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CPU-I/O Burst Cycle

Introduction

In an operating system, processes alternate between CPU execution and I/O operations. This behavior is known as the CPU-I/O Burst Cycle. Understanding this cycle is essential for designing efficient CPU scheduling algorithms, which help improve system performance.

1. What is the CPU-I/O Burst Cycle?

A process does not execute in the CPU continuously. Instead, it alternates between two phases:

  1. CPU Burst: The process executes in the CPU, performing computations such as arithmetic, logical operations, or instruction execution.
  2. I/O Burst: The process waits for an I/O operation, such as reading from disk, network communication, or user input.

This cycle repeats multiple times during the execution of a process until it terminates.

Example of CPU-I/O Burst Cycle:

TimeProcess StateActivity
0-3CPU BurstExecuting calculations
3-7I/O BurstWaiting for disk read
7-12CPU BurstProcessing data
12-16I/O BurstWriting output to file

2. Characteristics of CPU and I/O Bursts

a) CPU-Bound vs. I/O-Bound Processes

Processes can be classified based on how they use CPU and I/O resources:

  1. CPU-Bound Processes:
    • Spend more time in CPU bursts.
    • Perform heavy computations (e.g., scientific simulations, video rendering).
    • Require efficient CPU scheduling to maximize performance.
  2. I/O-Bound Processes:
    • Spend more time in I/O operations.
    • Frequently wait for user input, disk operations, or network access.
    • Need better I/O management to reduce waiting time.

Most processes are a mix of CPU-bound and I/O-bound tasks.

b) Distribution of CPU and I/O Bursts

  • Short CPU bursts are more common than long ones.
  • A typical program execution follows a random distribution of short CPU bursts and long I/O bursts.
  • The CPU scheduler must efficiently manage processes with different burst patterns.

Graph: CPU Burst Duration Distribution

| Number of CPU bursts
|       *
|      ***
|     *****
|    *******
|   *********
|*******************
|________________________
    Burst Duration →
  • The peak of the graph shows that most CPU bursts are short.

3. Role of CPU-I/O Burst Cycle in Process Scheduling

Since CPU and I/O bursts occur alternately, the operating system must efficiently schedule processes to keep the CPU busy. This is where CPU scheduling algorithms play a crucial role.

a) Short-Term Scheduling

  • The CPU scheduler selects which process will execute next based on the burst cycle.
  • When a process enters an I/O burst, another process is scheduled for CPU execution.

b) Scheduling Algorithms and Burst Cycles

Different scheduling algorithms manage CPU bursts differently:

AlgorithmHow it Handles CPU Bursts
First-Come, First-Served (FCFS)Executes processes in order of arrival, regardless of CPU burst length.
Shortest Job Next (SJN)Selects the process with the shortest next CPU burst first.
Round Robin (RR)Allocates the CPU for a fixed time (time quantum) before switching to another process.
Priority SchedulingProcesses with higher priority get CPU time first.

4. CPU-I/O Overlapping for Better Efficiency

Modern operating systems use multiprogramming to improve system performance:

  • When one process is in an I/O burst, another process can use the CPU.
  • This ensures that the CPU is never idle, maximizing system efficiency.

Direct Memory Access (DMA)

  • Some I/O operations use DMA (Direct Memory Access) to transfer data without CPU intervention.
  • This allows the CPU to continue executing other processes while I/O operations occur in parallel.

5. Real-World Example of CPU-I/O Burst Cycle

Consider a text editor:

  1. Typing (CPU Burst): The program processes keystrokes.
  2. Saving a file (I/O Burst): The system writes data to the disk.
  3. Auto-spell check (CPU Burst): The system checks spelling.
  4. Printing a document (I/O Burst): The system sends data to the printer.

6. Conclusion

  • The CPU-I/O Burst Cycle is a fundamental concept in process execution.
  • Efficient CPU scheduling ensures that processes utilize CPU and I/O resources optimally.
  • Understanding CPU bursts helps in designing better scheduling algorithms for improved system performance.