Planning Considerations for Other Virtual Planning Considerations for Virtual Machine
Operating Systems (Other than CMS) This section contains information about the following: • The V8/VS Handshaking Feature • Virtual Machines Using the Same Operating System • The RSCS Virtual • Using Channel Switching • Operating Systems Using Reserve/Release
The VM/VS Handshaking Feature
The Handshaking feature is a communication path between VM/370 and
certain other system control programs (such as OS/VS1) that makes each
system control program aware of certain capabilities and requirements of
the other. The VM/VS Handshaking feature consists of: • Processing pseudo page faults • Closing VM/370 spool files when the system control program's output
writer operation is complete • Providing an optional nonpaging mode for operating systems executing
under the control of VM/370 • Providing miscellaneous enhancements for an operating system's
virtual machine executing under the control of VM/370 A page fault is a program interrupt that occurs when a page that is
marked "not in storage" is referred to by an instruction in an active
page. The virtual machine requesting the page is placed in the wait
state while the requested page is brought into real storage. However,
with the handshaking feature, a multiprogramming operating system
executing under the control of VM/370 in a virtual machine may dispatch
one task while waiting for V8/370 to honor a page request for another
task. When the pseudo page fault portion of V8/VS Handshaking is
active, V8/370 sends a pseudo page fault (program interrupt X'14') to
the guest system. When the guest system recognizes a pseudo page fault,
it places only the task waiting for the page in page wait and can
dispatch any of its other tasks. Since no paging is done by the operating system using VM/VS handshaking, ISAM programs are treated by as if they are being
run from fixed storage locations. Therefore, in order to execute the ISAM program successfully, the virtual machine directory must include
the ISAM option. When the handshaking feature is active, the operating system using VM/VS handshaking closes the CP spool files by invoking the CP CLOSE command when the task or job has completed. Once these spool files are
closed, they can be processed by without operator intervention. Part 1. Planning for System Generation 39

Planning Considerations for Other Virtual Machines Operating systems using VM/VS handshaking can execute in nonpaging mode. Nonpaging mode exists when (1) the handshaking feature is active,
and (2) the operating systems virtual storage size equals the virtual
storage size of the VM/370 virtual machine. When the guest operating
system executes in nonpaging mode, fewer privileged instructions are
executed and duplicate paging is eliminated. Note that such a virtual
machine may have a larger working set when it is in non paging mode than
when it is not in nonpaging mode.
Also, there are some other enhancements for guest systems using VM/VS handshaking while executing under the control of VM/310@ With the
handshaking feature, the guest system avoids some of the instructions
and procedures that would be inefficient in the VM/310 environment. When the VM/VS Handshaking feature is active, the operation of a
system control program closely resembles the standalone operation
because much redundancy of function between VM/310 and the operating
system is eliminated. For instance: • One VS1 task can be dispatched while another is waiting for a page to
be brought into real storage. • There is less need for the virtual machine operator to intervene
because output files are automatically closed and processed. Even when the handshaking feature is active for a virtual machine,
the pseudo page fault portion of the handshaking feature is not
available until it is set on with the CP SET PAGEX ON command; this
command can set pseudo page fault handling on and off. MUltiple Virtual Machines Using the Same
Operating System
In general, an operating system which is to run in a virtual machine
should have as few options generated as possible. This is also true
when several virtual machines share a system residence volume. Very often, options that improve performance on a real machine have no effect
(or possibly an adverse effect) in a virtual machine. For example, seek
separation, which improves performance on the real machine, is redundant
in a virtual machine: CP itself issues a standalone seek for all disk
I/O. Sharing the system residence volume
multiple copies of the operating system
residence volume should be read-only.
makes it
online.
unnecessary to keep
The shared system
The CMS system residence volume, for example, is read-only, so it can
be shared among virtual machines. CMS discontiguous saved segments can
also be shared among all virtual machines; they are outside the virtual
storage of each of the sharing virtual machines. The eMS/DOS environment of CMS simulates DOS/VS supervisor and input/output
functions, thereby allowing execution of many DOS programs. DOS and OS systems can be shared among users if all data sets with write access are
removed from the system residence volume. Refer to the VK111Q Syste! for more details. 40 IBM VM/370 Planning and System Generation Guide