最近同事對(duì) ?.net core memcached 緩存客戶(hù)端 EnyimMemcachedCore 進(jìn)行了高并發(fā)下的壓力測(cè)試，發(fā)現(xiàn)在 linux 上高并發(fā)下使用 async 異步方法讀取緩存數(shù)據(jù)會(huì)出現(xiàn)大量失敗的情況，比如在一次測(cè)試中，100萬(wàn)次讀取緩存，只有12次成功，999988次失敗，好恐怖。如果改為同步方法，沒(méi)有一次失敗，100%成功。奇怪的是，同樣的壓力測(cè)試程序在 Windows 上異步讀取卻沒(méi)問(wèn)題，100%成功。

排查后發(fā)現(xiàn)是2個(gè)地方使用的鎖引起的，一個(gè)是 ManualResetEventSlim?，一個(gè)是?Semaphore?，這2個(gè)鎖是在同步方法中使用的，但 aync 異步方法中調(diào)用了這2個(gè)同步方法，我們來(lái)分別看一下。

使用 ManualResetEventSlim 是在創(chuàng)建 Socket 連接時(shí)用于控制連接超時(shí)

var args = new SocketAsyncEventArgs();
using (var mres = new ManualResetEventSlim())
{
args.Completed += (s, e) => mres.Set();
if (socket.ConnectAsync(args))
{
if (!mres.Wait(timeout))
{
throw new TimeoutException("Could not connect to " + endpoint);
}
}
}

使用?Semaphore 是在從 EnyimMemcachedCore 自己實(shí)現(xiàn)的 Socket 連接池獲取 Socket 連接時(shí)

if (!this.semaphore.WaitOne(this.queueTimeout))
{
message = "Pool is full, timeouting. " + _endPoint;
if (_isDebugEnabled) _logger.LogDebug(message);
result.Fail(message, new TimeoutException());

// everyone is so busy
return result;
}

為了棄用這個(gè)2個(gè)鎖造成的異步并發(fā)問(wèn)題，采取了下面2個(gè)改進(jìn)措施：

1）對(duì)于?ManualResetEventSlim ，參考 corefx 中 SqlClient 的?SNITcpHandle?的實(shí)現(xiàn)，改用?CancellationTokenSource 控制連接超時(shí)

var cts = new CancellationTokenSource();
cts.CancelAfter(timeout);
void Cancel()
{
if (!socket.Connected)
{
socket.Dispose();
}
}
cts.Token.Register(Cancel);

socket.Connect(endpoint);
if (socket.Connected)
{
connected = true;
}
else
{
socket.Dispose();
}

2）對(duì)于?Semaphore ，根據(jù)同事提交的 PR ，將?Semaphore 換成?SemaphoreSlim ，用?SemaphoreSlim.WaitAsync 方法等待信號(hào)量鎖

if (!await this.semaphore.WaitAsync(this.queueTimeout))
{
message = "Pool is full, timeouting. " + _endPoint;
if (_isDebugEnabled) _logger.LogDebug(message);
result.Fail(message, new TimeoutException());

// everyone is so busy
return result;
}

改進(jìn)后，壓力測(cè)試結(jié)果立馬與同步方法一樣，100% 成功！

為什么會(huì)這樣？

我們到 github 的 coreclr 倉(cāng)庫(kù)（針對(duì) .net core 2.2）中看看?ManualResetEventSlim?與?Semaphore?的實(shí)現(xiàn)源碼，看能否找到一些線索。

(一）

先看看 ManualResetEventSlim.Wait 方法的實(shí)現(xiàn)代碼（523開(kāi)始）：

1）先?SpinWait 等待

var spinner = new SpinWait();
while (spinner.Count < spinCount)
{
spinner.SpinOnce(sleep1Threshold: -1);

if (IsSet)
{
return true;
}
}

SpinWait 等待時(shí)間比較短，不會(huì)造成長(zhǎng)時(shí)間阻塞線程。

在高并發(fā)下大量線程在爭(zhēng)搶鎖，所以大量線程在這個(gè)階段等不到鎖。

2）然后?Monitor.Wait 等待

try
{
// ** the actual wait **
if (!Monitor.Wait(m_lock, realMillisecondsTimeout))
return false; //return immediately if the timeout has expired.
}
finally
{
// Clean up: we're done waiting.
Waiters = Waiters - 1;
}

Monitor.Wait 對(duì)應(yīng)的實(shí)現(xiàn)代碼

[MethodImplAttribute(MethodImplOptions.InternalCall)]
private static extern bool ObjWait(bool exitContext, int millisecondsTimeout, object obj);

public static bool Wait(object obj, int millisecondsTimeout, bool exitContext)
{
if (obj == null)
throw (new ArgumentNullException(nameof(obj)));
return ObjWait(exitContext, millisecondsTimeout, obj);
}

最終調(diào)用的是一個(gè)本地庫(kù)的?ObjWait 方法。

查閱一下?Monitor.Wait 方法的幫助文檔：

Releases the lock on an object and blocks the current thread until it reacquires the lock. If the specified time-out interval elapses, the thread enters the ready queue.

Monitor.Wait 的確會(huì)阻塞當(dāng)前線程，這在異步高并發(fā)下會(huì)帶來(lái)問(wèn)題，詳見(jiàn)一碼阻塞，萬(wàn)碼等待：ASP.NET Core 同步方法調(diào)用異步方法“死鎖”的真相。

（二）

再看看?Semaphore 的實(shí)現(xiàn)代碼，它繼承自?WaitHandle?，?Semaphore.Wait 實(shí)際調(diào)用的是 WaitHandle.Wait ，后者調(diào)用的是?WaitOneNative?，這是一個(gè)本地庫(kù)的方法

[MethodImplAttribute(MethodImplOptions.InternalCall)]
private static extern int WaitOneNative(SafeHandle waitableSafeHandle, uint millisecondsTimeout, bool hasThreadAffinity, bool exitContext);

.net core 3.0 中有些變化，這里調(diào)用的是?WaitOneCore 方法

[MethodImpl(MethodImplOptions.InternalCall)]
private static extern int WaitOneCore(IntPtr waitHandle, int millisecondsTimeout);

查閱一下 WaitHandle.Wait?方法的幫助文檔：?

