swchunk(5)                                                       swchunk(5)
                          Tunable Kernel Parameters



 NAME    [Toc]    [Back]
      swchunk - swap chunk size in 1 KB blocks

 VALUES    [Toc]    [Back]
    Default
      2048 blocks

    Allowed values    [Toc]    [Back]
      Minimum: 2048 blocks

      Maximum: 65536 blocks

 DESCRIPTION    [Toc]    [Back]
      Swap space in the kernel is managed using 'chunks' of physical device
      space.  These chunks contain one or more (usually more) pages of
      memory, but provide another layer of indexing (similar to inodes in
      file systems) to keep the global swap table relatively small, as
      opposed to a large table indexed by swap page.

      swchunk controls the size in physical disk blocks (which are defined
      as 1 KB) for each chunk.  The total bytes of swap space manageable by
      the system is swchunk * 1 KB * 16384 (the system maximum number of
      swap chunks in the swap table).

      The way to think of swchunk is not as the size of the I/O transactions
      in the swap system (in disk blocks), but as the number of blocks that
      will be placed on one swap device (or file system) before moving to
      the next device (assuming all priorities are equal).  This spreads the
      swap space over any devices and is called swap interleaving.  Swap
      interleaving spreads out the swap over many devices and reduces the
      possibility of one single device becoming a bottleneck for the entire
      system when swap usage is heavy.

    Who is Expected to Change This Tunable?
      This tunable should only be modified by those with a complete
      knowledge of both kernel behavior and underlying device hardware.

    Restrictions on Changing    [Toc]    [Back]
      Changes to this tunable take effect at the next reboot.

    When Should the Value of This Tunable Be Raised?
      If the system owner wishes to add more swap to the system, but the
      additional swap chunks needed are unavailable, raising this tunable
      will work around the problem.  By increasing the size of each chunk,
      fewer total chunks are needed.

    What Are the Side Effects of Raising the Value?
      The second level of the swap table (used to track pages within a
      chunk) will increase, resulting in more memory used by the kernel.  If
      swchunk is being increased to allow for mapping of a larger swap
      space, increased memory usage by the kernel to track the swap space is



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






 swchunk(5)                                                       swchunk(5)
                          Tunable Kernel Parameters



      unavoidable.

      This means that more swap is allocated to each device (or file system)
      using the round-robin interleaving scheme when all priorities are
      equal.  Increasing swchunk when the number of chunks needed to
      represent the system swap space is less than 16384 could hinder system
      performance by creating unneeded I/O bottlenecks.  For example, two
      pages that were in different chunks using the smaller value which were
      previously on different swap devices and thus accessible independently
      of one another (with no read head or controller issues) are now on the
      same device and can not be read concurrently, resulting in a longer
      access time for the second page.

    When Should the Value of This Tunable Be Lowered?
      If the amount of swap space mappable by the system is much larger than
      the total amount of swap space which is attached (or going to be
      attached) to the system, which is calculable by multiplying 16384 *
      swchunk* 1KB, then kernel memory usage can be reduced by lowering
      swchunk to fit the actual swap space.

    What Are the Side Effects of Lowering the Value?
      It may have to be raised back if more swap is added to the system and
      there is not enough room in the swap table to allow for the increased
      space.  If this is not the case, then there is a finer grain of
      interleaving on the system (assuming there is more than one swap
      device) that can provide a performance gain under heavy swap usage.

    What Other Tunable Values Should Be Changed at the Same Time?
      A change to swchunk is independent of any other tunables.

 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]
      swchunk was developed by HP.


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