SimpleScheduler - A Process Scheduler in C from Scratch

Design Document for Operating Systems (OS) Assignment 3

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Group Members & Contribution

2024138 - Atharva Singh Velpula

Contributions

• Scheduler loop
• Round Robin queue
• display_info() function
• SIGINT handler for Scheduler

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2024343 - Mayank Yadav

Contributions

• Shared memory and semaphore setup & cleanup
• Shell loop
• SIGCHLD handler for Shell
• dummy_main.h

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Joint Contribution

Both members contributed to:

• Error handling
• Bug fixing

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Implementation Details

• The codebase is well documented using:

  • Comments
  • Appropriate variable names
  • Meaningful function names

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SimpleShell.c

Functionality

• Runs an infinite loop to continuously accept user input.
• Terminates explicitly when the exit command is entered.

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Shared Memory & Semaphore Initialization

Shared Memory

• Initializes shared memory for maintaining the process table.

Semaphores

• Initializes semaphores for:

  • Mutual exclusion
  • Safe concurrent access while reading/writing shared memory

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Scheduler Creation

• Creates a separate scheduler process before starting the shell loop.

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SIGCHLD Handler

Purpose

• Updates the process table whenever a child process finishes execution.

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Process Submission

Supported Command

submit <executable>

Workflow

• Creates a child process for the executable.
• Submits the process to the scheduler for scheduling.

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Exit Handling

On exit Command

• Sends a termination signal to the scheduler.
• Waits for the scheduler process to terminate.
• Prints required execution details.
• Cleans up resources and exits gracefully.

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SimpleScheduler.c

Functionality

• Continuously reads the process table in an infinite scheduling loop.
• Picks NCPU processes for execution for TSLICE milliseconds.

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Scheduler Workflow

At Each Iteration

1. Pop NCPU processes from the queue.

2. Signal selected processes to execute.

3. Update:

   • Time spent on CPU
   • Time spent off CPU

4. Sleep for TSLICE milliseconds.

5. Signal running processes to stop.

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Scheduling Policy

Round Robin Scheduling

• Processes are scheduled in a Round Robin fashion.
• Ensures fair CPU allocation among submitted processes.

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Termination Logic

Case 1: Exit Signal + No Remaining Processes

• Scheduler terminates immediately.

Case 2: Exit Signal + Remaining Processes

• Scheduler continues executing remaining processes.
• Terminates only after all processes complete execution.

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SIGINT Handler

Purpose

• Helps manage graceful termination of the scheduler.

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Shared Resource Initialization

Before beginning the scheduling loop:

• Shared memory is initialized.
• Semaphores are initialized.

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dummy_main.h

Functionality

• Contains boilerplate code provided in the assignment.

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Modification Made

The following line was added:

raise(SIGSTOP);

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Purpose of SIGSTOP

• Ensures each process blocks immediately after exec() execution.
• Prevents processes from executing automatically.
• Makes the process wait for:

SIGCONT

from the scheduler before execution begins.

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Process Scheduling Workflow

User submits executable
          ↓
     SimpleShell
          ↓
 Creates child process
          ↓
 Process added to shared process table
          ↓
    SimpleScheduler
          ↓
 Selects NCPU processes
          ↓
 Sends SIGCONT
          ↓
 Processes execute for TSLICE
          ↓
 Sends stop signal
          ↓
 Round Robin continues

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Shared Memory Architecture

Shared Components

The following data is stored in shared memory:

• Process table
• Scheduling information
• CPU timing statistics

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Synchronization

Semaphores

Used to:

• Prevent race conditions
• Ensure safe concurrent access
• Synchronize Shell and Scheduler operations

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Signal Usage

Signal	Purpose
SIGSTOP	Pause newly created processes
SIGCONT	Resume scheduled processes
SIGCHLD	Detect child process completion
SIGINT	Graceful scheduler termination

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Key System Calls & Functions Used

• fork()
• exec()
• wait()
• signal()
• raise()
• kill()
• shm_open()
• mmap()
• sem_init()
• sem_wait()
• sem_post()

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Core Features

• Round Robin process scheduling
• Configurable CPU time slicing
• Shared memory based process table
• Semaphore synchronization
• Multi-process coordination using signals
• Graceful scheduler termination
• Execution tracking and statistics

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Notes

• Processes do not begin execution immediately after exec().

• Scheduler-controlled execution is achieved using:

  • SIGSTOP
  • SIGCONT

• Shared memory enables communication between:

  • Shell
  • Scheduler

• Semaphore synchronization ensures consistency of the shared process table.

• The scheduler guarantees completion of submitted processes before termination.