Monitor
Class
6
7
Keyword DASTAP SEEKS Data Collection Function
Periodically samples device I/O activity counts (SIOs), for
tape and DASD devices only. DASTAP samples only those
tapes and DASD devices that are online when the MONI­ TOR START command is issued.
It is possible that the number of DASD and tape devices
defined in DMKRIO may exceed 291 (the maximum number
of MONITOR DASTAP records that fit in a MONITOR buffer). The following algorithm determines which devices
are monitored:
1. If the total number of DASD and tape devices that are
online is less than or equal to 291, all online DASD and
tape devices are monitored.
2. If the total number of online DASD devices is less than
or equal to 291, all online DASD devices are monitored.
3. Otherwise, the first 291 online DASD devices are moni­
tored.
Collects data for every I/O request to DASD. Reveals
channel, control unit, or device contention and arm move­
ment interference problems.
Note: When NOTRANS is in effect for a virtual=real
machine, no meaningful data is collected.
No data is collected for TIO or HIO operations. For SIO operations, data is collected when the request for the I/O operation is initially handled and again when the request is
satisfied.
This means that a single SIO request could result in two MONITOR CALLs. For example, if the request gets queued
because the device is already busy, then a MONITOR CALL
would be issued as the request is queued. Later, when the
device becomes free and is restarted, a second MONITOR CALL is issued.
In general, the data collected is the same except that in the
first case nonzero counts are associated with queued
requests.
If the request for I/O is satisfied when it is initially handled
without being queued, only one MONITOR CALL results.
In both this case and the second of the two data collections
mentioned above, the count of I/O requests queued for the
device is zero.
Performance Observation and Analysis 55

Monitor
Class
8
Keyword SYSPROF Data Collection Function
Collects data complementary to the DAST AP and SCHED­ ULE classes in order to provide a more detailed "profile" of
system performance through a closer examination of DASD utilization.
VM/SP Monitor Response to Unusual Tape Conditions
Suspension
Unrecoverable Tape Error
End-of-Tape Condition
When I/O to the tape is requested, the device may still be busy from the previous
request. If this occurs, two data pages are full and data collection must be tempo­
rarily suspended. Control register 8 is saved and then set to zero to disable MON- 1ToR CALL program interruptions and timer data collection. A running count is
kept of the number of times suspension occurs. The current Monitor event is dis­
regarded. When the current tape I/O operation ends, the next full data page is
scheduled for output. MONITOR CALL interruptions are reenabled (control reg­
ister 8 is restored), a record containing the time of suspension, the time of
resumption, and the suspension count is recorded and data collection continues.
The suspension count is reset to zero when the MONITOR STOP TAPE is issued.
If a MONITOR command is issued at the time monitor is suspended, a message is
displayed to the invoker of the command stating, SEEK, STOP, OR CLOSE CMD IS IN PROGRESS, RETRY
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 discon­
tinued 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, DMKMON receives control. A tape mark
is written on the tape and it is rewound and unloaded. The VM/SP Monitor is
stopped and the operator is informed of the action taken.
VM/SP Monitor Considerations
Initial Program Load
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 (MONITOR 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 automat­
ic). The VM/SP Monitor tape data collection is off after IPL. If automatic per­
formance monitoring is specified in the SYSMON macro and IPL occurs within the
range of the TIME operand of the SYSMON macro, VM/SP monitor data col­
lection to a spool file is started.
56 VM/SP System Programmer's Guide