Changing the Priority Queue of Minix3

Question

I installed minix3 on a virtual machine and was hoping I could manipulate the current queue selection algorithm so that I could change it from a priority order to a priority order that includes a random assortment of lower priority jobs. I was able to find that the section of code I needed to change was in proc.c with the specific section being pick_proc.c.

/*===========================================================================*
 *                pick_proc                     * 
 *===========================================================================*/
PRIVATE struct proc * pick_proc(void)
{
    /* Decide who to run now.  A new process is selected and returned.
     * When a billable process is selected, record it in 'bill_ptr', so that the 
     * clock task can tell who to bill for system time.
     */
    register struct proc *rp;  /* process to run */
    int q;                     /* iterate over queues */

    /* Check each of the scheduling queues for ready processes. The number of
     * queues is defined in proc.h, and priorities are set in the task table.
     * The lowest queue contains IDLE, which is always ready.
     */
    for (q=0; q < NR_SCHED_QUEUES; q++) {    
        if(!(rp = rdy_head[q])) {
            TRACE(VF_PICKPROC, printf("queue %d empty\n", q););
            continue;
        }

        u64_t timecount;
        u32_t randdom;
        read_tsc_64(&timecount);
        rand = timecount.lo;

        #if DEBUG_RACE
        rp = random_process(rdy_head[q]);
        #endif

        TRACE(VF_PICKPROC, printf("found %s / %d on queue %d\n", 
              rp->p_name, rp->p_endpoint, q););
        assert(proc_is_runnable(rp));
        if (priv(rp)->s_flags & BILLABLE)         
            bill_ptr = rp;        /* bill for system time */
        return rp;
    } 
    return NULL;
}

I already put some code to start the randomization process but I don't know where to go from here. I know I need to add something to this file but I'm not sure which variables do what and which pointers I need to change. I was hoping someone could show me how to do this or point out which part I need to change to help me move along. Right now I'm pretty stuck.

score 0 · Accepted Answer · answered Oct 27 '14 at 13:43

0

Feels like a delicate question; changing the algorithm could leave high priority tasks unfulfilled. Q1: what is a "lower priority task"? Is that NR_SCHED_QUEUES/2? Q2: how many higher priority tasks must at most be waiting before you are willing to prefer a lower priority task? With this answered, you can change q=0 in the for loop to e.g. q=low_tasks and select a process from there.

for (p=0, q=0; q < NR_SCHED_QUEUES/2; q++)
    p += rdy_tail[q] - rdy_head[q]; // number of processes in this queue
if (p<some_value) q= NR_SCHED_QUEUES / 2; else q= 0;

for (; q < NR_SCHED_QUEUES; q++) {    
    if(!(rp = rdy_head[q])) {
        TRACE(VF_PICKPROC, printf("queue %d empty\n", q););
        continue;
    }

    TRACE(VF_PICKPROC, printf("found %s / %d on queue %d\n", 
          rp->p_name, rp->p_endpoint, q););
    assert(proc_is_runnable(rp));
    if (priv(rp)->s_flags & BILLABLE)         
        bill_ptr = rp;        /* bill for system time */
    return rp;
} 
return NULL;

Note: this is demo only and not guaranteed to work properly!

Note: you must also check there are lower priority tasks to run.

answered Oct 27 '14 at 13:43

Paul Ogilvie

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Thanks for the tips Paul. I'm a bit confused on how you were able to introduce randomness into the scheduling queue by adding the for loop. To answer your previous questions: 1. The lower priority tasks can be any lower priority task. It doesn't have to be something specific. 2. That number doesn't have to be fixed either. It can be random as well. So sometimes it can let a few higher priority tasks in and other times it can let in many. You mentioned changing q=0 to q = low_task, would I be able to incorporate my code that selected randomness into that? – John123 Oct 27 '14 at 15:56
I did not add randomness. It is just an example of deciding whether to allow lower priority tasks. You can make the whole process random, from highest to lowest task. It implies there will be chance the machine will appear hung (e.g. only scheduling low priority tasks). Hence I feel the scheduler may not be random, or only random under guards. – Paul Ogilvie Oct 27 '14 at 17:15
Thanks Paul! This helps me a lot. I appreciate your input and I'll make changes accordingly. – John123 Oct 27 '14 at 18:00

Changing the Priority Queue of Minix3

1 Answers1