What is Swap Space（Pseudo-Swap）?

Pseudo-swap space allows for the use of system memory as a third type of swap space. That is, HP-UX swap space can also consist of up to seven-eighths (87.5%) of system memory capacity.

For example, a computer with one GB of system memory and one GB of device and file system swap, can run up to 1.87 GB of processes. If any process attempts to grow or be created beyond this extended threshold, the process will fail.

When using pseudo-swap, since more processes can be created, the system load increases, causing more paging and deactivation activity.

By default, pseudo-swap space is configured to be available. If you do not wish to make use of it, you will need to re-set the tunable system parameter, swapmem_on, to 0 ("off"). (To modify a configurable parameter, see "Making Adjustments to Your System" in Chapter 1.)
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System memory used for swap space is called pseudo-swap space. It allows users to execute processes in memory without allocating physical swap. Pseudo-swap is controlled by an operating-system parameter; by default, swapmem_on is set to 1, enabling pseudo-swap.

Typically, when the system executes a process, swap space is reserved for the entire process, in case it must be paged out. According to this model, to run one gigabyte of processes, the system would have to have one gigabyte of configured swap space. Although this protects the system from running out of swap space, disk space reserved for swap is under-utilized if minimal or no swapping occurs.

To avoid such waste of resources, HP-UX is configured to access up to three-quarters of system memory capacity as pseudo-swap. This means that system memory serves two functions: as process-execution space and as swap space. By using pseudo-swap space, a one-gigabyte memory system with one-gigabyte of swap can run up to 1.75 GB of processes. As before, if a process attempts to grow or be created beyond this extended threshold, it will fail.

When using pseudo swap for swap, the pages are locked; as the amount of pseudo-swap increases, the amount of lockable memory decreases.

For factory-floor systems (such as controllers), which perform best when the entire application is resident in memory, pseudo-swap space can be used to enhance performance: you can either lock the application in memory or make sure the total number of processes created does not exceed three-quarters of system memory.

Pseudo-swap space is set to a maximum of three-quarters of system memory because the system can begin paging once three-quarters of system available memory has been used. The unused quarter of memory allows a buffer between the system and the swapper to give the system computational flexibility.

When the number of processes created approaches capacity, the system might exhibit thrashing and a decrease in system response time. If necessary, you can disable pseudo-swap space by setting the tunable parameter swapmem_on in /usr/conf/master.d/core-hpux to zero.

At the head of a doubly linked list of regions that have pseudo-swap allocated is a null terminated list called pswaplist.