Using Processor Resources CP allocates the processor resource to virtual machines according to
their operating characteristics, priority, and the system resources
available. Virtual machines are dynamically categorized at the end of each time slice as interactive or noninteractive, depending upon the frequency of
operations to or from either the virtual system console or a terminal
controlled by the virtual machine. Virtual machines are dispatched from one of two queues, called Queue
1 and Queue 2. In order to be dispatched fro. either a virtual machine must be considered executable (that is, not waiting for some activity or for some other system resource). Virtual machines are not
considered dispatchable if the virtual machine: • Enters a virtual wait state after an I/O operation has begun. • Is waiting for a page frame of real storage. • Is waiting for an I/O operation to be translated by CP and started. • Is waiting for CP to simulate its privileged instructions. • Is waiting for a CP console function to be performed. Queue 1 Virtual machines in Queue 1 (Q1) are considered conversational or
interactive users, and enter this queue when an interrupt from a
terminal is reflected to the virtual machine. Users are considered for
dispatching from this queue on a first-in-first-out (FIFO) basis. When a virtual machine uses more than a certain amount of processor time without entering a virtual wait state, that user is placed in Queue 2. Virtual machines are dropped fro. Q1 when they complete their ti.e slice of processor usage, and are placed in an "eligible list". Virtual machines entering CP command mode are also dropped from Q1. When the
virtual machine becomes executable again (returns to execution mode) it
is placed at the bottom of 01. Queue 2 Virtual machines in Queue 2 (02) are considered noninteractive users. Users are selected to enter Q2 from a list of eligible virtual machines (the "eligible list"). The list of eligible virtual machines is sorted
on a lIFO basis within user priority (normally defined in the user
record in the V8/310 directory, but may be altered by the system operator). Usually, a virtual aachine is selected to enter 02 only if its
"working set" is not greater than the number of real page frames available for allocation at the time. The working set of a virtual
aachine is calculated and saved each time a user is dropped froa 02 and
is based on the number of virtual pages referred to by the virtual
aachine during its stay in Q2, and the nuaber of its virtual pages that
are resident in real storage at the time it is dropped froa the queue.
CP Introduction 1-11

If the calculated working set of the highest priority virtual machine
in the eligible list is greater than the number of page frames available
for allocation, then 15 percent of the working set for that virtual machine is calculated. If the pages required for 15 percent of the
working set are available, the virtual machine is placed on 02. otherwise, the virtual machine remains on the eligible list until there
are no other users on 01 or 02. Executable virtual machines are sorted by "dispatching priority".
This priority is calculated each time a user is dropped from a queue and
is the ratio of processor time used while in the queue to elapsed time in the queue. Infrequent processor users are placed at the top of the
list and are followed by more frequent processor users. When a
nonexecutable user becomes executable, he is placed on the queue based
on his dispatching priority. When a virtual machine completes its time slice of processor usage,
it is dropped from 02 and placed in the eligible list by user priority. When a user request in 02 enters CP command mode, it is removed from 02. the tc .i=tual .achina execution mode), it is placed in the eligible list based on user
priority.
If a user's virtual machine is not in 01 or 02, it is because: • The virtual machine is on the "eligible list," waiting to be put on 02 -- or -- • The virtual machine execution is suspended because the user is in CP mode executing CP commands To leave CP mode and return his virtual machine to the "eligible
list" for 02, the user can issue one of the CP commands that transfer
control to the virtual machine operating system for execution (for example, BEGIN, IPL, EXTERNAL, and RESTART). In CP, interactive users (01), if any, are considered for dispatching
before noninteractive users (02). This means that CMS users entering commands that do not involve disk or tape I/O operations should get fast
responses from the VM/310 system even with a large number of active
users. An installation may choose to override the CP scheduling and
dispatching scheme and force allocation of the processor resource to a
specified user, regardless of its priority or operating characteristics.
The favored execution facility allows an installation to:
1. Specify that one particular virtual machine is to receive up to a
specified percentage of processor time. 2. Specify that any number of virtual machines are to remain in the
queues at all times. Assignment of the favored execution option is
discussed in the "Preferred Virtual Machines" section.
1-12 IBM VM/310 System Logic and Problem Deteraination--Voluae 1