如何解决perf工具中的非counted>问题？

Question

perf stat -d ./sample.out

产出如下：

Performance counter stats for './sample.out':

          0.586266 task-clock (msec)         #    0.007 CPUs utilized          
                 2 context-switches          #    0.003 M/sec                  
                 1 cpu-migrations            #    0.002 M/sec                  
               116 page-faults               #    0.198 M/sec                  
          7,35,790 cycles                    #    1.255 GHz                     [81.06%]
     <not counted> stalled-cycles-frontend 
   <not supported> stalled-cycles-backend  
     <not counted> instructions            
     <not counted> branches                
     <not counted> branch-misses           
   <not supported> L1-dcache-loads:HG      
     <not counted> L1-dcache-load-misses:HG
     <not counted> LLC-loads:HG            
   <not supported> LLC-load-misses:HG      

       0.088013919 seconds time elapsed

我读到了为什么<not supported>会出现在上。但我得到的<not counted>，甚至基本的计数器，如指示，分支等。谁能建议如何使它工作？

有趣的是：

苏多珀夫睡眠3

给出输出：

Performance counter stats for 'sleep 3':

          0.598484 task-clock (msec)         #    0.000 CPUs utilized          
                 2 context-switches          #    0.003 M/sec                  
                 0 cpu-migrations            #    0.000 K/sec                  
               181 page-faults               #    0.302 M/sec                  
     <not counted> cycles                  
     <not counted> stalled-cycles-frontend 
   <not supported> stalled-cycles-backend  
     <not counted> instructions            
     <not counted> branches                
     <not counted> branch-misses

-C 1睡眠3

 Performance counter stats for 'CPU(s) 1':

       3002.640578 task-clock (msec)         #    1.001 CPUs utilized           [100.00%]
               425 context-switches          #    0.142 K/sec                   [100.00%]
                 9 cpu-migrations            #    0.003 K/sec                   [100.00%]
                 5 page-faults               #    0.002 K/sec                  
       7,82,97,019 cycles                    #    0.026 GHz                     [33.32%]
       9,38,21,585 stalled-cycles-frontend   #  119.83% frontend cycles idle    [33.32%]
   <not supported> stalled-cycles-backend  
       3,09,81,643 instructions              #    0.40  insns per cycle        
                                             #    3.03  stalled cycles per insn [33.32%]
         70,15,390 branches                  #    2.336 M/sec                   [33.32%]
          6,38,644 branch-misses             #    9.10% of all branches         [33.32%]

       3.001075650 seconds time elapsed

为什么这件意外的事起作用了？

Answer 1

我想，您的处理器不支持这么多计数器，而且它们之间的切换太频繁了。

在最后一个例子中，您可以看到最后一列，其中计数器是多路复用的(只计算了33%的时间)。如果您使用足够小的任务(或者在更多的内核上使用？)，则不会计算它们，因为所有的时间都在使用其他任务。在您的第一个示例中，只有cycles在时间上被成功计算。