6. DATA SOURCES › 6.5 RMF LCU Summarized File › 6.5.1 I/O Queueing in the I/O Processor

6.5.1 I/O Queueing in the I/O Processor

The I/O processor, in addition to monitoring the status of
the I/O configuration, is also responsible for starting and
redriving queued I/O requests.  Each IOP has one initiative
queue, which is continuously scanned in an attempt to handle
active I/O requests.  When requests are blocked due to
contention, the IOP temporarily removes them from the
initiative queue rather than continuing to redrive them.

If the contention is for a device, the request is deferred
until the IOP receives notification (device end) that the
busy condition has subsided.

If the contention is for the control unit, the IOP places the
LCU that is requesting service on the control unit header
(CU-HDR) queue.  A request is placed on this queue when all
paths to the device are busy and at least one path to the
physical control unit is busy.  Once the contention is
resolved, the IOP requeues the request from the CU-HDR back
to the IOP initiative queue.  The length of the CU-HDR queue,
consequently, is a measure of contention at the control unit
level.