有关DataNode内存配置

最近Hadoop/HBase集群，DataNode节点大面积挂掉，查看日志
storageID=DS-1187529127-192.168.0.242-50010-1422253305779, infoPort=50075,
ipcPort=50020):DataXceiver
java.lang.OutOfMemoryError: unable to create new native thread
      at java.lang.Thread.start0(Native Method)
      at java.lang.Thread.start(Thread.java:691)
      at org.apache.hadoop.hdfs.server.datanode.BlockReceiver.receiveBlock(BlockReceiver.java:576)
      at org.apache.hadoop.hdfs.server.datanode.DataXceiver.writeBlock(DataXceiver.java:404)
      at org.apache.hadoop.hdfs.server.datanode.DataXceiver.run(DataXceiver.java:112)
      at java.lang.Thread.run(Thread.java:722)

发现datanode内存溢出(配置为1000m)，我们调整datanode最大内存为2048m，重新启动，运用jmap查看datanode进程内存状况如下：
using thread-local object allocation.
Parallel GC with 10 thread(s)

Heap Configuration:
MinHeapFreeRatio = 40
MaxHeapFreeRatio = 70
   MaxHeapSize    = 2147483648 (2048.0MB)
NewSize       = 1310720 (1.25MB)
MaxNewSize    = 17592186044415 MB
OldSize       = 5439488 (5.1875MB)
NewRatio       = 2
SurvivorRatio = 8
PermSize       = 21757952 (20.75MB)
MaxPermSize    = 85983232 (82.0MB)
G1HeapRegionSize = 0 (0.0MB)

Heap Usage:
PS Young Generation
Eden Space:
capacity = 16711680 (15.9375MB)
used    = 11330912 (10.805999755859375MB)
free    = 5380768 (5.131500244140625MB)
67.80235140931373% used
From Space:
capacity = 327680 (0.3125MB)
used    = 294912 (0.28125MB)
free    = 32768 (0.03125MB)
90.0% used
To Space:
capacity = 393216 (0.375MB)
used    = 0 (0.0MB)
free    = 393216 (0.375MB)
0.0% used
PS Old Generation
   capacity = 125829120 (120.0MB)
used    = 123925368 (118.18444061279297MB)
free    = 1903752 (1.8155593872070312MB)
98.48703384399414% used
PS Perm Generation
capacity = 38141952 (36.375MB)
used    = 17436280 (16.62853240966797MB)
free    = 20705672 (19.74646759033203MB)
45.7141784458226% used

7361 interned Strings occupying 619352 bytes.

我们在DataNode节点上配置-xmx为2048m，并未配置-xmn值，默认值网上说是1/4*MaxHeapSize ,按理：
MaxHeapSize  ＝　PS Young Generation + PS Old Generation
然而，相差甚远，另外的内存去哪了？如果这样DataNode是否仍会OutOfMemoryError？
望解答，先谢谢啦~