Oracle Server - Enterprise Edition - Version: 9.2.0.4 to 11.2.0.2 - Release: 9.2 to 11.2
Red Hat Enterprise Linux Advanced Server x86-64 (AMD Opteron Architecture)
x86 64 bit (for Enterprise Linux only)
SUSE / UnitedLinux x86-64

When trying to increase the SGA to approach half available RAM with an Oracle 64bit version on a Linux 64bit operating system, even though shmmax is set to match half the amount of RAM, you get the following error when trying to start the instance:

SQL> startup nomount 
ORA-27102: out of memory
Linux-x86_64 Error: 28: No space left on device

shmall is too small, most likely is set to the default setting of 2097152

$ cat /proc/sys/kernel/shmall
2097152

shmall is the total amount of shared memory, in pages, that the system can use at one time.

Set shmall equal to the sum of all the SGAs on the system, divided by the page size.

The page size can be determined using the following command:

$ getconf PAGE_SIZE 
4096

For example, if the sum of all the SGAs on the system is 16Gb and the result of '$ getconf PAGE_SIZE' is 4096 (4Kb) then set shmall to 4194304 pages

As the root user set the shmall to 4194304 in the /etc/sysctl.conf file:

kernel.shmall = 4194304

then run the following command:

# sysctl -p
# cat /proc/sys/kernel/shmall
4194304

NOTE:

The above command loads the new value and a reboot is not necessary

Switch back to being the oracle user and retry the startup command.