/// <summary>
/// Executes specified task and starts cooresponding callback necessary for cooperation.
/// </summary>
/// <param name="taskWithInformation">Task to be executed</param>
private void RunTask(PrioritizedLimitedTask taskWithInformation)
{
lock (schedulingLocker)
{
executingTasks.TryAdd(taskWithInformation.PrioritizedLimitedTaskIdentifier, taskWithInformation);
// If pending task was paused, than callback exist and does not need to be started again
if (!taskWithInformation.CooperationMechanism.IsPaused && !taskWithInformation.CooperationMechanism.IsResumed)
{
StartCallback(taskWithInformation,
() => executingTasks.Remove(taskWithInformation.PrioritizedLimitedTaskIdentifier, out _),
() => PauseCallback(taskWithInformation));
}
// Do not block a main thread.
new Task(() =>
{
if (taskWithInformation.CooperationMechanism.IsPaused)
{
taskWithInformation.CooperationMechanism.Resume();
}
else
{
TryExecuteTask(taskWithInformation);
}
}).Start();
}
}
/// <summary>
/// Using preemptive algorithm, runs tasks (number of tasks dependins on parallelism level).
/// Deadlock avoidance is enabled.
/// </summary>
public override void RunScheduling()
{
lock (schedulingLocker)
while (!pendingTasks.IsEmpty && executingTasks.Count < maxLevelOfParallelism)
{
// Get task that is next for execution (task with highest priority)
PrioritizedLimitedTask taskWithInformation = GetNextTaskWithDeadlockAvoidance();
executingTasks.TryAdd(taskWithInformation.PrioritizedLimitedTaskIdentifier, taskWithInformation);
// If pending task was paused, than callback exist and does not need to be created again
if (!taskWithInformation.CooperationMechanism.IsPaused && !taskWithInformation.CooperationMechanism.IsResumed)
{
StartCallback(taskWithInformation,
() => executingTasks.Remove(taskWithInformation.PrioritizedLimitedTaskIdentifier, out _),
() => PauseCallback(taskWithInformation));
}
// Do not block a main thread.
new Task(() =>
{
if (taskWithInformation.CooperationMechanism.IsPaused)
{
taskWithInformation.CooperationMechanism.Resume();
}
else
{
TryExecuteTask(taskWithInformation);
}
}).Start();
}
}
/// <summary>
/// Delays execution of a callback for specific amount of time.
/// </summary>
/// <param name="taskWithInformation">Task that contains information about that time.</param>
private void PauseCallback(PrioritizedLimitedTask taskWithInformation)
{
do
{
if (taskWithInformation.CooperationMechanism.IsPaused || taskWithInformation.CooperationMechanism.IsResumed)
{
Task.Delay(taskWithInformation.CooperationMechanism.PausedFor).Wait();
}
}while (taskWithInformation.CooperationMechanism.IsPaused);
}
/// <summary>
/// If a task uses no shared resources, deadlock will be avoided. Otherwise,
/// call is delegated to a manager that uses Banker's algorithm and decides whether deadlock can be avoided <see cref="IBankerAlgorithm"/>.
/// </summary>
private bool CanAvoidDeadlock(PrioritizedLimitedTask task)
{
lock (schedulingLocker)
{
RequestApproval approved = RequestApproval.Approved;
if (task.UsesSharedResources())
{
approved = sharedResourceManager.AllocateResources(task.PrioritizedLimitedTaskIdentifier
, task.SharedResources);
}
return(approved == RequestApproval.Approved);
}
}
/// <summary>
/// Starts (creates and schedules) task that is used to cooperatively cancels (and optionally pauses) user's task.
/// Is user's task used resources, resources are released.
/// This task is scheduled using default .NET scheduler.
