Specifications

Chapter 4. z/VM notes 71
4.9 Paging
If you run z/OS (or another System z operating system) under z/VM on a zPDT base
machine, remember that you have three levels of paging:
򐂰 The base Linux system pages whenever virtual memory usage exceeds the available real
memory. Our advice to have at least 1 GB more real memory than your defined for System
z memory size. This is intended to minimize Linux paging. A Linux page fault in the
primary 1090 CP process causes the CP to pause until the page fault is resolved.
򐂰 z/VM pages when its requirement for virtual memory exceeds the defined System z
memory (which is actually base Linux virtual memory). Each z/VM guest (whether a CMS
user or a whole z/OS system) resides in z/VM virtual memory. z/VM systems on larger
machines, running multiple significant guests, tend to page rather heavily.
򐂰 z/OS (or another System z operating system) pages when its need for real memory (which
is actually z/VM virtual memory
10
) exceeds whatever size was defined in the z/VM
directory for the z/OS guest.
A zPDT system based on a mobile computer has limited I/O bandwidth to its disk, and that
bandwidth is best used for running applications rather than for paging. Our advice is to
consider your memory usage carefully when planning to use z/VM for multiple guests.
10
While conceptually true (ignoring V=R and similar environments), the implementation details depend on the level
of assists enabled through the SIE instruction.