Scheduling priority function

Another scheduling approach is list scheduling, which schedules operations into control steps, one control step at a time. For the current control step, a list of data ready operators is constructed, containing those operators whose inputs are produced in earlier control steps, and that do not violate any resource constraints. This list is then sorted according to some priority function, the highest-priority operator is placed into the current control step, the list is updated, and the process continues until no more operators can be placed into that control step. This process is then repeated on the next control step, until the entire design is scheduled. Two common priority functions are mobility... 

The CSTEP control step scheduler uses list scheduling on a block-by-block basis, with timing constraint evaluation as the priority function. Operations are scheduled into control steps one basic block at a time, with the blocks scheduled in executidepth-first traversal of the control flow graph. For each basic block, data ready operator are considered for placement into the current control step, using a priority function that reflects whether or not that placement will violate timing constraints. Resource limits may be applied to limit the number of operators of a particular type in any one control step. 

Uses list scheduling, with expected freedom (mobility) as the priority function. 

Fisher [FisherSl] describes the list scheduling algorithm as it is used in microcode compaction. CSTEP uses list scheduling with a number of modifications that support scheduling for synthesis. First, it supports primitive estimation of combinational delay. Second, it uses this estimation to support multiple cycle operators and cascaded operators. Third, it uses a modified priority function in which negative priority values force placement of operators to be delayed. 

Timing constraint evaluation is used as the primary priority function. For an operator %a that is placed in a control step, timing constraints which involve Xa relative to previously scheduled operators are evaluated by counting the number of control steps between them. This value is compared to the value of each constraint to determine whether the constraint is satisfied. 

During the execution of the CSTEP algorithm, the primary priority function evaluates data-ready operators for scheduling into the current control step csjj. It first considers the impact of this placement on minimum-time constraints. If scheduling an operator into the current control step violates any minimum-time constraints, this placement is considered too early and must be delayed to a later control step. This is... 

In the case where it is acceptable to schedule an operator, but not imperative to do so immediately, the decision will be passed on to the secondary priority function. 

If the secondary function determines that placement of the current operator would violate the maximum hardware specifications, one of two actions will take place. If the user has chosen to run in "hard mode, the maxlist values are accepted as hard limits that cannot be violated by the scheduler, so a negative value is returned by the priority function so that placement is delayed. If, however, the user has chosen "soft mode," wherein the initial maxlist values imply a lower limit on hardware and CSTEP may choose higher limits according to utilization, the tertiary priority function is invoked. 

One commonly used approach is a list scheduling (LS) technique [1] where a hardware constraint is specified and the algorithm attempts to minimize the total execution time by using a local priority function to defer operations when resource conflicts occur. A different sjppTOxh,force-directed scheduling (FDS),... 

In force-directed list scheduling (FDLS), the approach is similar except that force is used as the priority function. More precisely, whenever a hardware constraint is exceeded in the course of regular scheduling, force calculations are used to... 

Because of the batchwise nature of electronic materials processing and the long process sequences, a supervisory system is essential. This system collects information on the state of the system gives status of work in progress schedules work based on process priorities, product requirement, equipment readiness, and materials availability and controls product and raw material inventories. Materials handling concerns the physical movement of wafers through the fabrication line via various mechanical means [32] and is therefore primarily a mechanical engineering problem. The control of the individual unit operations and the incorporation of local control functions in... 

Much of the extensive work that has been done on 5-hydroxytrypt-amine (HT serotonin) since its isolation and identification twenty years ago has been directed to uncovering its possible function in neural activity. As no short review can include references to all the numerous contributions, the authors have tried to select those references that are recent and include a significant bibliography. No attempt has been made to designate priorities for the observations. Two books and a symposium scheduled for publication in 1968 provide full references and detailed discussions of all aspects of the subject. 

Simple heuristics such as the five described above provide adequate results only for the simplest scheduling problems. Real-world scheduling problems usually require techniques that are significantly more sophisticated than a myopic priority rule that just orders the jobs according to a function of one or two parameters. There are various ways of formulating scheduling problems as well as various types of solution procedures. These are discussed next. 

Scheduling criteria. In designing schedulers, it is important to determine the desirable criteria for a scheduler. The low-level scheduler basically determines which task has the highest priority and then selects that task for execution it also maintains the ready queue. If the task currently executing has a lower priority than a ready task, the scheduler may preempt the executing task in favor of the waiting task. (This is particularly true in systems which have a real-time function.)... 

Many processes have known functions and durations and can be given a permanent, fixed priority. In process control applications, where the duration and function of each process is known at design time, it may be appropriate to assign processes fixed priorities and place them in a static order. In this case, the scheduler begins at the top of the queue of processes and searches down for the first runable process. 

Clinical engineering identifies life-support equipment in the AIMS database. The definition of life-support equipment adopted by clinical engineering is medical devices intended to sustain life and whose failure to perform its primary function, when used according to manufacturer s instructions and clinical protocol, is expected to result in imminent death in the absence of immediate intervention. The maintenance of life-support equipment receives the highest priority in the medical equipment management efforts. The inspection schedule compliance goal is 100%. 

---