Why is important for the scheduler to distinguish

Slack and float show you where there is flexibility in the schedule, and this can be useful when you need to gain time once the project is up and running. For a complex project, you may decide to produce a separate Gantt chart for each of the key stages. If you do this shortly before each key stage begins, you will be able to take any last-minute eventualities into account.

These charts provide a useful tool for monitoring and control as the project progresses. Various programs are available to assist project managers in scheduling and control.

This is more difficult if you are working manually. Many project managers use network diagrams when scheduling a project. The network diagram is a way to visualize the interrelationships of project activities. Network diagrams provide a graphical view of the tasks and how they relate to one another. The tasks in the network are the work packages of the WBS. All of the WBS tasks must be included in the network because they have to be accounted for in the schedule.

Leaving even one task out of the network could change the overall schedule duration, estimated costs, and resource allocation commitments. The first step is to arrange the tasks from your WBS into a sequence. Some tasks can be accomplished at any time throughout the project where other tasks depend on input from another task or are constrained by time or resources.

The WBS is not a schedule, but it is the basis for it. The network diagram is a schedule but is used primarily to identify key scheduling information that ultimately goes into user-friendly schedule formats, such as milestone and Gantt charts. The network diagram provides important information to the project team. It provides information about how the tasks are related Figure In our wedding planner example, Sally would look for relationships between tasks and determine what can be done in parallel and what activities need to wait for others to complete.

As an example, Figure Showing the activities in rectangles and their relationships as arrows is called a precedence diagramming method PDM. This kind of diagram is also called an activity-on-node AON diagram. Another way to show how tasks relate is with the activity-on-arrow AOA diagram. The main difference is the AOA diagram is traditionally drawn using circles as the nodes, with nodes representing the beginning and ending points of the arrows or tasks.

In the AOA network, the arrows represent the activities or tasks Figure All network diagrams have the advantages of showing task interdependencies, start and end times, and the critical path the longest path through the network but the AOA network diagram has some disadvantages that limit the use of the method. The critical path describes the sequence of tasks that would enable the project to be completed in the shortest possible time.

It is based on the idea that some tasks must be completed before others can begin. A critical path diagram is a useful tool for scheduling dependencies and controlling a project. In order to identify the critical path, the length of time that each task will take must be calculated. The stages can now be lined up to produce a network diagram that shows that there are three paths from start to finish and that the lines making up each path have a minimum duration Figure This is the minimum time in which it will be possible to complete the project.

In this example, the critical path is A—B—C—D—E—F—I—L, and the earliest completion date for the project is the sum of the estimated times for all the stages on the critical path — 28 weeks — from the point of securing the funding. All the key stages on the critical path must be completed on time if the project is to be finished on schedule. Mapping the critical path helps to identify the activities that need to be monitored most closely. Skip to content Main Body.

Susan : Steve, we need some help. Let me give her a call. Steve : We want everything to be perfect. Susan : There is so much to do!

Invitations, food, guests, and music. Susan : And it has to be done right. It is the calculation of the total time spent waiting to get into the memory, waiting in the queue and, executing on the CPU. The period between the time of process submission to the completion time is the turnaround time.

Timer interruption is a method that is closely related to preemption. When a certain process gets the CPU allocation, a timer may be set to a specified interval. Most of the multi-programmed operating system uses some form of a timer to prevent a process from tying up the system forever.

It is a module that provides control of the CPU to the process. The Dispatcher should be fast so that it can run on every context switch.

Dispatch latency is the amount of time needed by the CPU scheduler to stop one process and start another. It is the easiest and most simple CPU scheduling algorithm. This scheduling method can be managed with a FIFO queue. So, when CPU becomes free, it should be assigned to the process at the beginning of the queue.

The full form of SRT is Shortest remaining time. It is also known as SJF preemptive scheduling. In this method, the process will be allocated to the task, which is closest to its completion. This method prevents a newer ready state process from holding the completion of an older process.

Priority scheduling is a method of scheduling processes based on priority. In this method, the scheduler selects the tasks to work as per the priority. Priority scheduling also helps OS to involve priority assignments. The processes with higher priority should be carried out first, whereas jobs with equal priorities are carried out on a round-robin or FCFS basis.

Jobs that are scheduled to run only once will have only one instance, however, jobs that have a repeating schedule will have multiple instances, with each run of the job representing an instance.

For example, a job that is scheduled to run on Tuesday, Oct. A job that runs daily at noon for a week will have seven instances, one for each time the job ran. So, in this case, the run on Tuesday at noon represents one instance of the job. When a job is created, only one entry is added to the job table to represent the job, however, each time the job runs, an entry will be added to the job log.

Therefore, if you create a job that has a repeating schedule, you should find one entry in the job table and multiple entries in the job log. Each run of the job is assigned an instance ID that can be used to monitor job progress and manage future runs. They provide specific information about a particular run, for example, the job status and the start and end time.

