（1） 在init.rc中 系统定义了
Lowmeme_adj中各项数值代表阈值的警戒级数，lowmem_minfree代表对应级数的剩余内存
当系统的剩余内存为小于6MB时候，警戒级数为0，当系统内存剩余小于8M而大于6M的时候，警戒级数为1，当内存小于64M大于16MB的时候，警戒级数为12.Low memory killer 的规则就是根据当前系统的剩余内存多少来获取当前的警戒级数，如果进程的oom_adj大于警戒级数并且最大，进程将会被杀死（相同omm_adj的，则杀死占用内存较多的）。Omm_adj越小，代表进程越重要。一些前台的进程，oom_adj会比较小，而后台的服务，omm_adj会比较大，所以当内存不足的时候，Low memory killer 杀掉的必然先杀掉的是后台服务而不是前台的进程。
# Define the oom_adj values for the classes of processes that can be
# killed by the kernel. These are used in ActivityManagerService.
setprop ro.FOREGROUND_APP_ADJ 0
setprop ro.VISIBLE_APP_ADJ 1
setprop ro.PERCEPTIBLE_APP_ADJ 2
setprop ro.HEAVY_WEIGHT_APP_ADJ 3
setprop ro.SECONDARY_SERVER_ADJ 4
setprop ro.BACKUP_APP_ADJ 5
setprop ro.HOME_APP_ADJ 6
setprop ro.HIDDEN_APP_MIN_ADJ 7
setprop ro.EMPTY_APP_ADJ 15
# Define the memory thresholds at which the above process classes will
# be killed. These numbers are in pages (4k).
setprop ro.FOREGROUND_APP_MEM 2048
setprop ro.VISIBLE_APP_MEM 3072
setprop ro.PERCEPTIBLE_APP_MEM 4096
setprop ro.HEAVY_WEIGHT_APP_MEM 4096
setprop ro.SECONDARY_SERVER_MEM 6144
setprop ro.BACKUP_APP_MEM 6144
setprop ro.HOME_APP_MEM 6144
setprop ro.HIDDEN_APP_MEM 7168
setprop ro.EMPTY_APP_MEM 8192
（2） VM 的GC 一分钟轮循一次processGC列表
当满足以下条件时会回收内存
while (mProcessesToGc.size() > 0) {
ProcessRecord proc = mProcessesToGc.remove(0);
if (proc.curRawAdj > PERCEPTIBLE_APP_ADJ || proc.reportLowMemory) {
if ((proc.lastRequestedGc+GC_MIN_INTERVAL)
<= SystemClock.uptimeMillis()) {
// To avoid spamming the system, we will GC processes one
// at a time, waiting a few seconds between each.
performAppGcLocked(proc);
scheduleAppGcsLocked();
return;
} else {
// It hasn't been long enough since we last GCed this
// process... put it in the list to wait for its time.
addProcessToGcListLocked(proc);
break;
}
}
}
1 process 不是前台进程，可视进程，及优先级低于 PERCEPTIBLE，可以关闭该进程的后台程序及释放其内存
2 process 在进程关闭之前断开binder连接，通知VM关闭java程序申请的内存
（3）VM 回收内存类型
/* Not enough space for an "ordinary" Object to be allocated. */
GC_FOR_MALLOC,
/* Automatic GC triggered by exceeding a heap occupancy threshold. */
GC_CONCURRENT,
/* Explicit GC via Runtime.gc(), VMRuntime.gc(), or SIGUSR1. */
GC_EXPLICIT,
/* GC to try to reduce heap footprint to allow more non-GC'ed memory. */
GC_EXTERNAL_ALLOC,
/* GC to dump heap contents to a file, only used under WITH_HPROF */
GC_HPROF_DUMP_HEAP