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# Presentiaon task sheduling first come first serve FCFS

Published on: Mar 4, 2016
Published in: Education
Source: www.slideshare.net

#### Transcripts - Presentiaon task sheduling first come first serve FCFS

• 1. ‫الرحيم‬ ‫الرحمن‬ ‫هللا‬ ‫بسم‬ Elmashreq University Faculty of engineering Electronics engineering dept Prepared by: supervisor: 1.
• 2. Introduction: Process that requests the CPU FIRST is allocated the CPU FIRST. It’s simplest to understand and the simplest to implement . Other names of this algorithm are: • First-In-First-Out (FIFO) . • Run-to-Completion . • Run-Until-Done .  This is a non-preemptive algorithm.  Example on real life such as: Buying tickets !  Example on computer system likes: sharing printers.  Implementation: • FIFO queues. • A new process enters the tail of the queue. • The schedule selects from the head of the queue.
• 3. Scheduling Algorithms: First-Come, First-Served (FCFS) • Example: Three processes arrive in order P1, P2, P3. – P1 burst time: 24 – P2 burst time: 3 – P3 burst time: 3 • Waiting Time – P1: 0 – P2: 24 – P3: 27 • Completion Time: – P1: 24 – P2: 27 – P3: 30 • Average Waiting Time: (0+24+27)/3 = 17 • Average Completion Time: (24+27+30)/3 = 27 P1 P2 P3 0 24 27 30
• 4. Problems with FCFS: o Non-preemptive. o Not optimal AWT.  Cannot utilize resources in parallel: • Assume 1 process CPU bounded and many I/O bounded processes.  result: Convoy effect, low CPU and I/O Device utilization.  a convoy effect happens when a set of processes need to use a resource for a short time, and one process holds the resource for a long time, blocking all of the other processes. Essentially, it causes poor utilization of the other resources in the system.
• 5. Advantages and Disadvantages: Advantages: • simple • easy to understand • first come, first served Disadvantages: • Non-preemptive, that is, the process will run until it finishes. • short processes which are at the back of the queue have to wait for the long process at the front to finish. • Not good for time sharing systems. • Because of its simplicity, FCFS is not very efficient.