在地铁应用程序中,我需要执行许多 WCF 调用。有大量的调用要进行,所以我需要在并行循环中执行它们。 问题是并行循环在 WCF 调用全部完成之前退出。
您将如何重构它以按预期工作?
var ids = new List<string>() { "1", "2", "3", "4", "5", "6", "7", "8", "9", "10" };
var customers = new System.Collections.Concurrent.BlockingCollection<Customer>();
Parallel.ForEach(ids, async i =>
{
ICustomerRepo repo = new CustomerRepo();
var cust = await repo.GetCustomer(i);
customers.Add(cust);
});
foreach ( var customer in customers )
{
Console.WriteLine(customer.ID);
}
Console.ReadKey();
Parallel.ForEach()
背后的整个思想是你有一组线程,每个线程处理集合的一部分。正如您所注意到的,这不适用于 async
- await
,您希望在异步调用期间释放线程。
您可以通过阻止ForEach()
线程来"修复"它,但这违背了async
的全部意义 - await
.
你可以做的是使用TPL数据流而不是Parallel.ForEach()
,它很好地支持异步Task
。
具体来说,可以使用TransformBlock
编写代码,该使用async
lambda 将每个 id 转换为Customer
。此块可以配置为并行执行。您可以将该块链接到将每个Customer
写入控制台的ActionBlock
。设置块网络后,可以将每个 id Post()
到TransformBlock
。
在代码中:
var ids = new List<string> { "1", "2", "3", "4", "5", "6", "7", "8", "9", "10" };
var getCustomerBlock = new TransformBlock<string, Customer>(
async i =>
{
ICustomerRepo repo = new CustomerRepo();
return await repo.GetCustomer(i);
}, new ExecutionDataflowBlockOptions
{
MaxDegreeOfParallelism = DataflowBlockOptions.Unbounded
});
var writeCustomerBlock = new ActionBlock<Customer>(c => Console.WriteLine(c.ID));
getCustomerBlock.LinkTo(
writeCustomerBlock, new DataflowLinkOptions
{
PropagateCompletion = true
});
foreach (var id in ids)
getCustomerBlock.Post(id);
getCustomerBlock.Complete();
writeCustomerBlock.Completion.Wait();
尽管您可能希望将TransformBlock
的并行性限制为一些小常量。此外,您可以限制TransformBlock
的容量并使用 SendAsync()
异步添加项目,例如,如果集合太大。
与代码相比(如果它有效(的另一个好处是,一旦单个项目完成,编写就会开始,而不是等到所有处理完成。
Svick的回答(像往常一样(非常好。
但是,我发现当您实际有大量数据要传输时,数据流更有用。或者当您需要async
兼容的队列时。
在您的情况下,更简单的解决方案是仅使用 async
样式的并行性:
var ids = new List<string>() { "1", "2", "3", "4", "5", "6", "7", "8", "9", "10" };
var customerTasks = ids.Select(i =>
{
ICustomerRepo repo = new CustomerRepo();
return repo.GetCustomer(i);
});
var customers = await Task.WhenAll(customerTasks);
foreach (var customer in customers)
{
Console.WriteLine(customer.ID);
}
Console.ReadKey();
按照 svick 的建议使用 DataFlow 可能是矫枉过正的,斯蒂芬的回答并没有提供控制操作并发性的方法。但是,这可以相当简单地实现:
public static async Task RunWithMaxDegreeOfConcurrency<T>(
int maxDegreeOfConcurrency, IEnumerable<T> collection, Func<T, Task> taskFactory)
{
var activeTasks = new List<Task>(maxDegreeOfConcurrency);
foreach (var task in collection.Select(taskFactory))
{
activeTasks.Add(task);
if (activeTasks.Count == maxDegreeOfConcurrency)
{
await Task.WhenAny(activeTasks.ToArray());
//observe exceptions here
activeTasks.RemoveAll(t => t.IsCompleted);
}
}
await Task.WhenAll(activeTasks.ToArray()).ContinueWith(t =>
{
//observe exceptions in a manner consistent with the above
});
}
ToArray()
调用可以通过使用数组而不是列表并替换已完成的任务来优化,但我怀疑在大多数情况下它会有很大的不同。每个OP问题的示例用法:
RunWithMaxDegreeOfConcurrency(10, ids, async i =>
{
ICustomerRepo repo = new CustomerRepo();
var cust = await repo.GetCustomer(i);
customers.Add(cust);
});
编辑研究员SO用户和TPL专家Eli Arbel向我指出了Stephen Toub的一篇相关文章。像往常一样,他的实现既优雅又高效:
public static Task ForEachAsync<T>(
this IEnumerable<T> source, int dop, Func<T, Task> body)
{
return Task.WhenAll(
from partition in Partitioner.Create(source).GetPartitions(dop)
select Task.Run(async delegate {
using (partition)
while (partition.MoveNext())
await body(partition.Current).ContinueWith(t =>
{
//observe exceptions
});
}));
}
可以使用新的 AsyncEnumerator NuGet 包节省工作量,该包在 4 年前最初发布问题时并不存在。它允许您控制并行度:
using System.Collections.Async;
...
