CPU是系統非常重要的資源，在Android中，檢視CPU使用情況，可以使用top命令和dump cpuinfo。我記得很久以前，就發現這兩者存在不同，初步猜測應該是演算法上存在差異。最近需要採集應用CPU的使用率，看了一下兩種CPU的計算方法。

1、top

top是比較經典的CPU計算方法，top的程式碼在androidm/system/core/toolbox/top.c下面，輸出process的cpu使用率在print_procs裡面：

<code class="hljs haskell has-numbering"><span class="hljs-title">static</span> void print_procs(void) {
...
    for (i = <span class="hljs-number">0</span>; i < num_new_procs; i++) {
        <span class="hljs-keyword">if</span> (new_procs[i]) {
            old_proc = find_old_proc(new_procs[i]->pid, new_procs[i]->tid);
            <span class="hljs-keyword">if</span> (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;
            } <span class="hljs-keyword">else</span> {
                new_procs[i]->delta_utime = <span class="hljs-number">0</span>;
                new_procs[i]->delta_stime = <span class="hljs-number">0</span>;
            }
            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);

 ...

<span class="hljs-title">if</span> (!threads) {
            printf(<span class="hljs-string">"%5d %2d %3"</span> <span class="hljs-type">PRIu64</span> <span class="hljs-string">"%% %c %5d %6"</span> <span class="hljs-type">PRIu64</span> <span class="hljs-string">"K %6"</span> <span class="hljs-type">PRIu64</span> <span class="hljs-string">"K %3s %-8.8s %s\n"</span>,
                   <span class="hljs-keyword">proc</span>->pid, <span class="hljs-keyword">proc</span>->prs, <span class="hljs-keyword">proc</span>->delta_time * <span class="hljs-number">100</span> / total_delta_time, <span class="hljs-keyword">proc</span>->state, <span class="hljs-keyword">proc</span>->num_threads,
                   <span class="hljs-keyword">proc</span>->vss / <span class="hljs-number">1024</span>, <span class="hljs-keyword">proc</span>->rss * getpagesize() / <span class="hljs-number">1024</span>, <span class="hljs-keyword">proc</span>->policy, user_str, <span class="hljs-keyword">proc</span>->name[<span class="hljs-number">0</span>] != <span class="hljs-number">0</span> ? <span class="hljs-keyword">proc</span>->name : <span class="hljs-keyword">proc</span>->tname);
        } <span class="hljs-keyword">else</span> {
            printf(<span class="hljs-string">"%5d %5d %2d %3"</span> <span class="hljs-type">PRIu64</span> <span class="hljs-string">"%% %c %6"</span> <span class="hljs-type">PRIu64</span> <span class="hljs-string">"K %6"</span> <span class="hljs-type">PRIu64</span> <span class="hljs-string">"K %3s %-8.8s %-15s %s\n"</span>,
                   <span class="hljs-keyword">proc</span>->pid, <span class="hljs-keyword">proc</span>->tid, <span class="hljs-keyword">proc</span>->prs, <span class="hljs-keyword">proc</span>->delta_time * <span class="hljs-number">100</span> / total_delta_time, <span class="hljs-keyword">proc</span>->state,
                   <span class="hljs-keyword">proc</span>->vss / <span class="hljs-number">1024</span>, <span class="hljs-keyword">proc</span>->rss * getpagesize() / <span class="hljs-number">1024</span>, <span class="hljs-keyword">proc</span>->policy, user_str, <span class="hljs-keyword">proc</span>->tname, <span class="hljs-keyword">proc</span>->name);
        }
...
}
</code><ul style="" class="pre-numbering"><li>1</li><li>2</li><li>3</li><li>4</li><li>5</li><li>6</li><li>7</li><li>8</li><li>9</li><li>10</li><li>11</li><li>12</li><li>13</li><li>14</li><li>15</li><li>16</li><li>17</li><li>18</li><li>19</li><li>20</li><li>21</li><li>22</li><li>23</li><li>24</li><li>25</li><li>26</li><li>27</li><li>28</li><li>29</li><li>30</li><li>31</li><li>32</li><li>33</li><li>34</li><li>35</li></ul>

