title: What are stalled-cycles-frontend and stalled-cycles-backend in 'perf stat' result?
body: <p>Does anybody know what is the meaning of <em>stalled-cycles-frontend</em> and <em>stalled-cycles-backend</em> in perf stat result ? I searched on the internet but did not find the answer. Thanks</p>
  
  <pre><code>$ sudo perf stat ls                     
  
  Performance counter stats for 'ls':
  
        0.602144 task-clock                #    0.762 CPUs utilized          
               0 context-switches          #    0.000 K/sec                  
               0 CPU-migrations            #    0.000 K/sec                  
             236 page-faults               #    0.392 M/sec                  
          768956 cycles                    #    1.277 GHz                    
          962999 stalled-cycles-frontend   #  125.23% frontend cycles idle   
          634360 stalled-cycles-backend    #   82.50% backend  cycles idle
          890060 instructions              #    1.16  insns per cycle        
                                           #    1.08  stalled cycles per insn
          179378 branches                  #  297.899 M/sec                  
            9362 branch-misses             #    5.22% of all branches         [48.33%]
  
     0.000790562 seconds time elapsed
  </code></pre>