When an unrecoverable error occurs, DMKMON receives control and attempts to write two tape marks, rewind, and unload the tape. The use of the
tape is discontinued and data collection stops. The operator is informed
of the action taken. Whether or not the write-tape-marks, rewind, and
unload are successful, the tape drive is released. When an end-of-tape condition occurs, DKKMON receives control. A tape
.ark is written on the tape and it is rewound and unloaded. The VM/310 Monitor is stopped and the operator is informed of the action taken. VM/310 MONITOR CONSIDERATIONS The system programmer may want to set the TRACE(1} bit to a 1 in the LOCAL COpy file and reassemble DMKMCC to allow RESPONSE data (MOIITCH class 1) to be collected. See the information about security exposure
in "MONITOR ENABLE Restrictions" in the MONITOR command description. MONITOR START CPTRACE is active after real system IPL (manual or
automatic). The VM/370 Monitor tape data collection is off after IPL. If automatic performance monitoring is specified in the SYSMON macro and
IPL occurs within the range of the TIME operand of the 5YS80N macro. VM/370 monitor data collection to a spool file is started.
If the VM/370 Monitor data collection to a spool file is taking place, a
system shutdown causes closing of the file and termination of
monitoring. If data collection is to tape, a system shutdown implies a MONITOR STOP TAPE command. Normal command processing for the MCIITOS STOP TAPE function is performed by the If the VM/370 system fails and data collection to a spool file is
active, the spool file is closed and preserved, except for the last
buffer. If the VM/370 system fails and data collection is active on
tape, an attempt is made to write two tape marks, rewind, and unload the
tape. If the tape drive fails to rewind and unload, be sure to write a
tape mark befere rewinding and unloading the taFe. VM/370 Monitor data
collection is terminated by the system failure.
122 IBM System Programmer's Guide

If VM/370 monitor data collection is active using tape, a supported tape
drive must be dedicated to the system for the duration of the monitoring. For accounting purposes, all 110 is charged to the system. VM/370 MONITOR DATA VOLUME AND OVERHEAD Use of the VM/370 Monitor usually requires that three pages be locked in
storage for the entire time the VM/370 Monitor is active; however, only
two pages are required if the single buffer option is used with only the PERFORM class of data collection enabled. This reduces by three the Dumper of page frames available for paging. This significantly affects
the performance Qf the rest of the system when there is a limited number
of page frames available for paging. PERFORM This class of data collection is activated once every 60
seconds (or as defined by the MONITOR INTERVAL command). and
records system counters relevant to performance statistics.
It is, therefore, a very low overhead data collection option. RESPONSE This class collects terminal interaction data and, because cf
the human factor, has a very low rate of occurrence relative
to processor Consequently, this class causes
negligible overhead and produces a low volume of data. SCHEDULE This class records the queue manipulation activity of the
scheduler and generates a record every time a user is added to
the eligible list, added to queue1 or queue2, or removed from
queue. The recording overhead is very low. USER This class of data collection is active once every 60 seconds
(or as defined by the MONITOR INTERVAL command). Data is
extracted from each user's VMBLOK, including the system VMBLOK. The overhead incurred is comparable with that of the
statistical data of the PERFORM class; however, it increases
with the number of users logged onto the system. INSTSIM This class of data collection can give rise to large volumes
of data because of the frequency of privileged instructions in
some virtual machines. This may incur significant overhead.
It should be activated for short periods of time and
preferably, though not necessarily, when other classes of data
collection are inactive. If the Virtual Machine Assist
feature is active for the virtual machine, the data volume
and, consequently, the CP overhead may be reduced. DASTAP This class of data collection samples device activity counts
once every 60 seconds (or as defined by the MONITOR INTERVAL command) and is a very low source of overhead, similar to the PERFORM and USER classes. SEEKS This class of data collection can give rise to large volumes
of data because every start I/O request to DASD is recorded
via a MONITOR CALL. SYSPROF This class of data collection is complementary to the SCHEDULE and DASTAP classes and results in a small amount of additional
overhead. It obtains more refined data on DASD resource
usage. Part 2. Control Program (CP) 123