New versions of glibc present in RHEL6 include a new arena allocator design. In several clusters we've seen this new allocator cause huge amounts of virtual memory to be used, since when multiple threads perform allocations, they each get their own memory arena. On a 64-bit system, these arenas are 64M mappings, and the maximum number of arenas is 8 times the number of cores. We've observed a DN process using 14GB of vmem for only 300M of resident set. This causes all kinds of nasty issues for obvious reasons.
Setting MALLOC_ARENA_MAX to a low number will restrict the number of memory arenas and bound the virtual memory, with no noticeable downside in performance - we've been recommending MALLOC_ARENA_MAX=4. We should set this in hadoop-env.sh to avoid this issue as RHEL6 becomes more and more common.