**userId == UserHandle.USER_SYSTEM
,并且applicationInfo(应用可以在AndroidManifest.xml中配置)带有PERSISTENT_MASK相关flag,就会将ProcessRecord(一个进程存在一个该实例),设置为持久进程:r.setPersistent(true);
**
再来看看进程被kill的代码,AMS中调用forceStopPackage,killProcess进行杀进程,然后调用到ProcessList#removeProcessLocked,方法如下
boolean removeProcessLocked(ProcessRecord app, boolean callerWillRestart,
boolean allowRestart, int reasonCode, int subReason, String reason) {
final String name = app.processName;
final int uid = app.uid;
...
boolean needRestart = false;
final int pid = app.getPid();
if ((pid > 0 && pid != ActivityManagerService.MY_PID)
|| (pid == 0 && app.isPendingStart())) {
...
boolean willRestart = false;
Log.d(TAG, "removeProcessLocked: KeepAliveManager app:"+app.processName
+",isPersistent:"+app.isPersistent()+",app.isolated:"+app.isolated);
if (**app.isPersistent()** && !app.isolated) {
if (!callerWillRestart) {
**willRestart = true;**
} else {
**needRestart = true;**
}
}
app.killLocked(reason, reasonCode, subReason, true);
mService.handleAppDiedLocked(app, pid, willRestart, allowRestart,
false /* fromBinderDied */);
**if (willRestart)** {
removeLruProcessLocked(app);
**mService.addAppLocked(app.info, null, false, null /* ABI override */,
ZYGOTE_POLICY_FLAG_EMPTY);**
}
} else {
mRemovedProcesses.add(app);
}
return needRestart;
}
应用被杀的流程,会检查是否是持久进程,也就是**app.isPersistent()
,如果是则会将willRestart置为true,从而触发mService.addAppLocked
**,去启动应用(也就是重新走进程启动流程,会回到最开始的newProcessRecordLocked方法)