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 shmseg(5)                                                         shmseg(5)
                          Tunable Kernel Parameters

 NAME    [Toc]    [Back]
      shmseg - maximum number of System V shared memory segments per process

 VALUES    [Toc]    [Back]
      300 segments

    Allowed values    [Toc]    [Back]
      Minimum: 1

      Maximum: Any value less than or equal to shmmni.

 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.  Once attached,
      a process can read from or write to the segment depending on the
      permissions specified while attaching it.

      This tunable sets an upper limit to the number of segments which can
      be attached per process.

    Who is Expected to Change This Tunable?

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

    When Should the Value of This Tunable Be Raised?
      If user processes reach their limit, more segments are desired, and
      the current value is less than the total number of segments in the
      system (shmmni).

    What Are the Side Effects of Raising the Value?
      A single process will be able to acquire more segments, possibly
      starving another process which previously was able to acquire all the
      segments it needed.  In this case, shmmni should be raised if it is
      below it's maximum.

    When Should the Value of This Tunable Be Lowered?
      This tunable should only be lowered to enforce segment policy on user
      processes, or if one runaway process is hogging the global-segment
      pool.  Otherwise, keeping the maximum higher than the common usage is

    What Are the Side Effects of Lowering the Value?

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

 shmseg(5)                                                         shmseg(5)
                          Tunable Kernel Parameters

    What Other Tunable Values Should Be Changed at the Same Time?
      shmmni should be considered, as previously described.

 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

 AUTHOR    [Toc]    [Back]
      shmseg was developed by HP.

 SEE ALSO    [Toc]    [Back]

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