MVS Allocation

In systems that do not use MDS or the WLM functions of MVS™, jobs are presented to MVS based on criteria such as job class, priority, or workload mix. In these systems, a job's requirements are not known until the job entry subsystem selects the job for execution, and a system initiator begins the step allocation process. At each job step, MVS allocation attempts to satisfy the requirements for the step, in contention with every other job step currently executing on the same processor. If the requirements cannot be met, MVS allocation gives the operator the option of canceling the job or allowing it to wait for resources. Thus, in a system that does not use MDS, there may be jobs executing and other jobs waiting for resources.

The jobs waiting in MVS allocation hold critical resources (a system initiator, an address space, data sets, and possibly devices). Holding these resources longer than necessary makes it very difficult for the system programmer to determine how many initiators should be started to keep the system fully used, because at any given time, an unknown number of initiators may be waiting. MDS offers a solution to this problem.