CPU的計算是proc->delta_time * 100 / total_delta_time。

先看total_delta_time由：

<code class="hljs avrasm has-numbering">    total_delta_time = (new_cpu<span class="hljs-preprocessor">.utime</span> + new_cpu<span class="hljs-preprocessor">.ntime</span> + new_cpu<span class="hljs-preprocessor">.stime</span> + new_cpu<span class="hljs-preprocessor">.itime</span>
                        + new_cpu<span class="hljs-preprocessor">.iowtime</span> + new_cpu<span class="hljs-preprocessor">.irqtime</span> + new_cpu<span class="hljs-preprocessor">.sirqtime</span>)
                     - (old_cpu<span class="hljs-preprocessor">.utime</span> + old_cpu<span class="hljs-preprocessor">.ntime</span> + old_cpu<span class="hljs-preprocessor">.stime</span> + old_cpu<span class="hljs-preprocessor">.itime</span>
                        + old_cpu<span class="hljs-preprocessor">.iowtime</span> + old_cpu<span class="hljs-preprocessor">.irqtime</span> + old_cpu<span class="hljs-preprocessor">.sirqtime</span>)<span class="hljs-comment">;</span></code><ul style="" class="pre-numbering"><li>1</li><li>2</li><li>3</li><li>4</li></ul>

而這些變數的值，是在read_procs通過讀取/proc/stat的jiffies得到：

<code class="hljs perl has-numbering">static void read_procs(void) {
...
    proc_dir = <span class="hljs-keyword">opendir</span>(<span class="hljs-string">"/proc"</span>);
    <span class="hljs-keyword">if</span> (!proc_dir) <span class="hljs-keyword">die</span>(<span class="hljs-string">"Could not open /proc.\n"</span>);

    new_procs = calloc(INIT_PROCS * (threads ? THREAD_MULT : <span class="hljs-number">1</span>), sizeof(struct proc_info <span class="hljs-variable">*)</span>);
    num_new_procs = INIT_PROCS * (threads ? THREAD_MULT : <span class="hljs-number">1</span>);

    file = fopen(<span class="hljs-string">"/proc/stat"</span>, <span class="hljs-string">"r"</span>);
    <span class="hljs-keyword">if</span> (!file) <span class="hljs-keyword">die</span>(<span class="hljs-string">"Could not open /proc/stat.\n"</span>);
    fscanf(file, <span class="hljs-string">"cpu  <span class="hljs-variable">%lu</span> <span class="hljs-variable">%lu</span> <span class="hljs-variable">%lu</span> <span class="hljs-variable">%lu</span> <span class="hljs-variable">%lu</span> <span class="hljs-variable">%lu</span> <span class="hljs-variable">%lu</span>"</span>, &new_cpu.<span class="hljs-keyword">utime</span>, &new_cpu.ntime, &new_cpu.stime,
            &new_cpu.itime, &new_cpu.iowtime, &new_cpu.irqtime, &new_cpu.sirqtime);
    fclose(file);</code><ul style="" class="pre-numbering"><li>1</li><li>2</li><li>3</li><li>4</li><li>5</li><li>6</li><li>7</li><li>8</li><li>9</li><li>10</li><li>11</li><li>12</li><li>13</li></ul>

