Allocating 8GB memory to MySQL on a 64bit system

Question

Specs -

MySQL 5.0.x  ,  Redhat 5.9  ,  Physical memory - 16GB

I am trying to set innodb buffer pool size to 8GB (innodb_buffer_pool_size=8G). When i do, and start mysql, i get following error -

InnoDB: Error: cannot allocate 8589950976 bytes of
InnoDB: memory with malloc! Total allocated memory
InnoDB: by InnoDB 37679104 bytes. Operating system errno: 12
InnoDB: Check if you should increase the swap file or
InnoDB: ulimits of your operating system.

Here is output of free -m -

             total       used       free     shared    buffers     cached
Mem:         15922      15269        653          0        248       4275
-/+ buffers/cache:      10744       5178
Swap:         2047          0       2047

Here is output of ulimit -a -

core file size          (blocks, -c) 0
data seg size           (kbytes, -d) unlimited
scheduling priority             (-e) 0
file size               (blocks, -f) unlimited
pending signals                 (-i) 127352
max locked memory       (kbytes, -l) 32
max memory size         (kbytes, -m) unlimited
open files                      (-n) 1024
pipe size            (512 bytes, -p) 8
POSIX message queues     (bytes, -q) 819200
real-time priority              (-r) 0
stack size              (kbytes, -s) 10240
cpu time               (seconds, -t) unlimited
max user processes              (-u) 127352
virtual memory          (kbytes, -v) unlimited
file locks                      (-x) unlimited

I checked file '/etc/security/limits.conf'. Nothing in there. All lines are commented (start with #). Checked directory '/etc/security/limits.d/', empty.

Something is preventing allocation of more than 4GB of memory to MySQL. Not sure what.

Any ideas?

Danke.