parallel process consuming high memory

Database 11.2.0.1 on Oracle Linux 5.5.

below are all parallel process which are consuming high memory at OS level.

total memory consumption is >90% at OS level and daily its keep increasing.

any idea about any known issue on  parallel processing@11.2.0.1 version?

top - 09:42:30 up 229 days, 18:07,  1 user,  load average: 1.67, 2.93, 4.60

Tasks: 832 total,   2 running, 830 sleeping,   0 stopped,   0 zombie

Cpu(s):  7.8%us,  0.4%sy,  0.0%ni, 91.4%id,  0.3%wa,  0.0%hi,  0.1%si,  0.0%st

Mem:  74027752k total, 68773356k used,  5254396k free,   929928k buffers

Swap: 16771852k total,  2493244k used, 14278608k free, 54855564k cached

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND

18050 oracle    15   0 8422m 4.2g 4.2g S  0.0  6.0 944:00.79 oracle

17993 oracle    15   0 8403m 4.2g 4.2g S  0.0  6.0   1009:56 oracle