Tunable Kernel Parameters

Memory Paging Subsystem
Overview of Memory Paging Parameters
Chapter 7
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Overview of Memory Paging Parameters
Configurable kernel parameters for memory paging enforce operating rules and limits
related to virtual memory (swap space). They fall into the following categories:
Total System Swap
Maximum swap space that can be allocated, system-wide. Parameters
include: swchunk.
Device Swap
Swap space allocated on hard disk devices. Parameters include:
nswapdev.
File System Swap
Swap space allocated on mounted file systems. Parameters include:
allocate_fs_swapmap and nswapfs.
Pseudo-Swap
Use of installed RAM as pseudo-swap, allowing virtual memory space
allocation beyond the limit of swap space on disk devices. Parameters
include: swapmem_on.
Variable Page Sizes
The size of virtual memory pages can be altered to make swap
operations more efficient in particular applications. Parameters
include: vps_ceiling, vps_chatr_ceiling, and vps_pagesize.
Total System Swap
The swchunk parameter determines the size of each chunk of swap area that is created
on successive devices or file systems. Selecting an appropriate value for this parameter
requires extensive knowledge of kernel operation and system internals. Without such
knowledge, do not change swchunk to a nondefault value.
Device Swap
When devices are connected to the system and configured, part of the disk can be
reserved for device swap. The only configurable kernel parameter related to device swap
is nswapdev, which specifies how many devices have been configured with space
allocated for device swap. Data structure storage space is reserved in the kernel for
nswapdev devices.
• If the value of nswapdev is greater than the actual number of swap devices, the small
amount of memory space (<50 bytes per device) allocated for data structures to
support nonexistent devices is wasted and cannot be used for other purposes.
•Ifnswapdev is less than the number of available swap devices, only nswapdev
devices can be accessed because no data structure storage space is available for
supporting the remaining devices.