如何使用 C# 並行運行兩個任務？
[英]How to run two tasks in parallel with C#?
問題描述
假設我有這段代碼應該正確並行運行兩個任務，我對 C# 中的並行任務沒有一個好主意，我想從這段代碼開始理解這個概念，我想做同時運行兩個任務（異步）
public async Task Main() {
var task1 = Task.Run(() => DoWork());
var task2 = Task.Run(() => CleanIt());
await Task.WhenAll(task1, task2);
}
private void CleanIt() {
int sum = 0;
for (int i = 0; i < 10; i++) {
Console.WriteLine(" Thread two " + i);
}
}
private void DoWork() {
int sum = 0;
for (int i = 0; i < 10; i++) {
Console.WriteLine(" Thread one " + i);
}
}
我得到的結果：
Thread one 0
Thread two 0
Thread one 1
Thread one 2
Thread one 3
Thread one 4
Thread one 5
Thread one 6
Thread one 7
Thread one 8
Thread one 9
Thread two 1
Thread two 2
Thread two 3
Thread two 4
Thread two 5
Thread two 6
Thread two 7
Thread two 8
Thread two 9
我想顯示這樣的結果：
Thread one 0
Thread two 0
Thread one 1
Thread two 1
Thread one 2
Thread two 2
....
我怎樣才能達到這個結果？
2 個解決方案
解決方案1
0 2022-05-10 16:00:58
異步代碼的想法是告訴處理器不要等待這個任務完成，而是同時啟動其他任務。 由於“其他”任務也不會等待原始任務完成，因此如果不編寫同步代碼，就無法確保任務保持同步。 當您啟動多個異步任務時，您放棄了對這些任務何時執行的控制權及其處理器調度算法，並承諾它們最終會完成。
調度算法將選擇執行代碼的最有效方式，考慮到所有其他要求 CPU 時間的程序，並決定如何運行您的任務。 它可以選擇運行一個任務來完成，然后另一個任務。 或者，它可以選擇在不同的處理器內核上運行它們，或者在同一內核上切換它們。 這不取決於您，而是取決於操作系統來決定。
具體來說，在 C# 中，異步任務使用線程池運行，因此您甚至無法選擇是否運行多個線程。 如果任務使用同一個線程運行，它們就不能在多個核心上運行，這意味着它們不能並行運行。 如果您希望任務在多個內核上運行，則需要顯式使用Thread ，但無論您無法控制每個線程何時運行或運行在哪個內核上。
如果您有長時間運行的任務，您可能會看到輸出從一個切換到另一個。 每個任務運行的確切時間取決於許多您無法控制的事情，並且每次運行程序時可能會有所不同。
如果您需要您的代碼同步運行（即，在運行任務之前等待其他任務繼續執行），您需要編寫同步代碼，而不是異步代碼。
解決方案2
0 2022-05-10 16:02:22
你依次做兩件事：
(1) start task 1. (2) start task 2.
現在開始一個任務，即執行Task.Run() ，既慢又昂貴。 比如說，它可能需要 5 毫秒。 運行這個非常短的任務可能只需要 1 毫秒。 所以任務 1 早在任務 2 開始之前就完成了。
Time -> ...1ms...1ms...1ms...1ms...1ms...1ms..........1ms............1ms...1ms...1ms...1ms...1ms... main [set up task 1..................................][set up task 2............................................][main ends] task1 [DoWork() starts] [DoWork() ends] task2 [DoWorkToo() starts] [DoWorkToo() ends]
如果您想讓任務並行運行，則必須使它們的運行時間長於任務啟動時間。 一種解決方案是簡單地運行循環數千次，但這可能很笨拙。 更好的是讓任務在循環執行期間休眠，以便在任務 2 啟動時任務 1 仍在運行。
這是一個運行時間較長的任務的示例，很好地展示了執行交錯的方式：
using System;
using System.Threading;
using System.Threading.Tasks;
using System.Diagnostics; // stopwatch
namespace TwoTasks2
{
class Program
{
static Stopwatch globalStopWatch = new Stopwatch();
private static void DoWork()
{
Console.WriteLine("========================= Entering DoWork() after " + globalStopWatch.ElapsedMilliseconds);
int sum = 0;
Stopwatch sw = new Stopwatch();
sw.Start();
for (int i = 0; i < 100000; i++)
{
if (i % 10000 == 0)
{
Console.WriteLine("Task one " + 10000 + " cycles took " + sw.ElapsedMilliseconds + " ms");
sw.Stop();
sw.Start();
Console.WriteLine("Thread ID: " + Thread.CurrentThread.ManagedThreadId);
Console.WriteLine("CPU ID: " + Thread.GetCurrentProcessorId());
}
}
}
private static void DoWorkToo()
{
Console.WriteLine("========================= Entering DoWorkToo() after " + globalStopWatch.ElapsedMilliseconds);
int sum = 0;
Stopwatch sw = new Stopwatch();
sw.Start();
for (int i = 0; i < 100000; i++)
{
if (i % 10000 == 0)
{
Console.WriteLine(" Task two " + 10000 + " cycles took " + sw.ElapsedMilliseconds + " ms");
sw.Stop();
sw.Start();
Console.WriteLine(" Thread ID: " + Thread.CurrentThread.ManagedThreadId);
Console.WriteLine(" CPU ID: " + Thread.GetCurrentProcessorId());
