为什么perf报告缓存未命中？

Question

根据perf tutorials的说法，perf stat应该使用硬件计数器报告缓存未命中。但是，在我的系统(最新的Arch Linux)上，它不：

[joel@panda goog]$ perf stat ./hash

 Performance counter stats for './hash':

    869.447863 task-clock                #    0.997 CPUs utilized          
            92 context-switches          #    0.106 K/sec                  
             4 cpu-migrations            #    0.005 K/sec                  
         1,041 page-faults               #    0.001 M/sec                  
 2,628,646,296 cycles                    #    3.023 GHz                    
   819,269,992 stalled-cycles-frontend   #   31.17% frontend cycles idle   
   132,355,435 stalled-cycles-backend    #    5.04% backend  cycles idle   
 4,515,152,198 instructions              #    1.72  insns per cycle        
                                         #    0.18  stalled cycles per insn
 1,060,739,808 branches                  # 1220.015 M/sec                  
     2,653,157 branch-misses             #    0.25% of all branches        

   0.871766141 seconds time elapsed

我遗漏了什么？我已经搜索了手册页和网页，但没有找到任何明显的东西。

编辑:我的中央处理器是英特尔i5 2300K，如果这很重要的话。

Answer 1

在我的系统上，通过显式地请求这些事件，我能够让perf stat报告缓存引用和未命中的事件，如下所示

perf stat -B -e cache-references,cache-misses,cycles,instructions,branches,faults,migrations sleep 5
Performance counter stats for 'sleep 5':

         10573 cache-references                                            
          1949 cache-misses              #   18.434 % of all cache refs    
       1077328 cycles                    #    0.000 GHz                    
        715248 instructions              #    0.66  insns per cycle        
        151188 branches                                                    
           154 faults                                                      
             0 migrations                                                  

   5.002776842 seconds time elapsed

默认事件集不包括缓存事件，与您的结果匹配，我不知道原因

perf stat -B sleep 5

Performance counter stats for 'sleep 5':

      0.344308 task-clock                #    0.000 CPUs utilized          
             1 context-switches          #    0.003 M/sec                  
             0 CPU-migrations            #    0.000 M/sec                  
           154 page-faults               #    0.447 M/sec                  
        977183 cycles                    #    2.838 GHz                    
        586878 stalled-cycles-frontend   #   60.06% frontend cycles idle   
        430497 stalled-cycles-backend    #   44.05% backend  cycles idle   
        720815 instructions              #    0.74  insns per cycle        
                                         #    0.81  stalled cycles per insn
        152217 branches                  #  442.095 M/sec                  
          7646 branch-misses             #    5.02% of all branches        

   5.002763199 seconds time elapsed

Answer 2

在the latest source code中，默认事件不再包含cache-misses和cache-references：

struct perf_event_attr default_attrs[] = {

  { .type = PERF_TYPE_SOFTWARE, .config = PERF_COUNT_SW_TASK_CLOCK      },
  { .type = PERF_TYPE_SOFTWARE, .config = PERF_COUNT_SW_CONTEXT_SWITCHES    },
  { .type = PERF_TYPE_SOFTWARE, .config = PERF_COUNT_SW_CPU_MIGRATIONS      },
  { .type = PERF_TYPE_SOFTWARE, .config = PERF_COUNT_SW_PAGE_FAULTS     },

  { .type = PERF_TYPE_HARDWARE, .config = PERF_COUNT_HW_CPU_CYCLES      },
  { .type = PERF_TYPE_HARDWARE, .config = PERF_COUNT_HW_STALLED_CYCLES_FRONTEND },
  { .type = PERF_TYPE_HARDWARE, .config = PERF_COUNT_HW_STALLED_CYCLES_BACKEND  },
  { .type = PERF_TYPE_HARDWARE, .config = PERF_COUNT_HW_INSTRUCTIONS        },
  { .type = PERF_TYPE_HARDWARE, .config = PERF_COUNT_HW_BRANCH_INSTRUCTIONS },
  { .type = PERF_TYPE_HARDWARE, .config = PERF_COUNT_HW_BRANCH_MISSES       },

};

因此，到目前为止，man和大多数web都是过时的。

Answer 3

我花了几分钟来尝试理解perf。我通过首先记录，然后报告数据(这两个perf工具)找出了缓存未命中。

要查看事件列表，请执行以下操作：

perf list

例如，为了检查末级缓存加载未命中，您需要像这样使用事件LLC-loads-misses

perf record -e LLC-loads-misses ./your_program

然后报告结果

perf report -v