而proc->delta_time是兩次讀取/proc/pid/stat相減得到：

<code class="hljs haskell has-numbering"><span class="hljs-title">static</span> int read_stat(char *filename, struct proc_info *<span class="hljs-keyword">proc</span>) {
...
  /* <span class="hljs-type">Scan</span> rest <span class="hljs-keyword">of</span> string. */
    sscanf(close_paren + <span class="hljs-number">1</span>,
           <span class="hljs-string">" %c "</span> <span class="hljs-string">"%*d %*d %*d %*d %*d %*d %*d %*d %*d %*d "</span>
           <span class="hljs-string">"%"</span> <span class="hljs-type">SCNu64</span>
           <span class="hljs-string">"%"</span> <span class="hljs-type">SCNu64</span> <span class="hljs-string">"%*d %*d %*d %*d %*d %*d %*d "</span>
           <span class="hljs-string">"%"</span> <span class="hljs-type">SCNu64</span>
           <span class="hljs-string">"%"</span> <span class="hljs-type">SCNu64</span> <span class="hljs-string">"%*d %*d %*d %*d %*d %*d %*d %*d %*d %*d %*d %*d %*d %*d "</span>
           <span class="hljs-string">"%d"</span>,
           &<span class="hljs-keyword">proc</span>->state,
           &<span class="hljs-keyword">proc</span>->utime,
           &<span class="hljs-keyword">proc</span>->stime,
           &<span class="hljs-keyword">proc</span>->vss,
           &<span class="hljs-keyword">proc</span>->rss,
           &<span class="hljs-keyword">proc</span>->prs);

    return <span class="hljs-number">0</span>;
}</code><ul style="" class="pre-numbering"><li>1</li><li>2</li><li>3</li><li>4</li><li>5</li><li>6</li><li>7</li><li>8</li><li>9</li><li>10</li><li>11</li><li>12</li><li>13</li><li>14</li><li>15</li><li>16</li><li>17</li><li>18</li><li>19</li></ul>

可見，top是一段時間內，計算process的cpu jiffies與總的cpu jiffies差值得到。

2、dump cpuinfo

dump cpuinfo是Android特有的命令（我一直都android的各種dump、trace非常感興趣，快玩物喪志了。。。）。dump cpuinfo命令的實現在androidm/frameworks/base/core/java/com/android/internal/os/ProcessCpuTracker.java類裡面，方法是printCurrentState:

<code class="hljs avrasm has-numbering">final public String printCurrentState(long now) {
...
        int N = mWorkingProcs<span class="hljs-preprocessor">.size</span>()<span class="hljs-comment">;</span>
        for (int i=<span class="hljs-number">0</span><span class="hljs-comment">; i<N; i++) {</span>
            Stats <span class="hljs-keyword">st</span> = mWorkingProcs<span class="hljs-preprocessor">.get</span>(i)<span class="hljs-comment">;</span>
            printProcessCPU(pw, <span class="hljs-keyword">st</span><span class="hljs-preprocessor">.added</span> ? <span class="hljs-string">" +"</span> : (<span class="hljs-keyword">st</span><span class="hljs-preprocessor">.removed</span> ? <span class="hljs-string">" -"</span>: <span class="hljs-string">"  "</span>),
                    <span class="hljs-keyword">st</span><span class="hljs-preprocessor">.pid</span>, <span class="hljs-keyword">st</span><span class="hljs-preprocessor">.name</span>, (int)<span class="hljs-keyword">st</span><span class="hljs-preprocessor">.rel</span>_uptime,
                    <span class="hljs-keyword">st</span><span class="hljs-preprocessor">.rel</span>_utime, <span class="hljs-keyword">st</span><span class="hljs-preprocessor">.rel</span>_stime, <span class="hljs-number">0</span>, <span class="hljs-number">0</span>, <span class="hljs-number">0</span>, <span class="hljs-keyword">st</span><span class="hljs-preprocessor">.rel</span>_minfaults, <span class="hljs-keyword">st</span><span class="hljs-preprocessor">.rel</span>_majfaults)<span class="hljs-comment">;</span>
            if (!<span class="hljs-keyword">st</span><span class="hljs-preprocessor">.removed</span> && <span class="hljs-keyword">st</span><span class="hljs-preprocessor">.workingThreads</span> != null) {
                int M = <span class="hljs-keyword">st</span><span class="hljs-preprocessor">.workingThreads</span><span class="hljs-preprocessor">.size</span>()<span class="hljs-comment">;</span>
                for (int j=<span class="hljs-number">0</span><span class="hljs-comment">; j<M; j++) {</span>
                    Stats <span class="hljs-keyword">tst</span> = <span class="hljs-keyword">st</span><span class="hljs-preprocessor">.workingThreads</span><span class="hljs-preprocessor">.get</span>(j)<span class="hljs-comment">;</span>
                    printProcessCPU(pw,
                            <span class="hljs-keyword">tst</span><span class="hljs-preprocessor">.added</span> ? <span class="hljs-string">"   +"</span> : (<span class="hljs-keyword">tst</span><span class="hljs-preprocessor">.removed</span> ? <span class="hljs-string">"   -"</span>: <span class="hljs-string">"    "</span>),
                            <span class="hljs-keyword">tst</span><span class="hljs-preprocessor">.pid</span>, <span class="hljs-keyword">tst</span><span class="hljs-preprocessor">.name</span>, (int)<span class="hljs-keyword">st</span><span class="hljs-preprocessor">.rel</span>_uptime,
                            <span class="hljs-keyword">tst</span><span class="hljs-preprocessor">.rel</span>_utime, <span class="hljs-keyword">tst</span><span class="hljs-preprocessor">.rel</span>_stime, <span class="hljs-number">0</span>, <span class="hljs-number">0</span>, <span class="hljs-number">0</span>, <span class="hljs-number">0</span>, <span class="hljs-number">0</span>)<span class="hljs-comment">;</span>
                }
            }
        }

