Process management and FCFS scheduling

One CPU, a queue of programs

A program that is running is called a process. Process management makes sure each process gets the resources it needs, even when many are active, so they run without disturbing each other.

A process has three stages. Creation: you start a program and the OS loads it into memory with CPU time and memory. Execution: it runs and does its work. Termination: it is closed and the OS frees its resources for others.

Multitasking lets one user keep several programs open and switch between them, such as music, a document and a browser. Concurrency means many processes are active, but the CPU works on them one by one in very fast turns, like a chef moving between three dishes.

Because the CPU runs one process at a time, the OS must decide which runs first and for how long. This is scheduling. Your book uses First Come, First Served (FCFS): processes run in the exact order they arrive, like a queue at a shop counter.

In the book's example P1 runs 0–5 s, P2 runs 5–8 s and P3 runs 8–10 s. P3 needs only 2 s but must wait for the longer jobs. FCFS is easy and skips no process, but short jobs stuck behind long ones slow the system: the convoy effect.

FCFS: start time = when the CPU becomes free (or the arrival time, if later). Finish = start + time needed. Waiting time = start − arrival.

Key terms

Process

The unit the OS gives CPU time and memory to.

Concurrency

Running processes in fast turns so they seem simultaneous.

Scheduling

Choosing the order in which processes get the CPU.

Convoy effect

Short processes waiting a long time behind longer ones.

Short questions with model answers

1. Q1. Long Question 5 (p. 17): P1 arrives at 0 s (4 s), P2 at 1 s (3 s), P3 at 2 s (1 s). Find each waiting time and the average.

   • P1 → P2 → P3
   • P1: 0–4 s, P2: 4–7 s, P3: 7–8 s
   • Waits: 0 − 0 = 0, 4 − 1 = 3, 7 − 2 = 5
   • (0 + 3 + 5) ÷ 3 = 8/3 ≈ 2.67 s

   Waits 0 s, 3 s and 5 s; average ≈ 2.67 s. P3 needs only 1 s but waits 5 s: the convoy effect.

2. Q2. Long Question 2 (p. 17): explain the process life cycle using a web browser.

   • Creation.
   • Execution.
   • Termination.

   Creation (loaded, given resources) → execution (does its work) → termination (resources freed).

Common mistakes

• ✗ Running the shortest process first in an FCFS question.

  ✓ FCFS ignores length. Only arrival order counts, which is why P3 waits in your book's example.

• ✗ Starting each process at its arrival time even while another is running.

  ✓ A process starts when the CPU is free: at the previous finish time if that is later than its arrival.

• ✗ Saying concurrency means the CPU runs all processes at exactly the same moment.

  ✓ Your book says the CPU handles them one by one in extremely fast cycles, so they only seem simultaneous.

MCQs

1. 1. In the FCFS scheduling method, processes are served:

   1. (a) Randomly
   2. (b) Shortest job first
   3. (c) In the order they arrive
   4. (d) By priority only
   Show answer

   (c) Exercise MCQ 6 (p. 16); key p. 18; 1.3.3, p. 7.

2. 2. In the Activity on p. 9 (P1 0 s/4 s, P2 1 s/5 s, P3 2 s/3 s), when do all processes finish?

   1. (a) 9 s
   2. (b) 10 s
   3. (c) 15 s
   4. (d) 12 s
   Show answer

   (d) P1 0–4, P2 4–9, P3 9–12, so 12 s.

3. 3. Which stage of a process frees its memory and CPU time for others?

   1. (a) Creation
   2. (b) Termination
   3. (c) Execution
   4. (d) Scheduling
   Show answer

   (b) 1.3.1, p. 6.

4. 4. Listening to music while a document stays open and you browse the internet is an example of:

   1. (a) Multitasking
   2. (b) Termination
   3. (c) FCFS
   4. (d) A device driver
   Show answer

   (a) 1.3.2, p. 7.

5. 5. Short processes waiting a long time behind longer ones is called the:

   1. (a) Bottleneck
   2. (b) Life cycle
   3. (c) Convoy effect
   4. (d) Concurrency
   Show answer

   (c) Disadvantages of FCFS, p. 9.

Quick revision

• A process is a running program; it is created, executed and terminated, and the OS frees its resources at the end.
• Multitasking keeps many programs open for one user; concurrency runs many processes in fast turns on one CPU.
• FCFS runs processes in arrival order. It is simple and fair, but short jobs can wait long: the convoy effect.