/// </summary>
/// <param name="task">Corresponding user's task</param>
/// <param name="controlNumberOfExecutionTasksAction">Responsible for decrementing number of currently running tasks</param>
/// <param name="enablePauseAction">Responsible for pausing this callback for specific amount of time (optional)</param>
protected void StartCallback(PrioritizedLimitedTask task, Action controlNumberOfExecutionTasksAction, Action enablePauseAction = null)
=> Task.Factory.StartNew(() =>
{
Task.Delay(task.DurationInMiliseconds).Wait();
enablePauseAction?.Invoke();
task.CooperationMechanism.Cancel();
// Free shared resources
if (task.UsesSharedResources())
{
sharedResourceManager.FreeResources(task.PrioritizedLimitedTaskIdentifier, task.SharedResources);
}
controlNumberOfExecutionTasksAction.Invoke();
RunScheduling();
}, CancellationToken.None, TaskCreationOptions.None, Default);
/// <summary>
/// Using preemptive scheduling algorithm, runs task.
/// </summary>
public override void RunScheduling()
{
lock (schedulingLocker)
{
while (!pendingTasks.IsEmpty && currentlyRunningTasks < maxLevelOfParallelism)
{
PrioritizedLimitedTask taskWithInformation = GetNextTaskWithDeadlockAvoidance();
StartCallback(taskWithInformation, () => Interlocked.Decrement(ref currentlyRunningTasks));
Interlocked.Increment(ref currentlyRunningTasks);
// Do not block a main thread
new Task(() => TryExecuteTask(taskWithInformation)).Start();
}
}
}
/// <summary>
/// Finds and returns a task that has all necessary resources and allocates those resources.
/// It assumes that list is not empty, so a caller of this method needs to check that.
/// </summary>
protected PrioritizedLimitedTask GetNextTaskWithDeadlockAvoidance()
{
pendingTasks.TryDequeue(out PrioritizedLimitedTask taskWithInformation);
if (taskWithInformation.UsesSharedResources())
{
RequestApproval approved = sharedResourceManager.AllocateResources(taskWithInformation.PrioritizedLimitedTaskIdentifier
, taskWithInformation.SharedResources);
if (approved == RequestApproval.Wait)
{
PrioritizedLimitedTask nextTask = GetNextTaskWithDeadlockAvoidance();
pendingTasks.Enqueue(taskWithInformation);
taskWithInformation = nextTask;
}
}
return(taskWithInformation);
}
/// <summary>
/// This method simulates a context switch (pauses one task, and runs another) if there is a guarantee that deadlock won't happen.
/// If a task does not get necessary resources, normal scheduling is called (no context switching and no interruption).
/// </summary>
/// <param name="taskForExecution">Task to be executed</param>
/// <param name="taskToPause">Task to pause</param>
private void RequestContextSwitch(PrioritizedLimitedTask taskForExecution, PrioritizedLimitedTask taskToPause)
{
lock (schedulingLocker)
// TODO: Move
if (executingTasks.Count >= maxLevelOfParallelism && CanAvoidDeadlock(taskForExecution))
{
executingTasks.Remove(taskToPause.PrioritizedLimitedTaskIdentifier, out _);
taskToPause.CooperationMechanism.Pause(taskToPause.DurationInMiliseconds);
pendingTasks.Enqueue(taskToPause);
RunTask(taskForExecution);
}
else
{
SortPendingTasks();
RunScheduling();
}
}
/// <summary>
/// Schedules task based on a priority. If the task has a greater prirority than one of the currently running tasks,
/// and if one of the currently running tasks allows cooperative context-switching, than context-switching will happen.
/// </summary>
/// <param name="task">Task to be scheduled (must extend <see cref="PrioritizedLimitedTask"/>),
/// otherwise exception will be thrown</param>
/// <exception cref="InvalidTaskException"></exception>
protected override void QueueTask(Task task)
{
if (!(task is PrioritizedLimitedTask))
{
throw new InvalidTaskException();
}
pendingTasks.Enqueue(task as PrioritizedLimitedTask);
Priority priority = (task as PrioritizedLimitedTask).Priority;
PriorityComparer priorityComparer = new PriorityComparer();
PrioritizedLimitedTask taskToPause = executingTasks.Values
.Where(x => x.CooperationMechanism.CanBePaused)
.Min();
if (taskToPause != null && priority.CompareTo(taskToPause.Priority) > 0)
{
RequestContextSwitch(task as PrioritizedLimitedTask, taskToPause);
}
else
{
SortPendingTasks();
RunScheduling();
}
}
