SWCHUNK - man page.

swchunk    :          Tunable Kernel Parameters

 NAME:

      swchunk - swap chunk size in 1 KB blocks

VALUES:

    Default -   2048 blocks

    Allowed values  -          Minimum: 2048 blocks

                                            Maximum: 65536 blocks

 DESCRIPTION:

      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 * 2,147,483,648 (the system maximum number of swap chunks in the swap table). Note that the minimum (or default) value of swchunk therefore allows 4,096 TB of swap space.

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?

      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 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 2,147,483,648 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 cannot 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 2,147,483,648 * swchunk* 1 KB, 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

      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.