}
}
}
public static void Main()
{
globalStopWatch.Start();
var task1 = Task.Run(() => DoWork());
long ms = globalStopWatch.ElapsedMilliseconds;
Console.WriteLine("--------------------- RunTask 1 took " + ms);
var task2 = Task.Run(() => DoWorkToo());
Console.WriteLine("--------------------- RunTask 2 took " + (globalStopWatch.ElapsedMilliseconds-ms));
var tasks = new Task[] { task1, task2 };
Task.WaitAll(tasks);
}
}
}
我的機器上的示例輸出，調試構建：
--------------------- RunTask 1 took 23
========================= Entering DoWork() after 39
--------------------- RunTask 2 took 18
Task one 10000 cycles took 0 ms
Thread ID: 4
========================= Entering DoWorkToo() after 41
Task two 10000 cycles took 0 ms
Thread ID: 5
CPU ID: 1
CPU ID: 2
Task two 10000 cycles took 1 ms
Thread ID: 5
Task one 10000 cycles took 2 ms
Thread ID: 4
CPU ID: 4
CPU ID: 1
Task one 10000 cycles took 2 ms
Thread ID: 4
CPU ID: 4
Task two 10000 cycles took 2 ms
Thread ID: 5
Task one 10000 cycles took 2 ms
Thread ID: 4
CPU ID: 4
CPU ID: 1
Task one 10000 cycles took 2 ms
Thread ID: 4
CPU ID: 4
Task two 10000 cycles took 2 ms
Thread ID: 5
CPU ID: 1
Task one 10000 cycles took 2 ms
Thread ID: 4
CPU ID: 2
Task two 10000 cycles took 2 ms
Thread ID: 5
CPU ID: 1
Task one 10000 cycles took 3 ms
Thread ID: 4
CPU ID: 2
Task two 10000 cycles took 2 ms
Thread ID: 5
CPU ID: 1
Task one 10000 cycles took 3 ms
Thread ID: 4
CPU ID: 2
Task two 10000 cycles took 2 ms
Thread ID: 5
CPU ID: 1
Task one 10000 cycles took 3 ms
Thread ID: 4
CPU ID: 2
Task two 10000 cycles took 2 ms
Thread ID: 5
CPU ID: 3
Task one 10000 cycles took 3 ms
Thread ID: 4
CPU ID: 4
Task two 10000 cycles took 2 ms
Thread ID: 5
CPU ID: 10
Task two 10000 cycles took 3 ms
Thread ID: 5
CPU ID: 1
發布版本更加有序：
--------------------- RunTask 1 took 21
========================= Entering DoWork() after 37
--------------------- RunTask 2 took 16
Task one 10000 cycles took 0 ms
Thread ID: 4
CPU ID: 4
Task one 10000 cycles took 0 ms
Thread ID: 4
CPU ID: 6
Task one 10000 cycles took 1 ms
Thread ID: 4
CPU ID: 2
Task one 10000 cycles took 1 ms
Thread ID: 4
CPU ID: 2
Task one 10000 cycles took 1 ms
Thread ID: 4
CPU ID: 3
Task one 10000 cycles took 1 ms
Thread ID: 4
CPU ID: 6
Task one 10000 cycles took 2 ms
Thread ID: 4
CPU ID: 2
========================= Entering DoWorkToo() after 39
Task two 10000 cycles took 0 ms
Thread ID: 5
CPU ID: 11
Task one 10000 cycles took 2 ms
Thread ID: 4
CPU ID: 10
Task two 10000 cycles took 0 ms
Thread ID: 5
CPU ID: 11
Task one 10000 cycles took 2 ms
Thread ID: 4
CPU ID: 10
Task two 10000 cycles took 0 ms
Thread ID: 5
CPU ID: 11
Task one 10000 cycles took 2 ms
Thread ID: 4
CPU ID: 10
Task two 10000 cycles took 0 ms
Thread ID: 5
CPU ID: 1
Task two 10000 cycles took 0 ms
Thread ID: 5
CPU ID: 1
Task two 10000 cycles took 0 ms
Thread ID: 5
CPU ID: 11
Task two 10000 cycles took 0 ms
Thread ID: 5
CPU ID: 11
Task two 10000 cycles took 0 ms
Thread ID: 5
CPU ID: 11
Task two 10000 cycles took 0 ms
Thread ID: 5
CPU ID: 1
Task two 10000 cycles took 1 ms
Thread ID: 5
CPU ID: 1
第一個任務運行需要 35 毫秒！ 這是現代 CPU 的永恆。 然后第二個任務開始得更快。
這些任務甚至在打印到控制台的行之間輪流執行。 您還可以看到，即使是相同的線程也在 Windows 認為合適的情況下從一個核心跳到另一個核心。 （這實際上讓我感到驚訝。我的 Ryzen 有 6 個真正的內核，並且沒有任何顯着的負載，所以我會讓任務在它們所在的位置運行。）
在C＃中運行並行任務
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