On a Linux system, what do the parameters shmall, shmmax and shmmni define?

shmall
* indicates the total amount of shared memory that the system can use at one time (measured in pages)

shmmax
* defines the maximum size in bytes of a single shared memory segment that a Linux process can allocate in its virtual address space, in bytes.
Oracle recommends that more than half of the physical memory is assigned to shmmax

shmmni
* defines the system wide maximum number of shared memory segments