Ejemplo n.º 1
0
  /**
   * Suspends the thread that is currently on the processor on the specified event.
   *
   * <p>Note that the thread being suspended doesn't need to be running. It can also be waiting for
   * completion of a pagefault and be suspended on the IORB that is bringing the page in.
   *
   * <p>Thread's status must be changed to ThreadWaiting or higher, the processor set to idle, the
   * thread must be in the right waiting queue, and dispatch() must be called to give CPU control to
   * some other thread.
   *
   * @param event - event on which to suspend this thread. @OSPProject Threads
   */
  public void do_suspend(Event event) {
    // To suspend a thread, we must first figure out which state to suspend it to. As can
    // be seen from Figure 3.1, there are two candidates:

    // If the thread is running, then it is suspended to ThreadWaiting.
    if (this.getStatus() == ThreadRunning) {
      // Again steps 1-3 for changing ThreadRunning to another status.
      MMU.getPTBR().getTask().setCurrentThread(null);
      MMU.setPTBR(null);
      this.setStatus(ThreadWaiting);
    }

    // If it is already waiting, then the status is incremented
    // by 1. For instance, if the current status of the thread is ThreadWaiting then it should
    // become ThreadWaiting+1.
    else // if (this.getStatus() == ThreadWaiting) only accounted for threadWaiting not +1, +2, etc
    this.setStatus(this.getStatus() + 1); // Accounts for higher cases than +1; such as +2

    // We now must set the new thread status using the method
    // setStatus() and place it on the waiting queue to the event.
    // IF and ONLY IF it does not already exist on the queue.
    if (!(lowPriorityQueue.contains(this)) || !(highPriorityQueue.contains(this))) {
      // Move to waiting queue
      event.addThread(this);
    }

    // Finally, a new thread must be dispatched using a call to dispatch()
    dispatch();
  }
Ejemplo n.º 2
0
  /**
   * Selects a thread from the run queue and dispatches it.
   *
   * <p>If there is just one theread ready to run, reschedule the thread currently on the processor.
   *
   * <p>In addition to setting the correct thread status it must update the PTBR.
   *
   * @return SUCCESS or FAILURE @OSPProject Threads
   */
  public static int do_dispatch() {
    // Context switching. Passing control of the CPU from one thread to another is called
    // context switching. This has two distinct phases: preempting the currently running
    // thread and dispatching another thread. Preempting a thread involves the following steps:
    // 1. Changing of the state of the currently running thread from ThreadRunning to whatever
    // is appropriate in the particular case. For instance, if a thread looses control of the CPU
    // because it has to wait for I/O, then its status might become ThreadWaiting. If the
    // thread has used up its time quantum, then the new status should become ThreadReady.
    // Changing the status is done using the method setStatus() described later.
    // This step requires knowing the currently running thread. The call MMU.getPTBR()
    // (described below) lets you find the page table of the currently scheduled task. The
    // task itself can be obtained by applying the method getTask() to this page table. The
    // currently running thread is then determined using the method getCurrentThread().

    // 2. Setting the page table base register (PTBR) to null. PTBR is a register of the
    // memory management unit (a piece of hardware that controls memory access), or MMU,
    // which always points to the page table of the running thread. This is how MMU knows
    // which page table to use for address translation. In OSP 2 , PTBR can be accessed
    // using the static methods getPTBR() and setPTBR() of class MMU.

    // 3. Changing the current thread of the previously running task to null. The current
    // thread of a task can be set using the method setCurrentThread().

    // When a thread, t, is selected to run, it must be given control of the CPU. This is called
    // dispatching a thread and involves a sequence of steps similar to the steps for preempting
    // threads:
    // 1. The status of t is changed from ThreadReady to ThreadRunning.

    // 2. PTBR is set to point to the page table of the task that owns t. The page table of a
    // task can be obtained via the method getPageTable(), and the PTBR is set using the
    // method setPTBR() of class MMU.

    // 3. The current thread of the above task must be set to t using the method setCurrentThread().
    // In practice, context switch is performed as part of the dispatch() operation, and steps 2
    // and 3 in the first list above can be combined with steps 2 and 3 of the second list.
    // In the degenerate case, when the running thread, t, is suspended and no other thread
    // takes control of the CPU, consider it as a context switch from t to the imaginary “null
    // thread.” Likewise, if no process is running and the dispatcher chooses some ready-to-run
    // thread for execution, we can view it as a context switch from the null thread to t.

