nucleus is relocated and placed beyond the virtual=real machine in real
storage. This option is discussed in more detail in "VM/370 Performance nn";"ft"" " Since the entire address space required by the virtual machine is
locked, these page frames are not available for use by other virtual
machines except when the virtual=real machine is not on. This
option often increases the paging activity for other virtual machine
users, and in some cases for VM/370. (Paging activity on the system may
increase substantially, since all other virtual machine storage
requirements must be managed with fewer remaining real page frames.)
The virtual=real option may be desirable or mandatory in certain
situations. The virtual=real option is desirable when running a virtual
machine operating system (like DOS/VS or OS/VS) that performs paging of
its own because the possibility of double paging is eliminated. The
option must be used to allow programs that execute self-modifying
channel programs or have a certain degree of hardware timing
dependencies to run under VM/370. VM/370 Performance Options VM/370 provides a number of options an installation may use to improve
the performance of virtual machines and VM/370. Several options improve
the performance of installation specified virtual machines; other
options improve the performance of all virtual machines and VM/370. The
options, described in the following discussion are: I Favored execution I User priority I Reserved page frames I Virtual=real I to.! Affinity I Virtual machine assist I Extended Control-Program Support
Specifying a performance option may mean making a performance
trade-off; improving the performance of one virtual machine at the
expense of VM/370 and other virtual machines. For example, after an
operator specifies favored execution for a virtual machine, that virtual
machine receives more processor time than other virtual machines= Therefore, before specifying any performance option, identify the
option's performance trade-offs and assess their impact on system
performance. FAVORED EXECUTION The favored execution options allow an installation to modify the normal
scheduling algorithms and force the system to devote more of its
processor resources to a given virtual machine than would ordinarily be
the case. The options provided are: The basic favored execution option The favored execution percentage option
Part 2. Control Program (CP) 93
The basic favored execution option means that the virtual machine so
designated is not to be dropped from the active (in queue) subset by the
scheduler, unless it becomes nonexecutable. When the virtual machine is
executable, it is to be placed in the dispatchable list at its normal
priority position. However, any active virtual machine represents
either an explicit or implicit commitment of main storage. An explicit
storage commitment can be specified by either the virtual=real option or
the reserved page frames option. An implicit commitment exists if
neither of these options is specified, and the scheduler recomputes the
virtual machine's projected work-set at what it would normally have been
at queue-drop time. Multiple virtual machines can have the basic
favored execution option set. However, if their combined main storage
requirements exceed; the system's capacity, performance can suffer
because of thrashing.
If the favored task is highly compute bound and must compete for the
processor with many other tasks of the same type, an installation should
define the processor allocation to be made. In this case, the favored
execution percentage option can be selected for one virtual machine.
This option specifies that the selected virtual machine, in addition to
remaining in queue, is guaranteed a specified minimum percentage of the
total processor time if it can use it. To select the favored execution
option, specify the FAVORED operand on the class A, B, or F SET command.
After the option is invoked, VM/310 provides processor time for the
selected virtual machine as follows:
1. The in-queue time slice is multiplied by the specified percentage
to arrive at the virtual machine's guaranteed processor tiae. 2. The favored virtual machine, when it is executable, is always
placed at the top of the dispatchable list until it has obtained
its guaranteed processor time.
3. If the virtual machine obtains its guaranteed processor time before
the end of its in-queue time slice, it is placed in the
dispatchable list according to its calculated dispatching priority.
4. In either case (2 or 3), at the end of the in-queue time slice the
guarantee is recomputed as in step 1 and the process is repeated.
For a description of the SET command, see the Whether or not a percentage is specified, a virtual machine with the
favored execution option active is kept in the dispatching queues except
under the following conditions: Entering CP console function mode Loading a disabled PSW Loading an enabled PSW with no active I/O in process Logging on or off When the virtual machine becomes executable again, it is put back on the
executable list in Q1. If dropped from Q1, the virtual machine is
placed directly in Q2 and remains there even though it may exhaust its
allotted amount of processor usage. Virtual machines with this option
are thus considered for dispatching more frequently than other virtual
Note, however, that these options can impact the response time of
interactive users and that only one favored percentage user is allowed
at any given time.
94 IBM VM/370 System Programmer's Guide
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