Page 257 - DCAP103_Principle of operating system

Principles of Operating Systems

Notes 7.6.7 Three Rules of Swap Space Allocation
Start at the lowest priority swap device or file system. The lower the number, the higher priority;
that is, space is taken from a system with a zero priority before it is taken from a system with
a one priority.
If multiple devices have the same priority, swap space is allocated from the devices in a round-
robin fashion. Thus, to interleave swap requests between a number of devices, the devices should
be assigned the same priority. Similarly, if multiple file systems have the same priority, requests
for swap are interleaved between the file systems.
In the figure below swap requests are initially interleaved between the two swap devices at
priority 0.
If a device and a file system have the same swap priority, all the swap space from the device is
allocated before any file-system swap space. Thus, the device at priority 1 will be filled before
swap is allocated from the file system at priority 1.
Figure 7.7: Choosing a Swap Location

swdev_pri swdevt swaptab
0
1 dev1
dev2
10 dev3

swfs_pri
0 fswdevt
1
fs1

10

7.6.7.1 Swap Space Structures
Swapping is accomplished on HP-UX using the following data structures.

• Device swap priority array (swdev_pri[ ]), used to link together swap devices with the
same priority. That is, the entry in swdev_pri[n] is the head of a list of swap devices having
priority n. The first field in swdev_pri[ ] structure is the head of the list; the sw_next field
in the swdevt[ ] structure links each device into the appropriate priority list.

• File system swap priority array (swfs_pri[ ]), which serves the same purpose as swdev_pri[ ],
but for file system swap priority.
• Device swap table (struct swdevt), defined in conf.h to establish the fundamental swap
device information.
• File system swap table (struct fswdevt), defined in swap.h for supplimentary file-system
swap.
• Swap table of available chunks (struct swaptab), which keeps track of the available free
pages of swap space.
• Mapping of swap pages (struct swapmap), whose entries together with swaptab combine
for a swap disk block descriptor. The following table details the elements of the struct
swdevt.

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