Request processing with ring acceleration

Ring acceleration is available only when all systems in the ring are running MVS/SP Version 3 or higher. Ring acceleration also requires alternate links, except between systems in a sysplex, and IBM® recommends that the complex be a fully-connected complex. An installation where the complex is the same as the sysplex does not need to perform any additional setup to use ring acceleration; multisystem sysplexes must be fully connected. To achieve full connectivity in a mixed complex, however, see Designing a ring complex for details about how to define the necessary links.

Ring acceleration can significantly reduce the amount of time tasks spend waiting for global resources, especially in a large complex. It can, however, affect recovery. Ring acceleration (ACCELSYS) describes the factors involved in determining how your installation should use ring acceleration.

Using ring acceleration changes the processing of a global resource request. Figure 1 summarizes the processing of a global resource request with ring acceleration. The steps include:

1: The originating system suspends the requesting task.
2: When the incoming RSA-message arrives, the system places the request in the outgoing RSA-message, then passes the RSA-message on to the next system in the ring.
3: Your installation chooses the number of systems that must see the RSA-message before a system sends a “shoulder-tap”, an acknowledgement that it has received the RSA-message, to the originating system. Assuming that the number of systems is two, the next system in the ring sends the shoulder-tap to the originating system.
4: When the originating system receives the shoulder-tap, the ring acceleration signal, it grants access to the resource according to normal ENQ processing. That is, if the resource is available, the system grants the suspended task access to the resource and marks the task as ready to execute. If the resource is not available, the task continues to wait until it becomes available.
5: The RSA-message continues on its cycle around the ring so that each system in the ring knows about the request. The task that requested the resource, however, does not need to wait for the cycle to complete before obtaining access to the resource.

Figure 1. Global Resource Request Processing with Ring Acceleration