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Scheduling an Input/Output Channel allowing for Committed and/or Uncommitted Device Allocation Requests

IP.com Disclosure Number: IPCOM000081942D
Original Publication Date: 1974-Sep-01
Included in the Prior Art Database: 2005-Feb-28
Document File: 9 page(s) / 78K

Publishing Venue

IBM

Related People

Boggs, JK: AUTHOR

Abstract

Improved input/output (I/O) device scheduling techniques are described. A first approach is to address the support of response time demands that can be quite severe and quite critical in a human sense, to the maintenance of ongoing processing in an application. A second approach is to maximize the utilization of the I/O facilities of the system. These approaches are diametrically opposed because it is desirable to maximize the use of the I/O facilities, which could preclude sufficient responsiveness to critical applications. Providing critical response to critical applications requires setting aside and not using I/O facilities.

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Scheduling an Input/Output Channel allowing for Committed and/or Uncommitted Device Allocation Requests

Improved input/output (I/O) device scheduling techniques are described. A first approach is to address the support of response time demands that can be quite severe and quite critical in a human sense, to the maintenance of ongoing processing in an application. A second approach is to maximize the utilization of the I/O facilities of the system. These approaches are diametrically opposed because it is desirable to maximize the use of the I/O facilities, which could preclude sufficient responsiveness to critical applications. Providing critical response to critical applications requires setting aside and not using I/O facilities.

A third approach combines the techniques used in the first and second approaches, i.e., (1) heuristic, with device scheduling level determined by periodic sampling of actual rates achieved, and (2) committed, with response time guaranteed based upon maximum byte count and granularity parameters developed at session establishment time (uncommitted requests may be intermixed). Heuristic Determination of Device Rate.

Scheduling of I/O requests should, if adequate requests are presented, utilize the maximum available rate of an I/O device. However, the maximum available rate of a device changes, based upon several items including physical action at the device, buffering at the device, and channel or path contention of requests for different devices. This method is characterized by:

(1) Determining the available device rate.

(2) Distributing requests in time presented to the channel or

path which do not exceed available device rate.

(3) Adjusting available device rate up or down according to

whether requests are being processed on schedule, behind

schedule or ahead of schedule.

(4) Allowing control of device rates which may be used by the

scheduling mechanism.

(5) Using minimum scheduling overhead to achieve items 1 through

4 by lengthening the periods, as scheduling remains stable

and on schedule, used to measure rates.

Define:
(1) A channel or path control block (CB) containing:

(a) The summation of available device rates at this point

in time - (sigma Rav).

(b) The sampling level (Ls) currently active. The choice

of the number and the duration of sampling at a level

can be made appropriate to the channel or path service

rate maximum capability. For channels in the range

of 1-8 megabytes, levels 1-7 are 32, 64, 128...2048

milliseconds.

(c) The address of the next request to be processed by

this channel. This request in turn addresses the next

request, and thus is a queue of work in specified

sequence for this channel to process. (RQT).

1

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(d) Time of day that the current sample period ends (Tp).

(e) A list of device control blocks for devices connected

to this channel. (DCT).

The channel control-block is shown in Fig. 1.
(2) A device control block (DC) containing:

(a) The available rate that this...