Browse Prior Art Database

Static Unsuppressable Device End Presentation Queues

IP.com Disclosure Number: IPCOM000122217D
Original Publication Date: 1991-Nov-01
Included in the Prior Art Database: 2005-Apr-04
Document File: 6 page(s) / 216K

Publishing Venue

IBM

Related People

Legvold, VJ: AUTHOR [+2]

Abstract

A Description of 3990 Non-Airline Control Unit Static Scan Process is needed to better understand the benefits of the Static Unsuppressible Device End Presentation Queues Process (referred to as DEQ for the remainder of this article).

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 32% of the total text.

Static Unsuppressable Device End Presentation Queues

      A Description of 3990 Non-Airline Control Unit Static
Scan Process is needed to better understand the benefits of the
Static Unsuppressible Device End Presentation Queues Process
(referred to as DEQ for the remainder of this article).

      The Static Scan Process checks each device, one at a time, one
per idle loop, for owed status for the device. The scan starts with
device 0 and increments to the last possible device (31 for DLS mode
and 63 for DLSE mode) and then starts over with device 0.  The device
is checked regardless of whether it is installed or not.  If the
device is found to have owed status, a single Request In is raised.
If the channel cannot honor the request the device will be bypassed.
This means the device will wait for a minimum of 32 (64 if in DSLE
mode) idle loops before Static Scan will have a chance to try to
present its owed status again.  If the channel hardware is not
available for use when the device is checked, the device will be
rescheduled for the next idle loop scan.  The rescheduling will
continue until the channel hardware becomes available, which causes
all the other devices to have to wait.  The DEQ Process is designed
to reduce the disconnect time and to prevent the wasted idle loop
scans where there is Device Ends to present.

      For the 3990 Airline Microcode, during a read or write
operation, if the access to the physical device is required, the
microcode will disconnect from the channel.  After accessing the
physical device, the microcode will schedule a Device End for the
disconnected device so that the channel can reconnect and the
operation can complete.  To schedule the Device End, Owed Status is
placed in the Device's Control Block.  The 3990 Non-Airline microcode
uses a Static Scan Process to find the Owed Status.  This is done by
checking each device control block, one at a time, one per idle loop,
for the Owed Status.  If a device is found to have Owed Status and
the channel hardware for the cluster is not in use, a Request In is
raised for the device and, if the channel responds, the Owed Status
is presented and the CCW chain continues.  If the channel does not
respond the device will wait for its next Static Scan turn during
idle loop.  This causes long disconnect times which is solved by this
invention.

      The DEQ Process structures (refer to Figure 1) are made up of
Channel Queues, one queue per channel interface for each cluster.  A
Channel Queue Element contains a Next Element Pointer and flags.
There is one Queue Element per device with element 0 for device 0,
element 1 for device 1, and so forth.  Each Channel Queue has its own
Head and Tail Pointer which contains the same internal structure as a
Channel Queue Element.  There are two summary bytes per cluster that
reflect the status for the Channel Queues for the Cluster.  The
summary bytes are bit significant, having each bit represent a...