Pre Emptive And Cooperative Scheduling Essay

1237 words - 5 pages

Over the years, computer science kept evolving; leading to the emergence of what has become a standard in modern software development: Multitasking. Whether logical or physical, it has become a requirement for today's programs. In order to make it possible it became necessary to establish the notion of concurrency and scheduling. In this essay, concurrency will be discussed as well as two types of scheduling; pre-emptive used in threads and cooperative used in agents, their similarities and differences.

Concurrency is an essential part of multitasking; after all using concurrency signifies executing several tasks at the same time or at least to give that impression, “Concurrency is an illusion of parallelism. Thus, two tasks are concurrent if there is an illusion that they are being performed in parallel, whereas, in reality, only one of them may be performed at any time. ” (D.M.Dhamdere, 2006, p91).However, in the case of multi-core processors or multi-processors true parallelism can be achieved. In any case, in order to maintain the illusion of parallelism, executing alternately various tasks is essential, therefore, determining which task execute next is particularly serious. That is where scheduling comes forth since according to D.M.Dhamdere(2006,p91) scheduling is selecting the next process to be serviced, in order to do so different policies can be used, depending on the context in which the program is going to be used, of course combinations are also possible. Let’s focus mainly on pre-emptive scheduling commonly (used by threads) and cooperative scheduling (used by agents). Firstly pre-emptive scheduling, in this configuration, a process, thread or application can be pre-empted. This means that it can be suspended by the system and put into a queue in order to enable another application or process to run instead. It will be done whether or not the treatment that was being carried has reached completion, allowing proper use of the resources available. This type of scheduling is usually priority-based,” Pre-emptive scheduling is a round-robin, priority-based mechanism” (Microsoft MSDN, 2010), meaning that each task is given a priority and is processed according to it. For example, a task with a high priority will be executed before a task with a low priority. If a task is running and another task with a higher priority needs to be processed, then the task with the lower priority will go into suspended state while the higher one is being treated. The round robin attribute means that there is a rotation of the tasks being processed. A task will be executed for a given time and then suspended regardless of its state of advancement; leaving the place to the task that has been in a suspended state the longest. However, since the scheduling is priority based, the rotation will only be made amongst higher priority tasks, which could cause the other tasks to wait indefinitely, causing the situation called starvation. It is a drawback of the...

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