Enhancing XV-6

Name: Amogha A Halhalli

Roll No.: 2021101007

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Specification 1: System Calls

System Call 1: trace
    1. Added a sys_trace() function in kernel/sysproc.c 
    2. Added a new variable mask in struct proc in kernel/proc.h
    3. Initialised the new variable in allocproc in kernal/proc.c
    4. Added a new user program which calls the trace system call.
    5. Modified the syscall.c to print the output of the trace syscall.

System Call 2: sigalarm and sigreturn
    1. Added sys_sigalarm() and sys_sigreturn() in kernel/sysproc.c
    2. Added some new variables in struct proc in kernel/proc.h
    3. Initialised the new variables in allocproc in kernal/proc.c
    4. Modified the trap.c to handle the syscall sigalarm.

Specification 2: Scheduling

a. First Come First Serve (FCFS) --> Run as 'make qemu SCHEDULER=FCFS'
    1. Added new variables in struct proc in kernel/proc.h to store more information about the process.
    2. Modified the allocproc() function to set up the new values in kernel/proc.c
    3. Modified the scheduler() function in proc.c to separately handle the FCFS scheduling.
    4. Disabled the preemption of the process after the clock interrupts in kernel/trap.c

b. Lottery Based Scheduler (LBS) --> Run as 'make qemu SCHEDULER=LBS'
    1. Added new variables in struct proc in kernel/proc.h to store more information about the process.
    2. Modified the allocproc() function to set up the new values in kernel/proc.c
    3. Modified the scheduler() function in proc.c to separately handle the LBS scheduling.
    4. Added a syscall sys_settickets() in kernel/sysproc.c which calls int settickets(int number)
    5. The function settickets sets the number of tickets of the calling process.

c. Priority Based Scheduler (PBS) --> Run as 'make qemu SCHEDULER=PBS'
    1. Added new variables in struct proc in kernel/proc.h to store more information about the process.
    2. Modified the allocproc() function to set up the new values in kernel/proc.c
    3. Modified the scheduler() function in proc.c to separately handle the LBS scheduling.
    4. Added a syscall sys_set_priority which sets the static priority of the process.
    5. Added a user program setpriority.c which is used to set the priority of a process.

d. Multi Level Feedback Queue (MLFQ) --> Run as 'make qemu SCHEDULER=MLFQ'
    1. Added new variables in struct proc in kernel/proc.h to store more information about the process.
    2. Modified the allocproc() function to set up the new values in kernel/proc.c
    3. Defined new struct _queue in kernel/proc.h to store information of the queues.
    4. Added new functions enque, deque & update to manipulate the struct _queue Queue[5]
    5. Modified the scheduler() function in proc.c to separately handle the MLFQ scheduling.
    6. Enabled the preemption of the process after the time slice in kernel/trap.c
    7. Each process is pushed to higher priority queue after 128 ticks to avoid aging.

Specification 3: Copy-on-write fork

1. Added new function uvmcopy_cow in kernel/vm.c which will map the addresses of child process          pagetable to that of old process.
2. Added page_reference array in kernel/kalloc.c which keeps track of the number of processes sharing that same page.
3. Initialised page_reference array in kernel/main.c
4. Incremented page_reference of that pa in kalloc() in kernel/kalloc.c
5. Decremented page_reference of that pa in kfree() in kernel/kalloc.c, if zero, that page is freed.
6. Declared 6th bit as PTE_COW for addresses in kernel/riscv.h
7. Modified copyout function in kernel/vm.c to use the same pages if is a COW page
8. Modified usertrap function in kernel/trap.c. When a process tries to write on one of the COW pages, a page fault will be raised. The function will allocate it a page of physical memory, make the required changes and return to continue the process. 

Specification 4: Performance Comparision

Scheduler	Average run time	Average wait time
Round Robin	14	114
First Come First Serve	33	39
Lottery Based Scheduler	6	109
Priority Based Scheduler	17	107
Multi-Level Feedback Queue	14	143

Used 1 CPU for Multi-Level Feedback Queue.

Other Implementations:

1. Edited the procdump() function in kernel/proc.c for debugging.
2. Added a user program time.c and function update_time() in kernel/proc.c (tutorial)
3. Implemented a system call sys_waitx which return runtime and wait time of child process.(tutorial)
4. Added a user program schedulertest to test the running of scheduling algorithms.
5. Added a user program alarmtest to test working of system call sigalarm & sigreturn.
6. Added a user program cowtest to test working of copy on write.