Control Unit Queuing for Reserved Device Access Contention
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Solution Overview
Problem
In input/output processing systems, device contention issues arise when multiple operating systems attempt to access a reserved I/O device, leading to disparities in access times due to faster responding host systems blocking slower ones, resulting in inefficient communication throughput.
Innovation Solution
A control unit manages command messages by queuing them on a device busy queue when the device is reserved, monitoring for a device end indicator to service the queue, thereby reducing contention and ensuring equitable access for all operating systems.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If multiple operating systems attempt to access a reserved I/O device simultaneously, then the device can serve multiple systems, but faster responding host systems block slower ones, creating access disparity and reducing fairness
Solution Approach 1:
The control unit preliminarily queues access requests from multiple operating systems before the I/O device becomes available. When a device becomes free, the control unit checks the queue and grants access to the waiting OS in order of queue insertion, preventing race conditions and ensuring fair access regardless of response speed.
Solution Approach 2:
The control unit acts as an intermediary between multiple operating systems and the reserved I/O device. It receives access requests from multiple OSes, manages the queuing mechanism, and mediates access granting based on queue status and device availability, eliminating direct competition between OSes.
2Measurement precision
If the channel subsystem processes I/O operations on a per CCW basis, then detailed command tracking is achieved, but processing overhead increases significantly
Solution Approach 1:
The I/O processing is segmented into two distinct responsibilities: the channel subsystem handles high-level command submission and response reception, while the control unit handles detailed command parsing, state tracking, and processing. This segmentation moves the complex per-CCW processing burden from the channel subsystem to the control unit, reducing channel overhead while maintaining precise command tracking at the control unit level.
Data Source
AI summary
A computer program product, an apparatus, and a method for reducing reserved device access contention at a control unit in communication with a plurality of operating systems via one or more channels are provided. The computer program product includes a tangible storage medium readable by a processing circuit and storing instructions for execution by the processing circuit for performing a method that includes receiving a command message at the control unit from a first operating system, including an I/O operation command for a device. A device busy indicator is received, indicating that a second operating system has reserved the device. The command message is queued on a device busy queue in response to the device busy indicator. The control unit monitors for a device end indicator. The device busy queue is serviced to perform the I/O operation command in response to the device end indicator.