await ids.ParallelForEachAsync(async i =>
{
ICustomerRepo repo = new CustomerRepo();
var cust = await repo.GetCustomer(i);
customers.Add(cust);
},
maxDegreeOfParallelism: 10);
免责声明:我是AsyncEnumerator库的作者,该库是开源的,并在麻省理工学院下获得许可,我发布此消息只是为了帮助社区。
Parallel.Foreach
包装到Task.Run()
中,而不是使用 await
关键字,请使用 [yourasyncmethod].Result
(你需要做 Task.Run 的事情才能不阻塞 UI 线程(
像这样:
var yourForeachTask = Task.Run(() =>
{
Parallel.ForEach(ids, i =>
{
ICustomerRepo repo = new CustomerRepo();
var cust = repo.GetCustomer(i).Result;
customers.Add(cust);
});
});
await yourForeachTask;
这应该非常有效,并且比让整个TPL数据流工作更容易:
var customers = await ids.SelectAsync(async i =>
{
ICustomerRepo repo = new CustomerRepo();
return await repo.GetCustomer(i);
});
...
public static async Task<IList<TResult>> SelectAsync<TSource, TResult>(this IEnumerable<TSource> source, Func<TSource, Task<TResult>> selector, int maxDegreesOfParallelism = 4)
{
var results = new List<TResult>();
var activeTasks = new HashSet<Task<TResult>>();
foreach (var item in source)
{
activeTasks.Add(selector(item));
if (activeTasks.Count >= maxDegreesOfParallelism)
{
var completed = await Task.WhenAny(activeTasks);
activeTasks.Remove(completed);
results.Add(completed.Result);
}
}
results.AddRange(await Task.WhenAll(activeTasks));
return results;
}
扩展方法,它利用了SemaphoreSlim
,并且还允许设置最大并行度:
/// <summary>Concurrently Executes async actions for each item of
/// <see cref="IEnumerable<typeparamref name="T"/></summary>
/// <typeparam name="T">Type of IEnumerable</typeparam>
/// <param name="enumerable">instance of
/// <see cref="IEnumerable<typeparamref name="T"/>"/></param>
/// <param name="action">an async <see cref="Action" /> to execute</param>
/// <param name="maxDegreeOfParallelism">Optional, An integer that represents the
/// maximum degree of parallelism, Must be grater than 0</param>
/// <returns>A Task representing an async operation</returns>
/// <exception cref="ArgumentOutOfRangeException">If the maxActionsToRunInParallel
/// is less than 1</exception>
public static async Task ForEachAsyncConcurrent<T>(
this IEnumerable<T> enumerable,
Func<T, Task> action,
int? maxDegreeOfParallelism = null)
{
if (maxDegreeOfParallelism.HasValue)
{
using (var semaphoreSlim = new SemaphoreSlim(
maxDegreeOfParallelism.Value, maxDegreeOfParallelism.Value))
{
var tasksWithThrottler = new List<Task>();
foreach (var item in enumerable)
{
// Increment the number of currently running tasks and wait if they
// are more than limit.
await semaphoreSlim.WaitAsync();
tasksWithThrottler.Add(Task.Run(async () =>
{
await action(item).ContinueWith(res =>
{
// action is completed, so decrement the number of
// currently running tasks
semaphoreSlim.Release();
}, TaskScheduler.Default);
}));
}
// Wait for all tasks to complete.
