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 theVM/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 usingVM/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 theVM/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 betweenVM/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 CPSET 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
TheCMS 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. TheeMS/DOS environment of CMS simulates DOS/VS supervisor and input/output
functions, thereby allowing execution of manyDOS 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 theVK111Q Syste! for more details. 40 IBM VM/370 Planning and System Generation Guide
and (2) the operating systems virtual storage size equals the virtual
storage size of the
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
handshaking feature, the guest system avoids some of the instructions
and procedures that would be inefficient in the
system control program closely resembles the standalone operation
because much redundancy of function between
system is eliminated. For instance:
be brought into real storage.
because output files are automatically closed and processed.
the pseudo page fault portion of the handshaking feature is not
available until it is set on with the CP
command can set pseudo page fault handling on and off.
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.
(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.
multiple copies of the operating system
residence volume should be read-only.
makes it
online.
unnecessary to keep
The shared system
The
be shared among virtual machines.
also be shared among all virtual machines; they are outside the virtual
storage of each of the sharing virtual machines. The
functions, thereby allowing execution of many
removed from the system residence volume. Refer to the