swchunk(5) File Formats Manual swchunk(5)
NAME
swchunk - swap chunk size in 1 KB blocks
VALUES
Default
Allowed values
blocks
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 mem-
ory, 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.
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 sys-
tem is * 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
therefore allows 4,096 TB of swap space.
The way to think of 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 possibil-
ity 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 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 when the number of chunks needed to represent the system swap space is less than 2,147,483,648 could hinder system per-
formance 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 * 1 KB, then kernel memory usage can be reduced by lowering 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 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 tun-
able values, consult the documentation for the kernel software being installed. For information about optional kernel software that was
factory installed on your system, see at
AUTHOR
was developed by HP.
Tunable Kernel Parameters swchunk(5)