await Task.WhenAll(tasksWithThrottler.ToArray());
}
}
else
{
await Task.WhenAll(enumerable.Select(item => action(item)));
}
}
示例用法:
await enumerable.ForEachAsyncConcurrent(
async item =>
{
await SomeAsyncMethod(item);
},
5);
我参加派对有点晚了,但您可能需要考虑使用 GetAwaiter.GetResult(( 在同步上下文中运行异步代码,但如下所示;
Parallel.ForEach(ids, i =>
{
ICustomerRepo repo = new CustomerRepo();
// Run this in thread which Parallel library occupied.
var cust = repo.GetCustomer(i).GetAwaiter().GetResult();
customers.Add(cust);
});
在引入了一堆帮助程序方法之后,您将能够使用以下简单语法运行并行查询:
const int DegreeOfParallelism = 10;
IEnumerable<double> result = await Enumerable.Range(0, 1000000)
.Split(DegreeOfParallelism)
.SelectManyAsync(async i => await CalculateAsync(i).ConfigureAwait(false))
.ConfigureAwait(false);
这里发生的事情是:我们将源集合分成 10 个块 ( .Split(DegreeOfParallelism)
(,然后运行 10 个任务,每个任务一个接一个地处理其项目(.SelectManyAsync(...)
(,并将它们合并回一个列表。
值得一提的是,有一种更简单的方法:
double[] result2 = await Enumerable.Range(0, 1000000)
.Select(async i => await CalculateAsync(i).ConfigureAwait(false))
.WhenAll()
.ConfigureAwait(false);
但它需要采取预防措施:如果源集合太大,它将立即为每个项目安排Task
,这可能会导致显著的性能下降。
上述示例中使用的扩展方法如下所示:
public static class CollectionExtensions
{
/// <summary>
/// Splits collection into number of collections of nearly equal size.
/// </summary>
public static IEnumerable<List<T>> Split<T>(this IEnumerable<T> src, int slicesCount)
{
if (slicesCount <= 0) throw new ArgumentOutOfRangeException(nameof(slicesCount));
List<T> source = src.ToList();
var sourceIndex = 0;
for (var targetIndex = 0; targetIndex < slicesCount; targetIndex++)
{
var list = new List<T>();
int itemsLeft = source.Count - targetIndex;
while (slicesCount * list.Count < itemsLeft)
{
list.Add(source[sourceIndex++]);
}
yield return list;
}
}
/// <summary>
/// Takes collection of collections, projects those in parallel and merges results.
/// </summary>
public static async Task<IEnumerable<TResult>> SelectManyAsync<T, TResult>(
this IEnumerable<IEnumerable<T>> source,
Func<T, Task<TResult>> func)
{
List<TResult>[] slices = await source
.Select(async slice => await slice.SelectListAsync(func).ConfigureAwait(false))
.WhenAll()
.ConfigureAwait(false);
return slices.SelectMany(s => s);
}
/// <summary>Runs selector and awaits results.</summary>
public static async Task<List<TResult>> SelectListAsync<TSource, TResult>(this IEnumerable<TSource> source, Func<TSource, Task<TResult>> selector)
{
List<TResult> result = new List<TResult>();
foreach (TSource source1 in source)
{
TResult result1 = await selector(source1).ConfigureAwait(false);
result.Add(result1);
}
return result;
}
/// <summary>Wraps tasks with Task.WhenAll.</summary>
public static Task<TResult[]> WhenAll<TResult>(this IEnumerable<Task<TResult>> source)
{
return Task.WhenAll<TResult>(source);
}
}
.NET 6 中引入 Parallel.ForEachAsync
API 解决了并行化异步操作的问题,但使用较旧 .NET 平台的人可能仍然需要一个不错的替代品。实现一个的简单方法是使用 TPL 数据流库中的ActionBlock<T>
组件。此库包含在标准 .NET 库(.NET Core 和 .NET 5+(中,并作为 .NET Framework 的 NuGet 包提供。以下是它的使用方法:
public static Task Parallel_ForEachAsync<T>(ICollection<T> source,
int maxDegreeOfParallelism, Func<T, Task> action)
{