Blocks the current thread until the current WaitHandle receives a signal, using a 32-bit signed integer to specify the time interval in milliseconds.

WaitHandle.Wait 也會(huì)阻塞當(dāng)前線程。

2個(gè)地方在等待鎖時(shí)都會(huì)阻塞線程，難怪高并發(fā)下會(huì)出問(wèn)題。

(三）

接著閱讀?SemaphoreSlim 的源碼學(xué)習(xí)它是如何在?WaitAsync?中實(shí)現(xiàn)異步等待鎖的？

public Task<bool> WaitAsync(int millisecondsTimeout, CancellationToken cancellationToken)
{
//...

lock (m_lockObj!)
{
// If there are counts available, allow this waiter to succeed.
if (m_currentCount > 0)
{
--m_currentCount;
if (m_waitHandle != null && m_currentCount == 0) m_waitHandle.Reset();
return s_trueTask;
}
else if (millisecondsTimeout == 0)
{
// No counts, if timeout is zero fail fast
return s_falseTask;
}
// If there aren't, create and return a task to the caller.
// The task will be completed either when they've successfully acquired
// the semaphore or when the timeout expired or cancellation was requested.
else
{
Debug.Assert(m_currentCount == 0, "m_currentCount should never be negative");
var asyncWaiter = CreateAndAddAsyncWaiter();
return (millisecondsTimeout == Timeout.Infinite && !cancellationToken.CanBeCanceled) ?
asyncWaiter :
WaitUntilCountOrTimeoutAsync(asyncWaiter, millisecondsTimeout, cancellationToken);
}
}
}

重點(diǎn)看 else 部分的代碼，SemaphoreSlim.WaitAsync 造了一個(gè)專(zhuān)門(mén)用于等待鎖的 Task —— TaskNode ，CreateAndAddAsyncWaiter 就用于創(chuàng)建?TaskNode 的實(shí)例

private TaskNode CreateAndAddAsyncWaiter()
{
// Create the task
var task = new TaskNode();

// Add it to the linked list
if (m_asyncHead == null)
{
m_asyncHead = task;
m_asyncTail = task;
}
else
{
m_asyncTail.Next = task;
task.Prev = m_asyncTail;
m_asyncTail = task;
}

// Hand it back
return task;
}

從上面的代碼看到 TaskNode 用到了鏈表，神奇的等鎖專(zhuān)用 Task —— TaskNode 是如何實(shí)現(xiàn)的呢？

private sealed class TaskNode : Task<bool>
{
internal TaskNode? Prev, Next;
internal TaskNode() : base((object?)null, TaskCreationOptions.RunContinuationsAsynchronously) { }
}

好簡(jiǎn)單！

那?SemaphoreSlim.WaitAsync 如何用?TaskNode 實(shí)現(xiàn)指定了超時(shí)時(shí)間的鎖等待？

看?WaitUntilCountOrTimeoutAsync 方法的實(shí)現(xiàn)源碼：

private async Task<bool> WaitUntilCountOrTimeoutAsync(TaskNode asyncWaiter, int millisecondsTimeout, CancellationToken cancellationToken)
{
// Wait until either the task is completed, timeout occurs, or cancellation is requested.
// We need to ensure that the Task.Delay task is appropriately cleaned up if the await
// completes due to the asyncWaiter completing, so we use our own token that we can explicitly
// cancel, and we chain the caller's supplied token into it.
using (var cts = cancellationToken.CanBeCanceled ?
CancellationTokenSource.CreateLinkedTokenSource(cancellationToken, default(CancellationToken)) :
new CancellationTokenSource())
{
var waitCompleted = Task.WhenAny(asyncWaiter, Task.Delay(millisecondsTimeout, cts.Token));
if (asyncWaiter == await waitCompleted.ConfigureAwait(false))
{
cts.Cancel(); // ensure that the Task.Delay task is cleaned up
return true; // successfully acquired
}
}

// If we get here, the wait has timed out or been canceled.

// If the await completed synchronously, we still hold the lock. If it didn't,
// we no longer hold the lock. As such, acquire it.
lock (m_lockObj)
{
// Remove the task from the list. If we're successful in doing so,
// we know that no one else has tried to complete this waiter yet,
// so we can safely cancel or timeout.
if (RemoveAsyncWaiter(asyncWaiter))
{
cancellationToken.ThrowIfCancellationRequested(); // cancellation occurred
return false; // timeout occurred
}
}

// The waiter had already been removed, which means it's already completed or is about to
// complete, so let it, and don't return until it does.
return await asyncWaiter.ConfigureAwait(false);
}

用 Task.WhenAny 等待 TaskNode 與?Task.Delay ，等其中任一者先完成，簡(jiǎn)單到可怕。

又一次通過(guò) .net core 源碼欣賞了高手是怎么玩轉(zhuǎn) Task 的。

【2019-5-6更新】

今天將?Task.WhenAny +?Task.Delay 的招式用到了異步連接 Socket 的超時(shí)控制中

var connTask = _socket.ConnectAsync(_endpoint);
if (await Task.WhenAny(connTask, Task.Delay(_connectionTimeout)) == connTask)
{
await connTask;
}

原文地址：https://www.cnblogs.com/dudu/p/10812139.html

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