semmap(5)

NAME

semmap — number of entries in a System V IPC semaphore map

VALUES

Failsafe

(semmni+2)

Default

(semmni+2)

Allowed values

Minimum: 4

Maximum: semmni+2, or less than or equal to 32767.

DESCRIPTION

The semmap tunable specifies the size of a System V IPC message space resource map which tracks the free space in shared semaphore memory. Each resource map entry is an offset-space pair which points to the offset and size (bytes) of each contiguous series of unused semaphore space segments.

Free-space fragmentation generally increases as semaphore arrays are added. Since the resource map requires an entry pair for each fragment of free space, excess fragmentation can cause the free-space map to completely fill. If the map is full when message space is acquired or released, the system issues the DANGER: mfree map overflow warning message.

For more information about System V semaphores, refer to the "Overview" section of the mesg(5) manpage.

Who Is Expected to Change This Tunable?

This is a kernel-calculated value.

Restrictions on Changing

Changes to this tunable take effect at the next reboot.

When Should the Value of This Tunable Be Raised?

When the free-space map is full and the system issues the warning message DANGER: mfree map overflow. A larger value for semmap can correct this issue.

When Should the Value of This Tunable Be Lowered?

If requirements for semaphores has significantly decreased.

What Other Tunable Values Should Be Changed at the Same Time?

All the System V semaphore tunables are interrelated and should not be treated as independent variables. The tunables must be evaluated as a system to ensure they reflect the application requirements. The sema tunables include semaem, semmap, semmni, semmns, semmnu, semmsl, semume, semvmx, and sysv_hash_locks.

WARNINGS

All HP-UX kernel tunable parameters are release specific. This parameter may be removed or have its meaning changed in future releases of HP-UX.

AUTHOR

semmap was developed by AT&T.