...
}</code><ul style="" class="pre-numbering"><li>1</li><li>2</li><li>3</li><li>4</li><li>5</li><li>6</li><li>7</li><li>8</li><li>9</li><li>10</li><li>11</li><li>12</li><li>13</li><li>14</li><li>15</li><li>16</li><li>17</li><li>18</li><li>19</li><li>20</li><li>21</li><li>22</li></ul>

而printProcessCPU輸出process CPU的使用情況：

<code class="hljs cs has-numbering"><span class="hljs-keyword">private</span> <span class="hljs-keyword">void</span> <span class="hljs-title">printProcessCPU</span>(PrintWriter pw, String prefix, <span class="hljs-keyword">int</span> pid, String label,
            <span class="hljs-keyword">int</span> totalTime, <span class="hljs-keyword">int</span> user, <span class="hljs-keyword">int</span> system, <span class="hljs-keyword">int</span> iowait, <span class="hljs-keyword">int</span> irq, <span class="hljs-keyword">int</span> softIrq,
            <span class="hljs-keyword">int</span> minFaults, <span class="hljs-keyword">int</span> majFaults) {
        pw.print(prefix);
        <span class="hljs-keyword">if</span> (totalTime == <span class="hljs-number">0</span>) totalTime = <span class="hljs-number">1</span>;
        printRatio(pw, user+system+iowait+irq+softIrq, totalTime);
...
}</code><ul style="" class="pre-numbering"><li>1</li><li>2</li><li>3</li><li>4</li><li>5</li><li>6</li><li>7</li><li>8</li></ul>

user+system+iowait+irq+softIrq 比totalTime。
看下st個變數的賦值，在collectStats裡面：

<code class="hljs avrasm has-numbering">private int[] collectStats(String statsFile, int parentPid, boolean first,
            int[] curPids, ArrayList<Stats> allProcs) {

        int[] pids = Process<span class="hljs-preprocessor">.getPids</span>(statsFile, curPids)<span class="hljs-comment">;</span>
...
 final long uptime = SystemClock<span class="hljs-preprocessor">.uptimeMillis</span>()<span class="hljs-comment">;</span>
   final long[] procStats = mProcessStatsData<span class="hljs-comment">;</span>
                    if (!Process<span class="hljs-preprocessor">.readProcFile</span>(<span class="hljs-keyword">st</span><span class="hljs-preprocessor">.statFile</span><span class="hljs-preprocessor">.toString</span>(),
                            PROCESS_STATS_FORMAT, null, procStats, null)) {
                        continue<span class="hljs-comment">;</span>
                    }

