如何解决perf中的"未计数"？

Question

perf stat -d ./sample.out输出为:

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

我读了为什么会出现 .但我甚至得到了基本的指示器,如指令,分支机构等.任何人都可以建议如何让它工作？

有趣的是:

sudo perf stat sleep 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

sudo perf stat -C 1 sleep 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

为什么这出乎意料的工作呢？

谢谢

Answer 1

perf stat -d非常短的程序的典型问题不是统计抽样,而是多路复用(方括号中的百分比表示[33%]- 此计数器仅计算约33%的运行时间).

您要求PMU一次监视太多事件,并且perf无法同时在真实硬件(PMU - CPU的性能监视单元)上映射所有必需的计数器.典型的PMU可能具有4或7或8个独立计数器,但如果您启用了某些SMT技术(例如,HT - 超线程),则该数量可以除以2.

当你要求perf计算这么多计数器时(你的perf stat输出中有6个支持的HW事件),它会将它们分成更小的组.当perf_events有机会改变它们时,内核会在某些时间点更改组,例如在任务时钟滴答(~3 ms).

您可以使用较小的事件集将运行拆分为多个 - 每次运行任意数量的SW事件和2-4个HW事件:

perf stat -e task-clock,page-faults,cycles,stalled-cycles-frontend 
perf stat -e task-clock,page-faults,cycles,instructions            
perf stat -e task-clock,page-faults,branches,branch-misses           
perf stat -e task-clock,page-faults,L1-dcache-load-misses:HG,LLC-loads:HG