    ThreadCB queueThread = null;

    // First, some thread should be chosen from the ready queue (or the currently running
    // thread can be allowed to continue).
    // highPriorityQueue is not empty and it has been selected to retrieve queue item randomly

    // Pick an item from a queue. First case is when both queues are not empty. Choose a thread
    // from one of the queues where 90% of the time the one selected will be from highPriority.
    if (!(ThreadCB.lowPriorityQueue.isEmpty()) && !(ThreadCB.highPriorityQueue.isEmpty())) {
      if ((randomGeneratedNumber.nextInt((10 - 1) + 1) > 1))
        queueThread = ThreadCB.highPriorityQueue.elementAt(0);
      else queueThread = ThreadCB.lowPriorityQueue.elementAt(0);
    }

    // Case 2 only low priority has anything in its queue. Thread will be selected from the
    // lowPriority queue.
    else if (!(ThreadCB.lowPriorityQueue.isEmpty()) && ThreadCB.highPriorityQueue.isEmpty())
      queueThread = ThreadCB.lowPriorityQueue.elementAt(0);

    // High priority queue is the only queue containing any threads. Thread chosen from it.
    else if (ThreadCB.lowPriorityQueue.isEmpty() && !(ThreadCB.highPriorityQueue.isEmpty()))
      queueThread = ThreadCB.highPriorityQueue.elementAt(0);

    // Process currently in CPU and in a queue. Context switch with use of timer at end of time
    // quantum.
    if (MMU.getPTBR() != null
        && queueThread
            != null /*(!(ThreadCB.highPriorityQueue.isEmpty()) || !(ThreadCB.lowPriorityQueue.isEmpty()))*/) {
      // Context switching with steps 1-3 given above. We already know that a thread is running on
      // the CPU from getPTBR()
      // so we need to switch the currently running thread with the one on the queue after time
      // quantum has ended.
      if (HTimer.get() <= 0) {
        // Add the thread currently on the CPU to the end of the readyQueue
        if (MMU.getPTBR().getTask().getPriority() > 1)
          ThreadCB.highPriorityQueue.addElement(MMU.getPTBR().getTask().getCurrentThread());
        else ThreadCB.lowPriorityQueue.addElement(MMU.getPTBR().getTask().getCurrentThread());

        MMU.getPTBR().getTask().getCurrentThread().setStatus(ThreadReady);
        MMU.getPTBR().getTask().setCurrentThread(null);
        MMU.setPTBR(null);

        MMU.setPTBR(queueThread.getTask().getPageTable());

        queueThread.getTask().setCurrentThread(queueThread);
        MMU.getPTBR().getTask().getCurrentThread().setStatus(ThreadRunning);
        // Which queue contains the thread? Remove recently set thread to CPU from readyQueue
        if (ThreadCB.highPriorityQueue.contains(queueThread)) ThreadCB.highPriorityQueue.remove(0);
        else ThreadCB.lowPriorityQueue.remove(0);
      }
    }

    // There is a thread running in the CPU, but there is nothing left in the queues. Continue
    // running thread. Return SUCCESS
    // No switch needed as only thread is already in the CPU.
    else if (
    /*ThreadCB.highPriorityQueue.isEmpty() && ThreadCB.lowPriorityQueue.isEmpty()*/ queueThread
            == null
        && !(MMU.getPTBR() == null)) return SUCCESS;

    // Process is in a queue but not in the CPU
    else if (MMU.getPTBR() == null
        && queueThread
            != null /*(!(ThreadCB.highPriorityQueue.isEmpty()) || !(ThreadCB.lowPriorityQueue.isEmpty()))*/) {
      MMU.setPTBR(queueThread.getTask().getPageTable());
      MMU.getPTBR().getTask().setCurrentThread(queueThread);
      MMU.getPTBR().getTask().getCurrentThread().setStatus(ThreadRunning);
      // Which queue contains the thread? Remove recently set thread to CPU from readyQueue
      if (ThreadCB.highPriorityQueue.contains(queueThread))
        ThreadCB.highPriorityQueue.remove(queueThread);
      else ThreadCB.lowPriorityQueue.remove(queueThread);

      return SUCCESS;
    }

    // No thread is running and there are no threads in the queues. Return FAILURE
    else if (
    /*ThreadCB.highPriorityQueue.isEmpty() && ThreadCB.lowPriorityQueue.isEmpty()*/ queueThread
            == null
        && MMU.getPTBR() == null) return FAILURE;

    return FAILURE; // None of the above cases were true. Something went wrong.
  }