title: what does "vcore-seconds" in hadoop job log mean?
body: <pre><code>Job Counters
      Launched map tasks=3
      Launched reduce tasks=45
      Data-local map tasks=1
      Rack-local map tasks=2
      Total time spent by all maps in occupied slots (ms)=29338
      Total time spent by all reduces in occupied slots (ms)=200225
      Total time spent by all map tasks (ms)=29338
      Total time spent by all reduce tasks (ms)=200225
      Total vcore-seconds taken by all map tasks=29338
      Total vcore-seconds taken by all reduce tasks=200225
      Total megabyte-seconds taken by all map tasks=30042112
      Total megabyte-seconds taken by all reduce tasks=205030400
  </code></pre>
  
  <p>what does "vcore-seconds" mean and what is the difference between "vcore-seconds " and "Total time spent"</p>