xv6 System Enhancements

This repository contains enhancements made to the xv6 operating system, including new system calls, scheduling policies, and features such as copy-on-write fork.

System Calls

1. getreadcount

Description:

• Added the system call getreadcount to xv6.
• Returns the value of a counter which is incremented every time any process calls the read() system call.

Function Signature:

int getreadcount(void);

2. sigalarm and sigreturn

Description:

• Implemented a feature to periodically alert a process as it uses CPU time.
• Added a new system call sigalarm(interval, handler):
  • Calls the application function handler after every interval ticks of CPU time.
• Added system call sigreturn() to reset the process state after the handler is called.

Scheduling

1. First Come First Serve (FCFS)

Description:

• Implemented a policy that selects the process with the lowest creation time.
• Process runs until it no longer needs CPU time.

2. Multi-Level Feedback Queue (MLFQ)

Description:

• Implemented a preemptive MLFQ scheduler with four priority queues.
• Time slices:
  • Priority 0: 1 timer tick
  • Priority 1: 3 timer ticks
  • Priority 2: 9 timer ticks
  • Priority 3: 15 timer ticks
• Processes move between queues based on behavior and CPU bursts.
• Aging implemented to prevent starvation.
• Round-robin scheduler used for processes at the lowest priority queue.

3. Modified Priority-Based Scheduler

Description:

• Implemented a preemptive priority-based scheduler.
• Dynamic Priority (DP) based on Static Priority (SP) and Recent Behavior Index (RBI).
• RBI measures recent behavior (Running Time, Sleeping Time, Waiting Time).
• set_priority() system call to change Static Priority and reset RBI.
• User program setpriority to change priority via syscall.

Copy on Write Fork

Description:

• Implemented copy-on-write (COW) fork in xv6.
• Parent and child initially share all physical pages, mapped read-only.
• On write, kernel makes a copy of the page, updating page tables.
• Child and parent have separate copies for modified pages.