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State-Defined Recoverable Queues

IP.com Disclosure Number: IPCOM000049493D
Original Publication Date: 1982-Jun-01
Included in the Prior Art Database: 2005-Feb-09
Document File: 3 page(s) / 49K

Publishing Venue

IBM

Related People

Goldstein, BC: AUTHOR [+2]

Abstract

The following identifies the state of any item on a queue (or in queue buffer space). State transitions can be made without requiring any queue access serialization. The states of a given item of the queue are: (1) Physically on the queue (01) but not logically on (i.e., a DEQ'er could not receive this item). (2) Logically on the queue (11)-a DEQ'er could dequeue this item. (3) Logically taken from the queue (10) - as a result of DEQ but still physically residing on the queue. Subsequent DEQ's will not find this item. (4) Free (00) - this item represents a free item on the queue, and can be used to place physically an item on the queue. This is set forth pictorially in the figure.

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State-Defined Recoverable Queues

The following identifies the state of any item on a queue (or in queue buffer space). State transitions can be made without requiring any queue access serialization.

The states of a given item of the queue are: (1) Physically on the queue (01) but not logically on

(i.e., a DEQ'er could not receive this item).

(2) Logically on the queue (11)-a DEQ'er could dequeue

this item.

(3) Logically taken from the queue (10) - as a result of DEQ

but still physically residing on the queue.

Subsequent DEQ's will not find this item.

(4) Free (00) - this item represents a free item on the

queue, and can be used to place physically an

item on the queue.

This is set forth pictorially in the figure.

(1) When an item gets moved into physical queue space, space

is allocated for the item and a slot is allocated off

the free stack (through FREE-STACK HEAD). The STACK BITS

in the slot are set to 01.

(2) When the item is to be ENQUEUED onto the queue, then

the slot allocated in (1) is chained

onto the queue from TAIL and the state bits are set to

11. This item is now eligible for DEQ.

(3) When an item is to be DEQ'd (assuming FIF0 discipline),

then the item is removed from the queue chain

(using HEAD) and the state bits are set to 10.

(4) When the process that did the DEQ has successfully

completed (i.e., reached a COMMIT point in IMS

terminology) then the item space is freed and the slot

is returned to the free chain with state 00 (using

FREE-STACK TAIL).

There is a checkpoint process in IMS which writes all changed pages in the queue spa...