最近遇到 进程被杀的情况，当然这只是Android杀进程的其中一种机制。顺便分析，学习一下。
log如下:

ActivityManager: Killing 19893:com.xxx.xxx/u0a106 (adj 500): excessive cpu 7180 during 300080 dur=42644246 limit=2
ActivityManager: Killing 22598:com.xxx.xxx/u0a106 (adj 500): excessive cpu 21890 during 300019 dur=45344791 limit=2
ActivityManager: Killing 31530:com.xxx.xxx/u0a106 (adj 500): excessive cpu 19230 during 300017 dur=31239455 limit=2
ActivityManager: Killing 22598:com.xxx.xxx/u0a106 (adj 500): excessive cpu 21890 during 300019 dur=45344791 limit=2

AMS 每 5分钟(基于 Android 8.1源码) 检查一下异常耗电情况：

    final void checkExcessivePowerUsageLocked() {
        updateCpuStatsNow();

        BatteryStatsImpl stats = mBatteryStatsService.getActiveStatistics();
        boolean doCpuKills = true;
        if (mLastPowerCheckUptime == 0) {
            doCpuKills = false;
        }
        final long curUptime = SystemClock.uptimeMillis();//从开机到现在的毫秒数,不包括睡眠的时间
        final long uptimeSince = curUptime - mLastPowerCheckUptime;//距上次检查的时间间隔
        mLastPowerCheckUptime = curUptime;
        int i = mLruProcesses.size();
        while (i > 0) {
            i--;
            ProcessRecord app = mLruProcesses.get(i);
            if (app.setProcState >= ActivityManager.PROCESS_STATE_HOME) {
                if (app.lastCpuTime <= 0) {
                    continue;
                }
                long cputimeUsed = app.curCpuTime - app.lastCpuTime;//检查周期内进程CPU使用时长
                if (DEBUG_POWER) {
                    StringBuilder sb = new StringBuilder(128);
                    sb.append("CPU for ");
                    app.toShortString(sb);
                    sb.append(": over ");
                    TimeUtils.formatDuration(uptimeSince, sb);
                    sb.append(" used ");
                    TimeUtils.formatDuration(cputimeUsed, sb);
                    sb.append(" (");
                    sb.append((cputimeUsed*100)/uptimeSince);
                    sb.append("%)");
                    Slog.i(TAG_POWER, sb.toString());
                }
                //随着应用退到后台的时间越来越长，进程被允许使用的CPU占比越来越小
                // If the process has used too much CPU over the last duration, the
                // user probably doesn't want this, so kill!
                if (doCpuKills && uptimeSince > 0) {
                    // What is the limit for this process?
                    int cpuLimit;
                    long checkDur = curUptime - app.whenUnimportant;
                    if (checkDur <= mConstants.POWER_CHECK_INTERVAL) {
                        cpuLimit = mConstants.POWER_CHECK_MAX_CPU_1;//25
                    } else if (checkDur <= (mConstants.POWER_CHECK_INTERVAL*2)
                            || app.setProcState <= ActivityManager.PROCESS_STATE_HOME) {
                        cpuLimit = mConstants.POWER_CHECK_MAX_CPU_2;//25
                    } else if (checkDur <= (mConstants.POWER_CHECK_INTERVAL*3)) {
                        cpuLimit = mConstants.POWER_CHECK_MAX_CPU_3;//10
                    } else {
                        cpuLimit = mConstants.POWER_CHECK_MAX_CPU_4;//2
                    }
                    if (((cputimeUsed*100)/uptimeSince) >= cpuLimit) {
                        synchronized (stats) {
                            stats.reportExcessiveCpuLocked(app.info.uid, app.processName,
                                    uptimeSince, cputimeUsed);
                        }
                        app.kill("excessive cpu " + cputimeUsed + " during " + uptimeSince
                                + " dur=" + checkDur + " limit=" + cpuLimit, true);
                        app.baseProcessTracker.reportExcessiveCpu(app.pkgList);
                    }
                }
                app.lastCpuTime = app.curCpuTime;
            }
        }
    }

随着应用退到后台的时间越来越长，进程被允许使用的CPU占比越来越小：

这个占比的计算方式是：
rate = cputimeUsed*100/uptimeSince 一个检查周期内CPU的使用时长 / 检查周期时长
从上面的log看 是在 Xxx进程退到后台15分钟以后，使用CPU占比超过 2% 触发的。
譬如说进程退到后台15分钟后，在某次检查中，发现5分钟内CPU处于活动状态的时间是 100s, 而该进程占用CPU的时长如果超过 2s (100s * 2% ), 就会触发 杀进程机制。