Android CPU使用率：top和dump cpuinfo的不同

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CPU是系统非常重要的资源，在Android中，查看CPU使用情况，可以使用top命令和dump cpuinfo。我记得很久以前，就发现这两者存在不同，初步猜测应该是算法上存在差异。最近需要采集应用CPU的使用率，看了一下两种CPU的计算方法。

1、top

top是比较经典的CPU计算方法，top的代码在androidm/system/core/toolbox/top.c下面，输出process的cpu使用率在print_procs里面：

static void print_procs(void) { ... for (i = 0; i < num_new_procs; i++) { if (new_procs[i]) { old_proc = find_old_proc(new_procs[i]->pid, new_procs[i]->tid); if (old_proc) { new_procs[i]->delta_utime = new_procs[i]->utime - old_proc->utime; new_procs[i]->delta_stime = new_procs[i]->stime - old_proc->stime; } else { new_procs[i]->delta_utime = 0; new_procs[i]->delta_stime = 0; } new_procs[i]->delta_time = new_procs[i]->delta_utime + new_procs[i]->delta_stime; } } total_delta_time = (new_cpu.utime + new_cpu.ntime + new_cpu.stime + new_cpu.itime + new_cpu.iowtime + new_cpu.irqtime + new_cpu.sirqtime) - (old_cpu.utime + old_cpu.ntime + old_cpu.stime + old_cpu.itime + old_cpu.iowtime + old_cpu.irqtime + old_cpu.sirqtime); ...if (!threads) { printf("%5d %2d %3" PRIu64 "%% %c %5d %6" PRIu64 "K %6" PRIu64 "K %3s %-8.8s %s\n", proc->pid, proc->prs, proc->delta_time * 100 / total_delta_time, proc->state, proc->num_threads, proc->vss / 1024, proc->rss * getpagesize() / 1024, proc->policy, user_str, proc->name[0] != 0 ? proc->name : proc->tname); } else { printf("%5d %5d %2d %3" PRIu64 "%% %c %6" PRIu64 "K %6" PRIu64 "K %3s %-8.8s %-15s %s\n", proc->pid, proc->tid, proc->prs, proc->delta_time * 100 / total_delta_time, proc->state, proc->vss / 1024, proc->rss * getpagesize() / 1024, proc->policy, user_str, proc->tname, proc->name); } ... }

CPU的计算是proc->delta_time * 100 / total_delta_time。

先看total_delta_time由：

total_delta_time = (new_cpu.utime + new_cpu.ntime + new_cpu.stime + new_cpu.itime + new_cpu.iowtime + new_cpu.irqtime + new_cpu.sirqtime) - (old_cpu.utime + old_cpu.ntime + old_cpu.stime + old_cpu.itime + old_cpu.iowtime + old_cpu.irqtime + old_cpu.sirqtime);

而这些变量的值，是在read_procs通过读取/proc/stat的jiffies得到：

static void read_procs(void) { ... proc_dir = opendir("/proc"); if (!proc_dir) die("Could not open /proc.\n"); new_procs = calloc(INIT_PROCS * (threads ? THREAD_MULT : 1), sizeof(struct proc_info *)); num_new_procs = INIT_PROCS * (threads ? THREAD_MULT : 1); file = fopen("/proc/stat", "r"); if (!file) die("Could not open /proc/stat.\n"); fscanf(file, "cpu %lu %lu %lu %lu %lu %lu %lu", &new_cpu.utime, &new_cpu.ntime, &new_cpu.stime, &new_cpu.itime, &new_cpu.iowtime, &new_cpu.irqtime, &new_cpu.sirqtime); fclose(file);

而proc->delta_time是两次读取/proc/pid/stat相减得到：

static int read_stat(char *filename, struct proc_info *proc) { ... /* Scan rest of string. */ sscanf(close_paren + 1, " %c " "%*d %*d %*d %*d %*d %*d %*d %*d %*d %*d " "%" SCNu64 "%" SCNu64 "%*d %*d %*d %*d %*d %*d %*d " "%" SCNu64 "%" SCNu64 "%*d %*d %*d %*d %*d %*d %*d %*d %*d %*d %*d %*d %*d %*d " "%d", &proc->state, &proc->utime, &proc->stime, &proc->vss, &proc->rss, &proc->prs); return 0; }

可见，top是一段时间内，计算process的cpu jiffies与总的cpu jiffies差值得到。

2、dump cpuinfo

dump cpuinfo是Android特有的命令（我一直都android的各种dump、trace非常感兴趣，快玩物丧志了。。。）。dump cpuinfo命令的实现在androidm/frameworks/base/core/Java/com/android/internal/os/ProcessCpuTracker.java类里面，方法是printCurrentState:

final public String printCurrentState(long now) { ... int N = mWorkingProcs.size(); for (int i=0; i<N; i++) { Stats st = mWorkingProcs.get(i); printProcessCPU(pw, st.added ? " +" : (st.removed ? " -": " "), st.pid, st.name, (int)st.rel_uptime, st.rel_utime, st.rel_stime, 0, 0, 0, st.rel_minfaults, st.rel_majfaults); if (!st.removed && st.workingThreads != null) { int M = st.workingThreads.size(); for (int j=0; j<M; j++) { Stats tst = st.workingThreads.get(j); printProcessCPU(pw, tst.added ? " +" : (tst.removed ? " -": " "), tst.pid, tst.name, (int)st.rel_uptime, tst.rel_utime, tst.rel_stime, 0, 0, 0, 0, 0); } } }... }

而printProcessCPU输出process CPU的使用情况：

private void printProcessCPU(PrintWriter pw, String prefix, int pid, String label, int totalTime, int user, int system, int iowait, int irq, int softIrq, int minFaults, int majFaults) { pw.print(prefix); if (totalTime == 0) totalTime = 1; printRatio(pw, user+system+iowait+irq+softIrq, totalTime); ... }

user+system+iowait+irq+softIrq 比totalTime。
看下st个变量的赋值，在collectStats里面：

private int[] collectStats(String statsFile, int parentPid, boolean first, int[] curPids, ArrayList<Stats> allProcs) { int[] pids = Process.getPids(statsFile, curPids); ... final long uptime = SystemClock.uptimeMillis(); final long[] procStats = mProcessStatsData; if (!Process.readProcFile(st.statFile.toString(), PROCESS_STATS_FORMAT, null, procStats, null)) { continue; }... if (DEBUG) Slog.v("Load", "Stats changed " + st.name + " pid=" + st.pid + " utime=" + utime + "-" + st.base_utime + " stime=" + stime + "-" + st.base_stime + " minfaults=" + minfaults + "-" + st.base_minfaults + " majfaults=" + majfaults + "-" + st.base_majfaults); st.rel_uptime = uptime - st.base_uptime; st.base_uptime = uptime; st.rel_utime = (int)(utime - st.base_utime); st.rel_stime = (int)(stime - st.base_stime); st.base_utime = utime; st.base_stime = stime; st.rel_minfaults = (int)(minfaults - st.base_minfaults); st.rel_majfaults = (int)(majfaults - st.base_majfaults); st.base_minfaults = minfaults; st.base_majfaults = majfaults; st.working = true; ... }