See "How to View Scheduler Information" for more information. To determine when, where, and what will be executed in the database, you need to assign relationships among programs, jobs, and schedules. You can, however, leave a program, job, or schedule unassigned if you wish. You could, for example, create a job that calculates a year end inventory and leave it unassigned.

That could be J4 in Figure Similarly, schedules and programs could exist for year end tasks and be left unassigned. Figure illustrates the relationship among programs, jobs, and schedules. To understand Figure , consider a situation where tables are being analyzed.

The program takes in a variable for the table name. Two jobs, J1 and J2 , both point to the same program, which specifies a different table name. Finally, schedule S1 could have a value of 2AM for when the jobs should be performed. The end result is that the two tables named in J1 and J2 would be analyzed at 2AM. Note that P2 , P9 , J4 , and S2 are entities that have nothing pointed to them in Figure , so, for example, J4 could be self-contained with all relevant information inside the job itself.

Also note that more than one job can map to a program or schedule. Many Scheduler capabilities enable database administrators to control more advanced aspects of scheduling. Typically, these topics are not as important for application developers. A job class is a way of grouping jobs into larger entities, thus enabling you to prioritize access to the job slaves among those job classes.

This means that you can, for example, make sure a CEO's request will begin before a routine job. In addition to using job classes for resource allocation, you can also use them for setting job characteristics or behavior that must be the same for all the jobs in the class. When defining job classes, you should try to classify jobs by functionality.

Consider dividing jobs into groups that access similar data, such as marketing, production, sales, finance, and human resources.

Specify attributes at the class level. For example, you can specify the same policy for purging log entries of all payroll jobs. Specify the order in which a job is started. You can assign priority values of to individual jobs so that jobs with a higher relative priority start before jobs with a lower priority.

If two jobs have the same assigned value, the first job enqueued takes precedence. Ensure that a job is started only if there are no runnable jobs with a higher priority in that class.

In the case of resource contention, this ensures that you do not have a less important job preventing the timely completion of a more important one. A job must be part of exactly one class. When a job is created, you must specify which class the job is part of. If you do not specify which class the job is part of, it automatically becomes part of the default class. Dropping a class while there are still jobs in that class results in an error. You can force a class to be dropped even if there are still jobs that are members of that class, but all jobs referring to that class will automatically be disabled.

In this case, the class for these jobs will be reset to the default system class. Jobs belonging to the dropped class that are already running will continue to run under class settings determined at the start of the job. There is no guarantee that a high priority job in class A will be started before a low priority job in class B, even if they share the same schedule. Prioritizing among jobs of different classes is purely done on a class resource allocation basis.

A window enables you to change resource allocation during a time period such as time of day or time of the sales year. You do this so you can control which groups of users have what level of priority. For example, you could control access to a printer so product managers have a lower priority than executives. A window is represented by an interval of time with a well-defined beginning and end, such as "from 12am-6am". You can assign a priority with each window. If windows overlap, the window with the highest priority is chosen over other windows with lower priorities.

See "Scheduler Architecture" for details. Figure illustrates a simple scenario where separate windows have been assigned their own priorities. In Figure , Class A represents users with more need for resources in the morning while Class B represents users with more need for resources in the afternoon.

You use windows to guarantee that resources will be available when you need them. Therefore, you should try to avoid having more than one window active at the same time to minimize the potential for resources going to a less important task. It is not an error, however, for windows to overlap. Figure Overlapping Windows. There are three windows in Figure , so some rules need to be assigned for choosing which window will take precedence.

The order of precedence for windows is as follows:. A high priority is chosen over a low priority. Oracle recommends that you choose low as the normal setting so you can guarantee resources for important high priority jobs.

When a window finishes, the window with the highest percentage of its duration remaining is chosen. If at the end of a window there is still overlap with one or more windows, the Scheduler tries to switch to the window that has the highest priority and then, if the windows have the same priorities, to the one with the highest percentage of its duration remaining.

Overlapping windows are handled differently based on whether they have the same priority or not. When two or more windows of the same priority overlap, Oracle continues to run the window that was already running. After this window finishes running, Oracle switches to the one that still has the highest percentage of its duration left.

When high and low priority windows overlap, the Scheduler switches to the high priority window if it is currently running a low priority window. Jobs currently running that had a schedule naming the low priority window may be stopped depending on the behavior you assigned when you created the job. In addition, whenever two windows overlap, an entry is written in the Scheduler log. You can group jobs for ease of use.

An example would be to combine weekends, times from 12AM to 7AM, and holidays into something called "downtime". This downtime window group offers you more control over when jobs are run. In this case, perhaps the data warehousing department would only want their queries run during this time when they might be assigned a high percentage of available resources.

A typical example of a window group is to have windows for weeknight, weekend, and holidays. A window group could be for combining them. See "Creating Window Groups" for examples of creating window groups. Figure Scheduler Components.

The job table is a container for all the jobs, with one table per database.