about hadoop memory usage

[ch huang]

hi,all:
       if i allocate 2G memory to NN， and i get memory usage info from jmx
interface ,i add the max value of heapmemoryusage and nonheapmemorusage
,the total number is not equal 2G ,the value is 1950.5M ,why? ,BTW ,what's
the "committed" and "init" means?


  "HeapMemoryUsage" : {
      "committed" : 312344576,
      "init" : 258443648,
      "max" : 1908932608,
      "used" : 36133888
    },
    "NonHeapMemoryUsage" : {
      "committed" : 30408704,
      "init" : 24313856,
      "max" : 136314880,
      "used" : 29665296
    },