shmmni(5)

NAME

shmmni — number of System V shared memory segment identifiers in the system

VALUES

Default

400 identifiers

Allowed values

Minimum: 3

Maximum: 32768

DESCRIPTION

Shared memory is an efficient InterProcess Communications (IPC) mechanism. One process creates a shared memory segment and attaches it to its address space. Any processes looking to communicate with this process through the shared memory segment then attach the shared memory segment to their corresponding address spaces as well. Once attached, a process can read from or write to the segment depending on the permissions specified while attaching it.

This tunable effectively sets the number of unique segments creatable system wide, since each segment is assigned an identifier by the kernel. The identifier is simply a reference generated by the kernel such that any user process can request a particular segment for sharing with a simple integer, and let the kernel determine which segment this corresponds to.

Who is Expected to Change This Tunable?

Anyone.

Restrictions on Changing

Changes to this tunable take effect immediately.

Attempting to lower shmmni below the current number of identifiers in use or below the current value of shmseg will result in an EINVAL error message.

Attempts to raise the value of shmmni when insufficient memory is available to the kernel to create the needed structures will result in an ENOMEM error message.

When Should the Value of This Tunable Be Raised?

shmmni should be raised if users of System V shared memory are receiving the ENOSPC error message on shmget() calls.

What Are the Side Effects of Raising the Value?

Kernel memory usage will be slightly increased, as the data structures used to track the segments are allocated based on this tunable.

When Should the Value of This Tunable Be Lowered?

If kernel memory is at a premium, or it is known that few segments will be needed, a slight savings can be gained from decreasing this tunable, and thus decreasing the data structure memory usage associated with it.

What Are the Side Effects of Lowering the Value?

Kernel memory usage will be slightly reduced.

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

shmmax and shmseg should be considered. shmseg should be changed in the same manner as shmmni, since lowering the total number of segments but raising the number available per process only makes sense if you want a few processes taking all the segments.

shmmax is more complex and any changes to it really depend on the effect desired. Refer to the shmmax(5) manpage for more information before changing this tunable.

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.

Installation of optional kernel software, from HP or other vendors, may cause changes to tunable parameter values. After installation, some tunable parameters may no longer be at the default or recommended values. For information about the effects of installation on tunable values, consult the documentation for the kernel software being installed. For information about optional kernel software that was factory installed on your system, see HP-UX Release Notes at http://docs.hp.com.

AUTHOR

shmmni was developed by HP.