Performance Guidelines
In order to invoke the favored execution, priority, reserved page
frames, and locked pages options you must have a virtual machine defined
with the appropriate command privilege classes. Usually, the operator's
virtual machine has the appropriate command classes. Additional
planning is needed to support the virtual=real option and virtual
machine assist as well as VM/370:ECPS. All of these performance options
are described in detail in the Specifying a Virtual=Real Machine
Although the virtual=real option eliminates paging, its main function is
to bypass CCW translation. This is possible because I/O from a virtual
machine occupying a virtual=real space contains a list of CCWs whose data addresses reflect the real storage addresses.
The only exception is virtual page o. Virtual page 0 does not exist
as real page 0; it is relocated to the highest page of the virtual=real
area. In order for the virtual machine to perform input/output into
virtual page 0, the CCW addresses must be translated. When CP loads an operating system into a virtual=real area, it turns
on CCW translation. Once the operating system is loaded, the operator
of the virtual machine may issue a CP command to turn CCW translation
off. When the virtual machine is operating with CCW translation off, it
must not perform I/O into virtual page O. Most operating systems can be
generated so that they do not use this area for input/output. However,
violation of this restriction may cause damage to the entire VM/370 system.
The size of the virtual=real area is specified during CP system
generation. It must be large enough to contain the entire address space
of the largest virtual machine that you execute in the virtual=real
area. Only one virtual=real area can be defined. Only one virtual machine at a time can occupy the virtual=real area. Since the virtual=real option removes pages from the dynamic paging
area, it affects the performance of the other virtual machines. The virtual=real area is set up at V!/370 initial program load (1PL). It can be released by the primary system operator to be used as part of
the dynamic paging area. Once released, it cannot be reclaimed except
by reloading VM/370. The virtual=real area must be released in total,
that is, unused pages of the area cannot be selected for release. If a very large virtual=real area is released after V"/370 initialization, a system performance degradation may occur as more and
more users log on and use the released space. The reason for this is
that the number of pages allocated for CP fixed free storage during VM/370 initialization is based on real machine size minus virtual=real
size. Therefore, the number of fixed free pages allocated for a system
with a virtual=real area may not be enough to accommodate the larger
number of users of the released space, and system overhead may increase
as CP extends to get dynamic free storage pages. Part 1. Planning for System Generation 9
Performance Guidelines
This problem may be counteracted by using the FREE operand in the SYSCOR macro instruction in the system control (DMKSYS) file at system
generation. The SYSCOR macro is described in "Part 2. Defining Your V8/370 System." The examples used in the following discussions assume
that you are allowing V8/370 to determine the number of free storage
pages to allocate.
To use the virtual=real option effectively on a multiport
teleprocessing system with no CCW translation (SET NOTRANS ON), lines
must be dedicated to that system via the ATTACH command or by V8/370 directory assignment. Conversely, on a multiport teleprocessing
virtual=real operation, virtual 2701/2702/2703 lines, (that is, lines
assigned and used by CP's DEFINE and DIAL operate with CCW translation. If you issue the DIAL command while SET NOTRANS ON is in
effect, CCW translation is done for I/O involving that line.
Note that you cannot execute programs with dynamic or self-modifying
channel programs in a virtual=real area if you also use the DIAL
command. Also, you cannot load (via IPL) a shared into a virtqal machine running in the virtual=real area. For a virtual=real machine,
you must issue the IPL command with either a device address or the name
of a non shared system.
To generate CP so that it properly supports a virtual=real area, do
the following: • Specify the VIRT=REAL option in the
virtual machines in your installation
virtual=real area. VM/370 directory for all the
that you plan to run in the • Reserve enough DASD space for the CP nucleus. A CP nucleus that
supports a virtual=real area is larger than one that does not. • Make sure the virtual machine you are using to generate CP has
sufficient virtual storage. • Specify the amount of storage you want reserved for a virtual=real
area.
"Part 2. Defining Your V8/370 System" describes the Directory
program, including information about the VIRT=REAL operand of the OPTION control statement. RESERVING DASD SPACE FOR A CP NUCLEUS WITH A VIRTUAL=REAL AREA
A CP nucleus with the virtual=real option requires more DASD space for
system residence than a CP nucleus without the option. Use the
following formulas to calculate the number of cylinders needed for
system residence (disregard any remainders):
Number of 2314/2319 cylinders = (128+(VRSIZE/4»/32 Number of 3330/3333 cylinders = (171+(VRSIZE/4»/57 Number of 2305/3340 cylinders = (144+(VRSIZE/4»)/24 Number of 3350 cylinders = (240+(VRSIZE/4»/120 where VRSIZE is the size of the virtual=real storage area (in K bytes).
K represents 1024 bytes. This size must be at least 32K bytes. 10 IBM V8/370 Planning and System Generation Guide
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