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We have learned various process scheduling policies and examined their properties in the class. To better understand them, you will implement SJF, SRTF, round-robin, priority, priority + aging, priority + PIP, and priority + PCP scheduling policies on an educational scheduler simulation framework which imitates the scheduler core of modern operating systems.

• The framework maintains the time with ticks variable. It is increased by 1 when a scheduling is happened. You may read this variable but should not modify it.

• Firstly, we need a schedulable entity, and it is the process. The framework accepts a process description file as the argument, and it describes the processes to simulate. Following example shows an example description file for two processes (process 1 and process 2).

   process 1
   	start 0
   	lifespan 4
   	prio 0
   end
  
   process 2
   	start 5
   	lifespan 10
   	prio 10
   end
  

• The framework will create the process 1 at tick 0 (start 0) and the process should run for 4 ticks (lifespan 4) until it is completed. The process will be given a priority value 0 by default, and you may specify the priority with prio property (prio 0). The larger priority value implies the higher priority of processes. Likewise, process 2 will be forked at time tick 5 and run for 10 ticks with priority 10. This information is also shown at the beginning of the program execution as follow.

   - Process 1: Forked at tick 0 and run for 4 ticks with initial priority 0
   - Process 2: Forked at tick 5 and run for 10 ticks with initial priority 10
  

• The framework will realize the processes described in the description file with struct process defined in process.h. See the file for the fields that describes processes in the system. Note that some variables are forbidden for direct access.

• At any moment, struct process *current defined as a global variable points to the process that is currently running. You can use the variable to access the currently running process.

• The framework only implements scheduling mechanisms (e.g., replacing the current, counting ticks, ... ), and it interacts with scheduling policies that are defined with struct scheduler in sched.h. struct scheduler is a collection of function pointers. The framework will call the functions to ask the scheduling policy for making decisions. Have a look at fifo_scheduler in pa2.c which implements a FIFO scheduler. You may also find other scheduler instances in pa2.c that are waiting for your implementation.

• struct process *(*schedule)(void) is the key function for the scheduling policy. The framework invokes the function whenever it needs to schedule a process run next. Specifically, the function should return a process to run next or NULL to indicate there is no process to run. See fifo_schedule() in pa2.c.

• The framework has the ready queue struct list_head readyqueue which is supposed to keep the list of processes that are ready to run. Note that the current process should not be in the ready queue since it is currently running, not ready to run.

• The system has a number of system resources (16 in this PA) that can be assigned to processes exclusively. struct resource defines the system resources in resource.h. The process may ask the framework to acquire a resource and release it after use. Such a resource use is specified in the process description file using acquire property. For example, acquire 1 4 2 means the process will require resource #1 for 2 ticks when it is aged for 2 ticks. Have a look at testcases/resources for an example.

• When the framework gets the resource acquisition request, it calls acquire() function of the scheduler. Similarly, the framework calls release() function when the process releases a resource. You may find default FCFS acquire/release functions in pa2.c which are used by the FIFO scheduler.

• Non-priority-based scheduling policies should handle resource acquisition requests in a first-come-first-served way. On the other hand, priority-based scheduling policies should dispatch the releasing resource to the process with the highest priority. To this end, you may define your own acquire/release functions and associate them to your scheduler implementation to make a correct scheduling decision. If two processes with the same priority are requesting the same resource, the one came earlier receives the resource.

• The framework is waiting for your implementation of shortest-job first (SJF) scheduler, shortest-remaining time first (SRTF) scheduler, round-robin scheduler, base priority-based scheduler, priority-based scheduler with aging (PA), priority-based scheduler with priority ceiling protocol (PCP), and priority-based scheduler with priority inheritance protocol (PIP). You can select a scheduler to run with a starting option, and the framework will be set to use the corresponding scheduler automatically. Check the options by running the program (sched) without any option.

• When a process is created by the framework, forked() callback function will be invoked. Similarly, when the process is done, exiting() callback function is called.

• FCFS and SJF are supposed to be non-preemptive; even the framework may ask the scheduler to select next process to run at every tick, the scheduler should not change currently running process unless it is completed. SRTF scheduler can preempt the currently running process when a process with a higher priority arrives, but should keep the current process otherwise.

• For round-robin scheduler, you don't need to worry about managing the time quantum; the framework will automatically call the schedule() function whenever the time quantum expires. In other words, the time quantum coincides with the tick. Note that RR does not consider the priority of processes.

• The priority-based schedulers should handle processes with the same priority in the round-robin way; If two or more processes are with the same priority, they should be switched on each tick.

• For priority with aging scheduler, at the every scheduling moment, the priority of the current process is reset to its original priority, and all processes in the readyqueue receive a priority boost by 1. The priority can be boosted up to MAX_PRIO defined in process.h. The scheduler should pick the process with the highest adjusted priority at this point. Note that the processes with the same priority should be handled in a round-robin manner just like the original priority scheduler.

• To boost the priority of processes in PCP, use MAX_PRIO.

• When you implement PIP, make sure that the priority of a process is set properly when it releases a resource. There are complicated cases to implement PIP.

  • More than one processes with different priority values can wait for the releasing resource. Suppose one process is holding one resource type, and other process is to acquire the same resource type. And then, another process with higher (or lower) priority is to acquire the resource type again, and then ...
  • Many processes with different priority values are waiting for different resources held by a process. You will get the full points for PIP if and only if these cases are all handled properly. Hint: calculate the current priority of the releasing process by checking resource acquisition status.
  • See this for a comprehensive exposition.

• The grading system only examines the messages printed out to stderr. Thus, you can use printf as you want.
• Use dump_status() function to see the situation of processes and resources.
• Recall PA0 and PA1 for manipulating the list_head queues.
• Make sure you are using list_for_*_safe variants if an entry is removed from the list during the iteration, and list_del_init to remove an entry from the list. (Do some Internet search for their differences)

• Use PAsubmit for submission

  • 550 pts + 10 pts
  • You can submit up to 30 times.
  • The details of some testcase results are hidden and you can only check the final decision (i.e., pass/fail);

• Code: pa2.c (500 pts)

  • SJF scheduler: 20pts (tested using multi)
  • SRTF scheduler: 50pts (multi);
  • RR scheduler: 50pts (multi and prio)
  • Priority scheduler: 60pts (prio and resources-prio)
  • Priority scheduler + aging: 100pts (prio)
  • Priority scheduler + PCP: 70pts (resources-basic)
  • Priority scheduler + PIP: 150pts (resources-adv1 and resources-adv2)

• Document: One PDF document (50 pts) including;

  • Description how each scheduling policy is implemented
    • Do not explain the code itself. Instead, focus on explaining your idea and approach.
    • Please, do not put screenshots of your code.
  • Show the changes of the priorities of processes over time for aging and PIP.
    • Use prio testcase for PA scheduler, and resources-adv2 for PIP.
    • Explain from the beginning of the simulation to the 12nd tick.
  • Lesson learned
    • No need to recite what is explained in the class.
  • No more than five pages

• Git repository (10 pts)

  • Register http URL and with a deploy token and password.
  • Start the repository by cloning this repository.
  • Make sure the token is valid through May 7 (due + 3 slip days + 4 day)

• WILL NOT ANSWER THE QUESTIONS ABOUT THOSE ALREADY SPECIFIED ON THE HANDOUT.

• QUESTIONS OVER EMAIL WILL BE IGNORED UNLESS IT CONCERNS YOUR PRIVACY.