Idle状态的CPU在做什么?

What Does an Idle CPU Do?这篇文章介绍了CPUIdle状态下在干什么,讲解地很清晰。我总结一下:

当操作系统“无事可做”时,就会运行Idle进程。以LinuxIdle进程代码为例:

while (1) {
    while(!need_resched()) {
        cpuidle_idle_call();
    }

    /*
      [Note: Switch to a different task. We will return to this loop when the
      idle task is again selected to run.]
    */
    schedule_preempt_disabled();
}

可以看到只要没有“调度”需求(need_resched),就会执行cpuidle_idle_call函数。而对于Intel处理器而言,保持Idle状态意味着执行hlt指令。

 

使用vmstat命令监控CPU使用

vmstat命令可以用来监控CPU的使用状况。举例如下:

# vmstat 1
procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu-----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
 1  0      0 5201924   1328 5578060    0    0     0     0 1582 6952  2  1 98  0  0
 1  0      0 5200984   1328 5577996    0    0     0     0 2020 20567  9  1 90  0  0
 0  0      0 5198668   1328 5577952    0    0     0     0 1568 7617  5  1 94  0  0
 0  0      0 5194844   1328 5578000    0    0     0   187 1249 7057  1  1 98  0  0
 0  0      0 5199956   1328 5578232    0    0     0     0 1496 7306  4  1 95  0  0

上述命令每隔1秒输出系统状态,最后5列是描述的是CPU状况。man手册上关于这5列的含义描述的很清楚:

CPU
       These are percentages of total CPU time.
       us: Time spent running non-kernel code.  (user time, including nice time)
       sy: Time spent running kernel code.  (system time)
       id: Time spent idle.  Prior to Linux 2.5.41, this includes IO-wait time.
       wa: Time spent waiting for IO.  Prior to Linux 2.5.41, included in idle.
       st: Time stolen from a virtual machine.  Prior to Linux 2.6.11, unknown.

vmstat实质上是从/proc/stat文件获得系统状态:

# cat /proc/stat
cpu  381584 711 299364 1398303520 429839 0 251 0 0 0
cpu0 90740 58 44641 174627550 131209 0 120 0 0 0
cpu1 43141 26 22925 174746812 108219 0 10 0 0 0
cpu2 41308 35 25097 174831161 25877 0 40 0 0 0
cpu3 39301 70 27514 174836084 27792 0 4 0 0 0
cpu4 39187 78 46191 174750027 109013 0 0 0 0 0
......

需要注意的是这里数字的单位是Jiffies

另外,vmstat计算CPU时间百分比使用的是“四舍五入”算法(vmstat.c):

static void new_format(void){
    ......
    duse = *cpu_use + *cpu_nic;
    dsys = *cpu_sys + *cpu_xxx + *cpu_yyy;
    didl = *cpu_idl;
    diow = *cpu_iow;
    dstl = *cpu_zzz;
    Div = duse + dsys + didl + diow + dstl;
    if (!Div) Div = 1, didl = 1;
    divo2 = Div / 2UL;
    printf(w_option ? wide_format : format,
           running, blocked,
           unitConvert(kb_swap_used), unitConvert(kb_main_free),
           unitConvert(a_option?kb_inactive:kb_main_buffers),
           unitConvert(a_option?kb_active:kb_main_cached),
           (unsigned)( (unitConvert(*pswpin  * kb_per_page) * hz + divo2) / Div ),
           (unsigned)( (unitConvert(*pswpout * kb_per_page) * hz + divo2) / Div ),
           (unsigned)( (*pgpgin        * hz + divo2) / Div ),
           (unsigned)( (*pgpgout           * hz + divo2) / Div ),
           (unsigned)( (*intr          * hz + divo2) / Div ),
           (unsigned)( (*ctxt          * hz + divo2) / Div ),
           (unsigned)( (100*duse            + divo2) / Div ),
           (unsigned)( (100*dsys            + divo2) / Div ),
           (unsigned)( (100*didl            + divo2) / Div ),
           (unsigned)( (100*diow            + divo2) / Div ),
           (unsigned)( (100*dstl            + divo2) / Div )
    );
    ......
}

所以会出现CPU利用百分比相加大于100的情况:2 + 1 + 98 = 101

另外,在Linux系统上,r字段表示的是当前正在运行和等待运行的task的总和。

 

参考资料:
/proc/stat explained
procps