xv6 Custom Scheduling and Processes.

To run this you need to install qemu first. After getting it installed:

make
make qemu-nox

Also use the following command to make the folder clean again before compiling again:

make clean

To exit from non-graphical qemu (qemu-nox):

Ctrl+A x

User syscalls:

ps:

Prints the pid, status, and priority of all processes.

xpri pid pri:

Changes priority of process with PID = pid to pri

new:

Crates new parent-child process that takes quite a long time to execute.

pinfo:

Prints pid, runtime, number of runs and current queue of all active/inuse processes.

Other syscalls:

waitx:

Returns wait time and run time of the process. Its just extension of predefined wait syscall but with two more arguments for returning the wait time and run time of process.

Custom scheduling:

Default scheduler for xv6 is ROUND ROBIN, and to use this scheduler no new CLARG is to be passed.

For other schedulers following format should be used:

make qemu-nox SCHEDULER=VALUE

Where VALUE is:

DEFAULT : For Round Robin
FCFS    : For FCFS
PBS     : For Priority based
MLFQ    : For MLFQ scheduling

Implementations:

FCFS:

During the development of waitx, three more attributes namely ctime, rtime and etime denoting creation time, run time, end time were created. So for FCFS we look for process with smallest ctime.

Also this scheduling implemented here is non-preemptive. To implement this, we compare the ctime of currently running process, and that of smallest ctime and preemp only when the second is smaller than first.

PBS:

Similar to FCFS but here comparing priority attribute of the process (Added to implement this). Default priority is set to 60, lower the value of this attribute higher is the Process priority. Max value is set to 100. To change the priority of a process use xpri user syscall.

MLFQ:

The procedure is as follows:

1. On the initiation of a process, push it to the end of the highest priority queue.
2. The highest priority queue should be run always, if not empty.
3. If the process completes, it leaves the system.
4. If the process used the complete time slice assigned for its current priority queue, it is pre-empted and ​ inserted at the end of the next lower level queue.
5. If a process voluntarily relinquishes control of the CPU, it leaves the queuing network, and when the process becomes ready again after the I/O, it is inserted at the tail of the same queue, from which it is relinquished earlier.
6. A round-robin scheduler should be used for processes at the lowest priority queue.
7. To prevent starvation, implemented the aging :-
   • If the wait time of a process in lower priority queues exceeds a given limit(assign a suitable limit to prevent starvation), their priority is increased and they are pushed to the next higher priority queue.
   • The wait time is defined as the difference between the current time and the time at which the process was last executed.