In this machine problem, you'll be putting a new scheduler into xv6. It is called a lottery scheduler, and the full version is described in this chapter of OSTEP; you'll be building a simpler one. The basic idea is simple: assign each running process a slice of the processor based in proportion to the number of tickets it has; the more tickets a process has, the more it runs. Each time slice, a randomized lottery determines the winner of the lottery; that winning process is the one that runs for that time slice.

After completing this machine problem, you should be able to:

• Explain the workings of a real OS CPU scheduler
• Explain how a lottery scheduler works
• Implement an OS scheduler

You'll need two new system calls to implement this scheduler. The first is int settickets(int number), which sets the number of tickets of the calling process. By default, each process should get one ticket; calling this routine makes it such that a process can raise the number of tickets it receives, and thus receive a higher proportion of CPU cycles. This routine should return 0 if successful, and -1 otherwise (if, for example, the caller passes in a number less than one).

The second is int getpinfo(struct pstat *). This routine returns some information about all running processes, including how many times each has been chosen to run and the process ID of each. You can use this system call to build a variant of the command line program ps, which can then be called to see what is going on. The structure pstat is defined below; note, you cannot change this structure, and must use it exactly as is. This routine should return 0 if successful, and -1 otherwise (if, for example, a bad or NULL pointer is passed into the kernel).

Most of the code for the scheduler is quite localized and can be found in proc.c; the associated header file, proc.h is also quite useful to examine. To change the scheduler, not much needs to be done; study its control flow and then try some small changes.

You'll need to assign tickets to a process when it is created. Specfically, you'll need to make sure a child process inherits the same number of tickets as its parents. Thus, if the parent has 10 tickets, and calls fork to create a child process, the child should also get 10 tickets.

You'll also need to figure out how to generate random numbers in the kernel; some searching should lead you to a simple pseudo-random number generator, which you can then include in the kernel and use as appropriate.

Finally, you'll need to understand how to fill in the structure pstat in the kernel and pass the results to user space. The structure should look like what you see here, in a file you'll have to include called pstat.h:

#ifndef _PSTAT_H_
#define _PSTAT_H_

#include "param.h"

struct pstat {
  int inuse[NPROC];   // whether this slot of the process table is in use (1 or 0)
  int tickets[NPROC]; // the number of tickets this process has
  int pid[NPROC];     // the PID of each process 
  int ticks[NPROC];   // the number of ticks each process has accumulated 
};

#endif // _PSTAT_H_

As with the previous machine problem, we include a test suite which you can run with the command make test-mp2. The test source files (which you should not alter --- we'll be using our own pristine copies in any case) can be found in the "tests/mp2/ctests" directory if you're interested.

If a test fails, it'll stop immediately with an error report. If you want to continue running all tests even after hitting a failure, do make test-mp2-cont instead.

There are a total of 7 tests. Tests 1, 2, 4, and 7 are worth 10 points each, while test 3, 5, and 6 are worth 20 points each, for a maximum of 100 points. If your code fails to build, you will not earn any points at all. If you do not pass a test, you will not earn any points for it.

To submit your work, simply commit all your changes and push your work to your GitHub repository.