Page 254 - DCAP103_Principle of operating system

Unit 7: Secondary Storage Structure

device (primary swap) must be present on the system. During system startup, the location Notes
(disk block number) and size of each swap device is displayed in 512 KB blocks. The swapper
reserves swap space at process creation time, but does not allocate swap space from the disk
until pages need to go out to disk. Reserving swap at process creation protects the swapper
from running out of swap space. You can add or remove swap as needed (that is, dynamically)
while the system is running, without having to regenerate the kernel. HP-UX uses both physical
and pseudo swap to enable efficient execution of programs.
7.6.1 Pseudo-Swap Space

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.
7.6.2 Physical Swap Space

There are two kinds of physical swap space—device swap and file-system swap.

7.6.2.1 Device Swap Space

Device swap space resides in its own reserved area (an entire disk or logical volume of an LVM
disk) and is faster than file-system swap because the system can write an entire request (256
KB) to a device at once.

7.6.2.2 File-system Swap Space

File-system swap space is located on a mounted file system and can vary in size with the
system’s swapping activity. However, its throughput is slower than device swap, because
free file-system blocks may not always be contiguous; therefore, separate read/write requests

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