perf stat用于运行指令，并分析其统计结果。虽然perf top也可以指定pid，但是必须先启动应用才能查看信息。
perf stat能完整统计应用整个生命周期的信息。
命令格式为：
perf stat [-e <EVENT> | --event=EVENT] [-a] <command>
perf stat [-e <EVENT> | --event=EVENT] [-a] — <command> [<options>]
下面简单看一下perf stat 的输出：
al@al-System-Product-Name:~/perf$ sudo perf stat
^C
Performance counter stats for \'system wide\':
40904.820871 cpu-clock (msec) # 5.000 CPUs utilized
18,132 context-switches # 0.443 K/sec
1,053 cpu-migrations # 0.026 K/sec
2,420 page-faults # 0.059 K/sec
3,958,376,712 cycles # 0.097 GHz (49.99%)
574,598,403 stalled-cycles-frontend # 14.52% frontend cycles idle (49.98%)
9,392,982,910 stalled-cycles-backend # 237.29% backend cycles idle (50.00%)
1,653,185,883 instructions # 0.42 insn per cycle
# 5.68 stalled cycles per insn (50.01%)
237,061,366 branches # 5.795 M/sec (50.02%)
18,333,168 branch-misses # 7.73% of all branches (50.00%)
8.181521203 seconds time elapsed
输出解释如下：
cpu-clock：任务真正占用的处理器时间，单位为ms。CPUs utilized = task-clock / time elapsed，CPU的占用率。
context-switches：程序在运行过程中上下文的切换次数。