and then suspending execution until the next message arrives. VM/SP queue drops
the VT AM service machine when it suspends execution. When the next message
arrives, all the VT AM service machine's pages must be revalidated. If the message
rate is moderate to high, the overhead of repeated queue dropping exceeds the
benefit.
The CP class A command "SET QDROP userid ON/OFF [USERS]" allows the
installation to control this situation. If SET QDROP OFF is in effect for a virtual
machine, the virtual machine remains active in the queue and its pages are not
scanned or flushed. The page stealing mechanism is the only way the pages can be
removed from storage. (Page stealing is invoked only if the flush list is empty.)
Specifying SET QDROP OFF for a service virtual machine may improve system
performance and throughput when queue dropping would otherwise occur rapidly.
But applying SET QDROP OFF indiscriminately may degrade system throughput
by defeating the page flush mechanism and forcing page stealing to take place.
There can also be a relatively large overhead associated with a virtual machine
being queue dropped during communications with a service machine for which the
QDROP OFF specification is in effect. This can occur in small systems in which
there is a high degree of virtual machine intercommunications. Specifying SET QDROP userid OFF USERS addresses this problem by providing for the tempo­
rary extension of the QDROP OFF status to any virtual machine communicating
via VMCF or IUCV to the service virtual machine specified. The QDROP status
for the "served" virtual machine remains in effect only while messages are out­
standing between it and the service machine. Thus performance gains can be real­
ized in systems with heavy usage of products such as IFS or PVM (invoked via the CMS PASSTHRU command). No additional performance gains will be realized in
systems in which PVM is invoked via CP DIAL or with the SNA VTAM service
machine, since the communication is with CP rather than another virtual machine.
The QUERY QDROP command (CP class A and E) may be used to list the userids
for which SET QDROP OFF and the USERS parameter have been specified.
Virtual Machine Assist Feature
The Virtual Machine Assist Feature is a processor hardware feature that improves
the performance of VM/SP. Virtual storage operating systems, which run in prob­
lem state under the control of VM/SP, use many privileged instructions and SVCs that cause interrupts that VM/SP must handle. When the virtual machine assist
feature is used, many of these interrupts are intercepted and handled by the
processor. Consequently, VM/SP performance is improved.
The Virtual Machine Assist Feature intercepts and handles interruptions caused by SVCs (other than SVC 76), invalid page conditions, and several privileged
instructions. An SVC 76 is never handled by the assist feature; it is always handled
by CPo The processing of the following privileged instructions is handled by this
feature:
LRA STCTL RRB ISK SSK IPK STNSM (load real address)
(store control)
(reset reference bit)
(insert storage key)
(set storage key)
(insert PSW key)
(store then AND system mask)
Performance Guidelines 37

STOSM SSM LPSW SPKA (store then OR system mask)
(set system mask)
(load PSW) (set PSW key from address)
Although the assist feature was designed to improve the performance of VM/SP, virtual machines may see a performance improvement because more resources are
available for virtual machine users. For a list of processors on which the Virtual
Machine Assist Feature is available, see the VM / SP Planning Guide and Reference. , Using the Virtual Machine Assist Feature
Whenever you IPL VM/SP on a processor with the virtual machine assist feature,
the feature is available for all VM/SP virtual machines. However, the system
operator's SET command can make the feature unavailable to VM/SP and, subse­
quently, available again for all users. If you do not know whether or not the virtual
machine assist feature is available to VM/SP, use the class A and E QUERY com­
mand. For a complete description of the Class A and E QUERY and SET com­
mands, see the VM / SP Operator's Guide.
If the virtual machine assist feature is available to VM/SP when you log on your
virtual machine, it is also supported for your virtual machine unless you are running
a second level VM/370 or VM/SP system in your virtual machine. If your VM/SP directory entry has the SVCOFF option, the SVC handling portion of the assist
feature is not available when you log on. The class G SET command can disable
the assist feature (or only disable SVC handling). It can also enable the assist fea­
ture, or if the assist feature is available, enable the SVC handling. You can use the
class G QUERY SET command line to find whether you have full, partial, or none
of the assist feature available. For a complete description of the Class G QUERY
and SET commands, see the VM / SP CP Command Reference for General Users.
Restricted Use of the Virtual Machine Assist Feature
Certain interrupts must be handled by VM/SP. Consequently, the assist feature is
not available under certain circumstances. VM/SP automatically turns off the
assist feature in a virtual machine that:
Has set an instruction address stop
Is tracing SVC and program interrupts Since an address stop is recognized by an SVC interrupt, VM/SP must handle SVC interrupts while address stops are set. Whenever you issue the ADSTOP command, VM/SP automatically turns off the SVC handling portion of the assist feature for
your virtual machine. The assist feature is turned on again after the instruction is
encountered and the address stop removed. If you issue the QUERY SET com­
mand line while an address stop is in effect, the response indicates that the SVC handling portion of the assist feature is off.
Whenever a virtual machine issues a TRACE command with the SVC, PRIV, BRANCH, INSTRUCT, or ALL operands, the virtual assist feature is automat­
ically turned off for that virtual machine. The assist feature is turned on again
when the tracing is completed. If the QUERY SET command line is issued while SVCs or program interrupts are being traced, the response indicates the assist fea­
ture is off.
38 VM/SP System Programmer's Guide