shmmax(5)                                                         shmmax(5)
                          Tunable Kernel Parameters



 NAME    [Toc]    [Back]
      shmmax - maximum size (in bytes) for a System V shared memory segment

 VALUES    [Toc]    [Back]
    Default
      1 GB

    Allowed values    [Toc]    [Back]
      Minimum: 2048

      Maximum: 0x40000000000

 DESCRIPTION    [Toc]    [Back]
      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 sets the maximum size for such a segment within the
      system, and is dynamic as of HP-UX 11i.

    Who is Expected to Change This Tunable?
      Anyone.

    Restrictions on Changing    [Toc]    [Back]
      Changes to this tunable take effect immediately.

    When Should the Value of This Tunable Be Raised?
      shmmax should be raised if it is below the maximum and user programs
      are attempting to shmget segments larger than the current value, and
      receiving an [EINVAL] error message.

    What Are the Side Effects of Raising the Value?
      The only effect is that user programs can use shmget to get larger
      segments.

    When Should the Value of This Tunable Be Lowered?
      If you wish to enforce behavior on the user's code, limiting the
      maximum size of System V segments.

    What Are the Side Effects of Lowering the Value?
      None.

    What Other Tunable Values Should Be Changed at the Same Time?
      shmseg and shmmni should be considered because enforcing smaller
      segments may cause the user's code to try to create more segments to
      accomplish the task.




 Hewlett-Packard Company            - 1 -      HP-UX 11i Version 2: Sep 2004






 shmmax(5)                                                         shmmax(5)
                          Tunable Kernel Parameters



 WARNINGS    [Toc]    [Back]
      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    [Toc]    [Back]
      shmmax was developed by HP.

 SEE ALSO    [Toc]    [Back]
      shmem(5), shmmni(5), shmseg(5).


 Hewlett-Packard Company            - 2 -      HP-UX 11i Version 2: Sep 2004