refine the cpu usage calc, add total_delta.

@@ -193,6 +193,7 @@ SrsProcSystemStat::SrsProcSystemStat()
     ok = false;
     sample_time = 0;
     percent = 0;
+    total_delta = 0;
     memset(label, 0, sizeof(label));
     user = 0;
     nice = 0;
@@ -205,6 +206,11 @@ SrsProcSystemStat::SrsProcSystemStat()
     guest = 0;
 }
 
+int64_t SrsProcSystemStat::total()
+{
+    return user + nice + sys + idle + iowait + irq + softirq + steal + guest;
+}
+
 SrsProcSelfStat* srs_get_self_proc_stat()
 {
     return &_srs_system_cpu_self_stat;
@@ -307,11 +313,13 @@ void srs_update_proc_stat()
         SrsProcSystemStat& o = _srs_system_cpu_system_stat;
         
         // @see: http://blog.csdn.net/nineday/article/details/1928847
-        int64_t total = (r.user + r.nice + r.sys + r.idle + r.iowait + r.irq + r.softirq + r.steal + r.guest) 
-            - (o.user + o.nice + o.sys + o.idle + o.iowait + o.irq + o.softirq + o.steal + o.guest);
-        int64_t idle = r.idle - o.idle;
-        if (total > 0) {
-            r.percent = (float)(1 - idle / (double)total);
+        // @see: http://stackoverflow.com/questions/16011677/calculating-cpu-usage-using-proc-files
+        if (o.total() > 0) {
+            r.total_delta = r.total() - o.total();
+        }
+        if (r.total_delta > 0) {
+            int64_t idle = r.idle - o.idle;
+            r.percent = (float)(1 - idle / (double)r.total_delta);
         }
         
         // upate cache.

@@ -238,6 +238,20 @@ public:
 *           = 523331687 * 1/100 (seconds)
 *           = 5233316.87 seconds
 * the cpu total seconds almost the uptime, the delta is more precise.
+* 
+* we run the command about 26minutes:
+*       [winlin@SRS ~]$ cat /proc/uptime && cat /proc/stat
+*           5276739.83 4701090.76
+*           cpu  43514105 973 8548948 466278556 4150480 190899 804937 0 0
+* where the uptime is 5276739.83s
+* cpu total = 43514105+973+8548948+466278556+4150480+190899+804937+0+0 (USER_HZ)
+*           = 523488898 (USER_HZ)
+*           = 523488898 * 1/100 (seconds)
+*           = 5234888.98 seconds
+* where:
+*       uptime delta = 1586.82s
+*       cpu total delta = 1572.11s
+* the deviation is more smaller.
 */
 class SrsProcSystemStat
 {
@@ -248,10 +262,16 @@ public:
     int64_t sample_time;
     // the percent of usage. 0.153 is 15.3%.
     float percent;
+    // the total cpu time units
+    // @remark, zero for the previous total() is zero.
+    //          the usaged_cpu_delta = total_delta * percent
+    //          previous cpu total = this->total() - total_delta
+    int64_t total_delta;
     
     // always be cpu
     char label[32];
     
+public:
     // The amount of time, measured in units  of  USER_HZ  
     // (1/100ths  of  a  second  on  most  architectures,  use
     // sysconf(_SC_CLK_TCK)  to  obtain  the  right value)
@@ -285,6 +305,9 @@ public:
     unsigned long guest;
 
     SrsProcSystemStat();
+    
+    // get total cpu units.
+    int64_t total();
 };
 
 // get system cpu stat, use cache to avoid performance problem.