任务与连续调度

本文关键字：调度 连续 任务 | 更新日期: 2024-10-24 03:51:53

假设我有许多CPU密集型任务被转储到默认调度程序上（所有任务都同时运行，例如通过具有默认调度程序的Task.Run或Task.Factory.StartNew）。每个任务都有一个Continuation。调度程序将启动一些任务，并将它们置于运行状态。当这些任务完成时，调度程序将在其队列中混合原始任务和连续任务进行调度，并且必须在它们之间进行选择。与运行更多的组织任务相比，它如何优先考虑延续。特别是，如果一个任务已经完成，并且它有一个延续，那么该延续在调度程序中是否比已经排队的其他任务具有优先级。

例如，假设调度程序运行许多（T1…Tn）排队任务中的2个（T1&T2）。当其中一个完成时，调度器是否必须运行延续C1，或者是否决定启动T3？它到底会选择哪一个是决定性的吗？调度器是否可能选择运行更多的任务，并且因此在T1的结束和它的延续C1的开始之间可能有相当大的延迟？

更新：我运行了一些示例代码，并添加了一个答案-，但我仍然想知道这种观察到的行为是否得到了保证

任务与连续调度

所以我尝试了一个示例：

var continuations = new List<Task>();
for (int i = 0; i < 20; i++) {
    int counter = i;
    var continuation = Task.Run(() => {
        Console.WriteLine($"T{counter}: Start.");
        Thread.Sleep(500);
        Console.WriteLine($"T{counter}: Complete.");
    }).ContinueWith(t => {
        Console.WriteLine($"C{counter}: Start.");
        Thread.Sleep(50);
        Console.WriteLine($"C{counter}: Complete.");
    });
    continuations.Add(continuation);
}
Task.WaitAll(continuations.ToArray());

哪个输出以下-意味着调度器确实优先考虑继续而不是开始新任务（但这有保证吗？）：

T1: Start.
T2: Start.
T3: Start.
T0: Start.
T1: Complete.
T2: Complete.
T3: Complete.
C1: Start.
C3: Start.
T0: Complete.
C0: Start.
C2: Start.
C1: Complete.
T4: Start.
C3: Complete.
T5: Start.
C0: Complete.
T6: Start.
C2: Complete.
T7: Start.
T4: Complete.
C4: Start.
T5: Complete.
C5: Start.
T6: Complete.
C6: Start.
T7: Complete.
C7: Start.
C4: Complete.
C5: Complete.
T9: Start.
T8: Start.
C7: Complete.
T10: Start.
C6: Complete.
T11: Start.
T8: Complete.
C8: Start.
T9: Complete.
C9: Start.
T11: Complete.
C11: Start.
T10: Complete.
C10: Start.
C9: Complete.
C8: Complete.
T13: Start.
T12: Start.
C11: Complete.
T14: Start.
C10: Complete.

一种变体。在这里，在启动Task和注册Continuation之间，调用线程上还有额外的执行。

var continuations = new List<Task>();
for (int i = 0; i < 20; i++) {
    int counter = i;
    var task = Task.Run(() => {
        Console.WriteLine($"T{counter}: Start.");
        Thread.Sleep(500);
        Console.WriteLine($"T{counter}: Complete.");
    });
    Thread.Sleep(100); // Do some stuff before registering continuation.
    var continuation = task.ContinueWith(t => {
    Console.WriteLine($"C{counter}: Start.");
        Thread.Sleep(150);
        Console.WriteLine($"C{counter}: Complete.");
    });
    continuations.Add(continuation);
}
Task.WaitAll(continuations.ToArray());

结果与上述情况相同，即任务N的继续被赋予其他排队任务的优先级。

T0: Start.
T1: Start.
T2: Start.
T3: Start.
T0: Complete.
C0: Start.
T1: Complete.
C1: Start.
C0: Complete.
T4: Start.
T2: Complete.
C2: Start.
C1: Complete.
T5: Start.
T3: Complete.
C3: Start.
C2: Complete.
T6: Start.
C3: Complete.
T7: Start.
T4: Complete.
C4: Start.
T5: Complete.
C5: Start.
C4: Complete.
T8: Start.
T6: Complete.
C6: Start.
C5: Complete.
T9: Start.
T7: Complete.
C7: Start.
C6: Complete.
T10: Start.
C7: Complete.
T11: Start.
T8: Complete.
C8: Start.
T9: Complete.
C9: Start.
C8: Complete.
T12: Start.
T10: Complete.
C10: Start.
C9: Complete.
T13: Start.
T11: Complete.
C11: Start.
T14: Start.
C10: Complete.
T15: Start.
C11: Complete.
T16: Start.
T12: Complete.
C12: Start.
T13: Complete.
C13: Start.
T14: Complete.
C14: Start.
C12: Complete.
T17: Start.
T15: Complete.
C15: Start.......

另一种变体。在这里，在注册延续之前，所有任务都会先在scheduler上排队（并且可以安排运行）。

const int taskCount = 20;
var tasks = new List<Task>();
for (int i = 0; i < taskCount; i++) {
    int counter = i;
    var task = Task.Run(() => {
        Console.WriteLine($"T{counter}: Start.");
        Thread.Sleep(500);
        Console.WriteLine($"T{counter}: Complete.");
    });
    tasks.Add(task);
}
Thread.Sleep(400);
var continuations = new List<Task>();
for (int i = 0; i < taskCount; i++) {
    int counter = i;
    var continuation = tasks[i].ContinueWith(t => {
        Console.WriteLine($"C{counter}: Start.");
        Thread.Sleep(150);
        Console.WriteLine($"C{counter}: Complete.");
    });
    continuations.Add(continuation);
}
Task.WaitAll(continuations.ToArray());

结果和以前一样-继续获得优先权。

T0: Start.
T2: Start.
T3: Start.
T1: Start.
T1: Complete.
T3: Complete.
T0: Complete.
T2: Complete.
C2: Start.
C0: Start.
C3: Start.
C1: Start.
C2: Complete.
C1: Complete.
C0: Complete.
C3: Complete.
T7: Start.
T4: Start.
T6: Start.
T5: Start.
T6: Complete.
T5: Complete.
T7: Complete.
T4: Complete.
C6: Start.
C5: Start.
C7: Start.
C4: Start.
C7: Complete.
T8: Start.
C5: Complete.
T9: Start.
C6: Complete.
T10: Start.
C4: Complete.
T11: Start.
T8: Complete.
C8: Start.
T10: Complete.
C10: Start.
T9: Complete.
C9: Start.
T11: Complete.
C11: Start.
C8: Complete.
T12: Start.
C9: Complete.
T13: Start.
C10: Complete.
T14: Start.
C11: Complete.
T15: Start.
T14: Complete.
T12: Complete.
C12: Start.
T13: Complete.
C13: Start.
C14: Start.
T15: Complete.
C15: Start.
T16: Start.
C12: Complete.
C13: Complete.
T18: Start.
T17: Start.
C14: Complete.
T19: Start.
C15: Complete.
T16: Complete.
C16: Start.
T18: Complete.
C18: Start.
T17: Complete.
T19: Complete.
C19: Start.
C16: Complete.
C17: Start.
C18: Complete.
C19: Complete.
C17: Complete.
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