/*
* Create a thread object using the default constructor ThreadCB() and
* associate this newly created thread with a task (provided as an
* argument to the do create() method).
*
*/
public static ThreadCB do_create(TaskCB task) {
// It is important to keep in mind that each time control is transferred to
// the operating system, it is seen as an opportunity to schedule a thread
// to run. Therefore, regardless of whether the new thread was created
// successfully, the dispatcher must be called (or else a warning will be issued)
// There is a global constant (in IflThreadCB), called MaxThreadsPerTask.
// If this number of threads per task is exceeded, no new thread should be
// created for that task, and null should be returned.
if (MaxThreadsPerTask <= task.getThreadCount()) {
dispatch();
return null;
}
// To link a thread to its task, the method addThread() of class
// IflTaskCB should be used and the thread’s task must be set using
// the method setTask() of IflThreadCB.*/
ThreadCB thread = new ThreadCB();
// null should also be returned if addThread() returns FAILURE. You can
// find out the number of threads a task currently has by calling the
// method getThreadCount() on that task.
if (task.addThread(thread) == FAILURE) {
dispatch();
return null;
}
// Apparently I needed to add this after testing. 2 hours of debugging. Wow
task.addThread(thread);
thread.setTask(task);
// If priority scheduling needs to be implemented, the do create() method must cor-
// rectly assign the thread’s initial priority. The actual value of the priority depends
// on the particular scheduling policy used. OSP 2 provides methods for setting and
// querying the priority of both tasks and threads. The methods are setPriority() and
// getPriority() in classes TaskCB and ThreadCB, respectively.
thread.setPriority(randomGeneratedNumber.nextInt((10 - 1) + 1));
// Finally, the status of the new thread should be set to ThreadReady
// and it should be placed in the ready queue.
thread.setStatus(ThreadReady);
// If priority is above below 10% or equal to move into the low priority thread.
if (task.getPriority() <= 1) ThreadCB.lowPriorityQueue.addElement(thread);
else ThreadCB.highPriorityQueue.addElement(thread);
// If all is well, the thread object created by this method should be returned.
dispatch();
return thread;
}
/**
* 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.
}