var options = new ExecutionDataflowBlockOptions();
options.MaxDegreeOfParallelism = maxDegreeOfParallelism;
var block = new ActionBlock<T>(action, options);
foreach (var item in source) block.Post(item);
block.Complete();
return block.Completion;
}
此解决方案仅适用于具体化source
序列,因此参数的类型是ICollection<T>
而不是更常见的IEnumerable<T>
。它还具有忽略action
抛出的任何OperationCanceledException
的惊人行为。解决这些细微差别并尝试精确复制Parallel.ForEachAsync
的功能和行为是可行的,但它需要的代码几乎与使用更原始的工具一样多。我已经在这个答案的第 9 次修订版中发布了这样的尝试。
下面是实现 Parallel.ForEachAsync
方法的不同尝试,提供与 .NET 6 API 完全相同的功能,并尽可能模仿其行为。它仅使用基本的 TPL 工具。这个想法是创建许多等于理想MaxDegreeOfParallelism
的工作任务,每个任务以同步的方式枚举相同的枚举器。这类似于内部实现Parallel.ForEachAsync
的方式。不同之处在于,.NET 6 API 从单个辅助角色开始,然后逐步添加更多工作线程,而下面的实现从一开始就创建所有辅助角色:
public static Task Parallel_ForEachAsync<T>(IEnumerable<T> source,
ParallelOptions parallelOptions,
Func<T, CancellationToken, Task> body)
{
if (source == null) throw new ArgumentNullException("source");
if (parallelOptions == null) throw new ArgumentNullException("parallelOptions");
if (body == null) throw new ArgumentNullException("body");
int dop = parallelOptions.MaxDegreeOfParallelism;
if (dop < 0) dop = Environment.ProcessorCount;
CancellationToken cancellationToken = parallelOptions.CancellationToken;
TaskScheduler scheduler = parallelOptions.TaskScheduler ?? TaskScheduler.Current;
IEnumerator<T> enumerator = source.GetEnumerator();
CancellationTokenSource cts = CancellationTokenSource
.CreateLinkedTokenSource(cancellationToken);
var semaphore = new SemaphoreSlim(1, 1); // Synchronizes the enumeration
var workerTasks = new Task[dop];
for (int i = 0; i < dop; i++)
{
workerTasks[i] = Task.Factory.StartNew(async () =>
{
try
{
while (true)
{
if (cts.IsCancellationRequested)
{
cancellationToken.ThrowIfCancellationRequested();
break;
}
T item;
await semaphore.WaitAsync(); // Continue on captured context.
try
{
if (!enumerator.MoveNext()) break;
item = enumerator.Current;
}
finally { semaphore.Release(); }
await body(item, cts.Token); // Continue on captured context.
}
}
catch { cts.Cancel(); throw; }
}, CancellationToken.None, TaskCreationOptions.DenyChildAttach, scheduler)
.Unwrap();
}
return Task.WhenAll(workerTasks).ContinueWith(t =>
{
// Clean up
try { semaphore.Dispose(); cts.Dispose(); } finally { enumerator.Dispose(); }
return t;
}, CancellationToken.None, TaskContinuationOptions.DenyChildAttach |
TaskContinuationOptions.ExecuteSynchronously, TaskScheduler.Default).Unwrap();
}
签名有所不同。body
参数的类型为 Func<TSource, CancellationToken, Task>
而不是 Func<TSource, CancellationToken, ValueTask>
。这是因为值任务是一个相对较新的功能,在 .NET Framework 中不可用。
行为也有所不同。此实现通过完成取消来对body
抛出的OperationCanceledException
做出反应。正确的行为是将这些异常作为单个错误传播,并作为错误完成。修复这个小缺陷是可行的,但我宁愿不要使这个相对较短且可读的实现进一步复杂化。
没有TPL的简单原生方式:
int totalThreads = 0; int maxThreads = 3;
foreach (var item in YouList)
{
while (totalThreads >= maxThreads) await Task.Delay(500);
Interlocked.Increment(ref totalThreads);
MyAsyncTask(item).ContinueWith((res) => Interlocked.Decrement(ref totalThreads));
}
您可以使用下一个任务检查此解决方案:
async static Task MyAsyncTask(string item)
{
await Task.Delay(2500);
Console.WriteLine(item);
}