Check these two Metalink notes for what you should set these values to:
Maximum SHMMAX values for Linux x86 and x86-64 (Doc ID 567506.1)
Upon startup of Linux database get ORA-27102: out of memory Linux-X86_64 Error: 28: No space left on device (Doc ID 301830.1)
The second document discusses what your shmall should be set to which depends on your page size.
Also consider Huge pages...there are a number of metalink articles which discuss this:
HugePages on Oracle Linux 64-bit [ID 361468.1]
HugePages on Linux: What It Is... and What It Is Not... [ID 361323.1]