...
                    if (DEBUG) Slog<span class="hljs-preprocessor">.v</span>(<span class="hljs-string">"Load"</span>, <span class="hljs-string">"Stats changed "</span> + <span class="hljs-keyword">st</span><span class="hljs-preprocessor">.name</span> + <span class="hljs-string">" pid="</span> + <span class="hljs-keyword">st</span><span class="hljs-preprocessor">.pid</span>
                            + <span class="hljs-string">" utime="</span> + utime + <span class="hljs-string">"-"</span> + <span class="hljs-keyword">st</span><span class="hljs-preprocessor">.base</span>_utime
                            + <span class="hljs-string">" stime="</span> + stime + <span class="hljs-string">"-"</span> + <span class="hljs-keyword">st</span><span class="hljs-preprocessor">.base</span>_stime
                            + <span class="hljs-string">" minfaults="</span> + minfaults + <span class="hljs-string">"-"</span> + <span class="hljs-keyword">st</span><span class="hljs-preprocessor">.base</span>_minfaults
                            + <span class="hljs-string">" majfaults="</span> + majfaults + <span class="hljs-string">"-"</span> + <span class="hljs-keyword">st</span><span class="hljs-preprocessor">.base</span>_majfaults)<span class="hljs-comment">;</span>

                    <span class="hljs-keyword">st</span><span class="hljs-preprocessor">.rel</span>_uptime = uptime - <span class="hljs-keyword">st</span><span class="hljs-preprocessor">.base</span>_uptime<span class="hljs-comment">;</span>
                    <span class="hljs-keyword">st</span><span class="hljs-preprocessor">.base</span>_uptime = uptime<span class="hljs-comment">;</span>
                    <span class="hljs-keyword">st</span><span class="hljs-preprocessor">.rel</span>_utime = (int)(utime - <span class="hljs-keyword">st</span><span class="hljs-preprocessor">.base</span>_utime)<span class="hljs-comment">;</span>
                    <span class="hljs-keyword">st</span><span class="hljs-preprocessor">.rel</span>_stime = (int)(stime - <span class="hljs-keyword">st</span><span class="hljs-preprocessor">.base</span>_stime)<span class="hljs-comment">;</span>
                    <span class="hljs-keyword">st</span><span class="hljs-preprocessor">.base</span>_utime = utime<span class="hljs-comment">;</span>
                    <span class="hljs-keyword">st</span><span class="hljs-preprocessor">.base</span>_stime = stime<span class="hljs-comment">;</span>
                    <span class="hljs-keyword">st</span><span class="hljs-preprocessor">.rel</span>_minfaults = (int)(minfaults - <span class="hljs-keyword">st</span><span class="hljs-preprocessor">.base</span>_minfaults)<span class="hljs-comment">;</span>
                    <span class="hljs-keyword">st</span><span class="hljs-preprocessor">.rel</span>_majfaults = (int)(majfaults - <span class="hljs-keyword">st</span><span class="hljs-preprocessor">.base</span>_majfaults)<span class="hljs-comment">;</span>
                    <span class="hljs-keyword">st</span><span class="hljs-preprocessor">.base</span>_minfaults = minfaults<span class="hljs-comment">;</span>
                    <span class="hljs-keyword">st</span><span class="hljs-preprocessor">.base</span>_majfaults = majfaults<span class="hljs-comment">;</span>
                    <span class="hljs-keyword">st</span><span class="hljs-preprocessor">.working</span> = true<span class="hljs-comment">;</span>
...
}</code><ul style="" class="pre-numbering"><li>1</li><li>2</li><li>3</li><li>4</li><li>5</li><li>6</li><li>7</li><li>8</li><li>9</li><li>10</li><li>11</li><li>12</li><li>13</li><li>14</li><li>15</li><li>16</li><li>17</li><li>18</li><li>19</li><li>20</li><li>21</li><li>22</li><li>23</li><li>24</li><li>25</li><li>26</li><li>27</li><li>28</li><li>29</li><li>30</li><li>31</li><li>32</li></ul>

st.rel_utime 和 st.rel_stime還是通過讀/proc/pid/stat相減得到，而st.rel_uptime卻是通過 SystemClock.uptimeMillis()差值，並不是跟top一樣，通過得到總CPU jiffies。

看到這，也就能明白，top跟dump cpuinfo的區別在於：top分母有的是總測CPU jiffies，而dump cpuinfo是uptime，是時間，而並非jiffies，也能解釋為什麼top出來的cpu，大部分時間會比dump cpuinfo的原因。