/**
* 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();
}
// This method destroys threads. To destroy a thread, its status must be set to ThreadKill
// and a number of other actions must be performed depending on the current status of
// the thread. (The status of a thread can be obtained via the method getStatus().)
// If the thread is ready, then it must be removed from the ready queue. If a running
// thread is being destroyed, then it must be removed from controlling the CPU, as
// described earlier.
public void do_kill() {
if (this.getStatus() == ThreadReady) {
ThreadCB.lowPriorityQueue.remove(this);
ThreadCB.highPriorityQueue.remove(this);
}
if (this.getStatus() == ThreadRunning) {
// Following steps 1-3 change the current status from ThreadRunning to
// ThreadKill pages 55-56 of the manual
// Step 2
MMU.setPTBR(null);
// Step 3
this.getTask().setCurrentThread(null);
}
if (this.getStatus() == ThreadWaiting) {
// Cancel the I/O request by removing the corresponding
// IORB from its device queue. This can be done by scanning all devices in the device
// table and executing the method cancelPendingIO() on each device. The device table
// is an array of size Device.getTableSize() (starting with device 0), where device i
// can be obtained with a call to Device.get().
for (int device = 0; Device.getTableSize() > device; ++device)
Device.get(device).cancelPendingIO(this);
this.setStatus(ThreadKill);
}
// Includes threadWaiting
this.setStatus(ThreadKill);
// When a thread is killed, those resources must be released into the common
// pool so that other threads could use them. This is done using the static method
// giveupResources() of class ResourceCB, which accepts the thread be killed as a
// parameter.
this.getTask().removeThread(this);
ResourceCB.giveupResources(this);
// Two things remain to be done now. First, we must dispatch a new thread, since we
// should use every interrupt or a system call as an opportunity to optimize CPU usage.
dispatch();
// Second, since we have just killed a thread, we must check if the corresponding task still
// has any threads left. A task with no threads is considered dead and must be destroyed
// with the kill() method of class TaskCB. To find out the number of threads a task
// has, use the method getThreadCount() of TaskCB
if (this.getTask().getThreadCount() == 0) this.getTask().kill();
}
/**
* 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.
}