The semmnu parameter specifies the number of ''undo'' structures system-wide. An undo structure, which is set up on a per-process basis, keeps track of process operations on semaphores so that the operations may be ''undone'' if the structure terminates abnormally. This helps to ensure that an abnormally terminated process does not cause other processes to wait indefinitely for a change to a semaphore.
This parameter is specified in /var/sysgen/mtune/sem.