By Scheduling (English for "schedule production "), also timing price increase mentioned, one understands the production of a flow diagram (schedule), processes temporally limits resources assigns.

Scheduling finds mainly in the management economics and computer science application. In the marketing and management Scheduling specifies usually, which orders when and at which production machines are implemented. In computer science Scheduling specifies usually, which process when and how much processor time and main memory receive.

Criteria

A good Scheduling procedure is characterised by the fact that it optimizes the following criteria:

• Throughput. As much as possible processes are processed in as short a time as possible. Instead of maximizing directly the throughput, many procedures optimize instead the efficiency, i.e. resources the available are as completely as possible charged to capacity. Both criteria are equivalent, i.e. the fulfilment one always brings the fulfilment of the other one with itself.
• Fairness. Resources are fairly assigned to the processes, i.e. no process is durably neglected. One says also, the procedure avoids "the Verhungern "(starvation) of processes.
• Date observance. Processes, which must be terminated to a certain date, are planned in such a way that the date is kept. While in the marketing and management precise dates which can be kept are called "DEAD LINEs "and dates approximately which can be kept "completion dates ", one speaks in computer science only of DEAD LINEs and differentiates instead following developments "of the real time ability ": "Hard real time "does not keep all DEAD LINEs precisely, "soft real time "to some extent keeps DEAD LINEs and best EFF place ("as well as possible ") assures adherence to the DEAD LINEs.

Apart from these general optimization criteria occasionally further secondary conditions are required, e.g.:

• Retention time. Processes should be as fast as possible terminated.

Preemptive and non-preemptive procedures

One differentiates between preemptive (preemptive, "prematurely emptying ") of non-preemptive procedures. A non-preemptive Scheduling procedure hands necessary resources to a process over and waits, until it is completely processed. A preemptive procedure can extract resources before completion from the process again around it in the meantime different processes apportionable. The process is interrupted thereby in its execution and remained in its momentary condition, until again resources are assigned to it.

Special terms of the management economics

Management economics and computer science have different terminologies for the same circumstances. In the management economics one uses the following terms:

• Order (job) is equivalent to "process "and designates the execution of certain operations using machines. They are more near specified by the following data:
• Operating time (processing time) is the temporal duration, which an order at (determined) a machine must be worked on.
• Reading in time (release DATE) is the time, to which the order in the system arrives, i.e. the time, at which at the earliest with the treatment can be begun.
• Weight (weight) is equivalently to the Nebenkriterium "retention time "and designates a priority factor, which describes the urgency of an order compared with other orders in the system.
• Completion date (due DATE) designates the time, at which an order should be processed. Here completion dates only absolutely which can be kept are called '' DEAD LINE ''.

Both jobs, which are processed before the planned completion date, and jobs, which cannot keep it and are terminated only later, cause costs. These are called "early costs" and "tardy costs". One calls the order, in which a job goes through several machines, way ("route").

With the solution of Scheduling problems various restrictions ("constraints") must be considered - in such a way e.g. for execution by jobs resources (e.g. machines, mechanics, processors etc.) are used, which are available only to limited extent.

One differentiates frequently additionally between hard restrictions ("hard constraints"), which absolutely to be kept is, and soft restrictions ("softly constraints"). Among the hard restrictions ranks among other things the above example and all restrictions of physical nature (e.g. preparation times). Soft restrictions are such, which serve for the optimization of the plans, but not to be necessarily kept must. Thus if necessary the possibility exists of taking after full extent of utilization of the existing personnel capacities additional capacity in the form of overtime in requirement.

Further typical restrictions are given of planning the completion dates, which represent however usually weaker restrictions than the resources-conditioned or technical restrictions, as well as reading in times, which are to prevent that with production one begins, although necessary materials are not yet present.

Scheduling problems

Scheduling problems become frequently by the system configuration, which defines given restrictions and the which is the basis objective.

The simplest system configuration is the single machine Model. It exists only one machine, on which jobs must be taken into account. The model is very frequently to find - one gave for example a system configuration with several machines, with which there is however an individual bottleneck machine, so that the Scheduling of the other machines must depend on the plan of the bottleneck, the available problem attributed to the single machine problem. By the small complexity it is possible to achieve by means of simple priority rules determined goals with security.

The parallel machine model is a generalization one machine of the model. Several machines of the same type work parallel. An arriving job can be worked on by each of these machines.

Often jobs must go through different operations at different machines, so that they exhibit different ways. One calls such an environment job Shop model. To job Shop of problems arise e.g. in the semiconductor industry during the wafer manufacturing; likewise one in addition, a hospital can regard model as typical example of a job Shop: The patients are following the jobs, different ways, in different places in the hospital (registration, waiting room, physician area, Roentgen area,"Â…) are treated.

If all jobs go through the same machines in the same order, i.e. if its ways are identical, one speaks of a flow Shop model. A flow Shop model is thus a reduced job Shop model.

One finds typical flow Shops for example in the metal manufacture industry or the load and flow production in food production.

Scheduling problems arise in many places in production procedures and are in most cases only very with difficulty optimally solvable, since they fall frequently into the class of the NP-complete problems. In practice however often good approximation solutions are sufficient.

A frequently arising and practice-relevant problem represents the single machine early/tardy problem. Into single machine environment a row jobs on a machine are to be taken into account, so that the arising are as minimal as possible early costs and tardy costs. The objective covers itself with the goal of the Just in time production. This problem is NP complete.

The addressed Scheduling problems can be formulated all as integral optimization problems. One tries predominantly to solve such problems with so-called Branch and Bound procedure or the John on algorithm.

Scheduling in computer science

Scheduling (dt.: Task management/resources dispatching/scheduling) is a task of an operating system. It designates the fair administration of several processes, which are implemented on a computer

Scheduling procedure

• Roofridge Come roofridge Serve (FCFS)
• Roofridge in - roofridge Out (FIFO)
• Sporadic Scheduling
• Round Robin strategy
• Tarpaulins by searches
• Generalized Processor sharing (government inspection department)
• Shortest job roofridge
• Shortest Process NEXT one (SPN)
• Shortest Remaining Time (SRT)
• Highest Response reason NEXT one (HRRN)
• Feedback
• Leases Laxity roofridge (tarpaulins after clearance)
• Guess/advise Monotonic Scheduling (RMS) (tarpaulins after monotonous rates)
• Earliest DEAD LINE roofridge (EDF) (tarpaulins after periods)
• DEAD LINE Monotonic Scheduling (DMS)
• COVERT (Cost Over Time)
• Lotterie Scheduling

Goals

• To maximize throughput of the plant
• Anwortzeiten minimize
• Resources extent of utilization maximize
• to show predictable behavior
• to need small-possible administration expense (overhead)
• to be determined to guarantee i.e. that a process is processed within finite time,
• to carry out also under high load of meaningful work and
• to be fair to all users (processes).

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