C#多線程編程中的鎖體系（四）：自旋鎖。本站提示廣大學習愛好者：（C#多線程編程中的鎖體系（四）：自旋鎖）文章只能為提供參考，不一定能成為您想要的結果。以下是C#多線程編程中的鎖體系（四）：自旋鎖正文
目次
一：基本
二：自旋鎖示例
三：SpinLock
四：持續SpinLock
五：總結
一：基本
內核鎖：基於內查對象結構的鎖機制，就是平日說的內核結構形式。用戶形式結構和內核形式結構
長處：cpu應用最年夜化。它發明資本被鎖住，要求就列隊等待。線程切換到別處干活，直到接收到可用旌旗燈號，線程再切回來持續處置要求。
缺陷：托管代碼->用戶形式代碼->內核代碼消耗、線程高低文切換消耗。
在鎖的時光比擬短時，體系頻仍忙於休眠、切換，是個很年夜的機能消耗。
自旋鎖：原子操作+自輪回。平日說的用戶結構形式。 線程不休眠，一向輪回測驗考試對資本拜訪，直到可用。
長處：完善處理內核鎖的缺陷。
缺陷：長時光一向輪回會招致cpu的白白糟蹋，高並發競爭下、CPU的消費特殊嚴重。
混雜鎖：內核鎖+自旋鎖。 混雜鎖是先自旋鎖一段時光或自旋若干次，再轉成內核鎖。
長處：內核鎖和自旋鎖的折衷計劃，應用前兩者長處，防止湧現極端情形（自旋時光太長，內核鎖時光太短）。
缺陷： 自旋若干時光、自旋若干次，這些戰略很難把控。
ps：操作體系或net框架，這塊算法戰略做的曾經異常優了，有些API函數也供給了時光及次數可設置裝備擺設項，閃開發者依據需求自行斷定。
二:自旋鎖示例
來看下我們本身簡略完成的自旋鎖：
int signal = 0;
var li = new List<int>();
Parallel.For(0, 1000 * 10000, r =>
{
while (Interlocked.Exchange(ref signal, 1) != 0)//加自旋鎖
{
//黑魔法
}
li.Add(r);
Interlocked.Exchange(ref signal, 0); //釋放鎖
});
Console.WriteLine(li.Count);
//輸入：10000000
下面就是自旋鎖：Interlocked.Exchange+while
1：界說signal 0可用，1弗成用。
2：Parallel模仿並發競爭，原子更改signal狀況。 後續線程自旋拜訪signal，能否可用。
3：A線程應用完後，更改signal為0。 殘剩線程競爭拜訪資本，B線程成功後，更改signal為1，掉敗線程持續自旋，直到可用。
三：SpinLock
SpinLock是net4.0後體系幫我們完成的自旋鎖，外部做了優化。
簡略看下實例：
var li = new List<int>();
var sl = new SpinLock();
Parallel.For(0, 1000 * 10000, r =>
{
bool gotLock = false; //釋放勝利
sl.Enter(ref gotLock); //進入鎖
li.Add(r);
if (gotLock) sl.Exit(); //釋放
});
Console.WriteLine(li.Count);
//輸入：10000000
四：持續SpinLock
new SpinLock(false) 這個結構函數重要用來幫我們檢討逝世鎖用，true是開啟。
開啟狀況下，假如產生逝世鎖會直接拋異常的。
貼了一部門源碼(已折疊)，我們來看下：
public void Enter(ref bool lockTaken)
{
if (lockTaken)
{
lockTaken = false;
throw new System.ArgumentException(Environment.GetResourceString("SpinLock_TryReliableEnter_ArgumentException"));
}
// Fast path to acquire the lock if the lock is released
// If the thread tracking enabled set the new owner to the current thread id
// Id not, set the anonymous bit lock
int observedOwner = m_owner;
int newOwner = 0;
bool threadTrackingEnabled = (m_owner & LOCK_ID_DISABLE_MASK) == 0;
if (threadTrackingEnabled)
{
if (observedOwner == LOCK_UNOWNED)
newOwner = Thread.CurrentThread.ManagedThreadId;
}
else if ((observedOwner & LOCK_ANONYMOUS_OWNED) == LOCK_UNOWNED)
{
newOwner = observedOwner | LOCK_ANONYMOUS_OWNED; // set the lock bit
}
if (newOwner != 0)
{
#if !FEATURE_CORECLR
Thread.BeginCriticalRegion();
#endif
#if PFX_LEGACY_3_5
if (Interlocked.CompareExchange(ref m_owner, newOwner, observedOwner) == observedOwner)
{
lockTaken = true;
return;
}
#else
if (Interlocked.CompareExchange(ref m_owner, newOwner, observedOwner, ref lockTaken) == observedOwner)
{
// Fast path succeeded
return;
}
#endif
#if !FEATURE_CORECLR
Thread.EndCriticalRegion();
#endif
}
//Fast path failed, try slow path
ContinueTryEnter(Timeout.Infinite, ref lockTaken);
}
private void ContinueTryEnter(int millisecondsTimeout, ref bool lockTaken)
{
long startTicks = 0;
if (millisecondsTimeout != Timeout.Infinite && millisecondsTimeout != 0)
{
startTicks = DateTime.UtcNow.Ticks;
}
#if !FEATURE_PAL && !FEATURE_CORECLR // PAL doesn't support eventing, and we don't compile CDS providers for Coreclr
if (CdsSyncEtwBCLProvider.Log.IsEnabled())
{
CdsSyncEtwBCLProvider.Log.SpinLock_FastPathFailed(m_owner);
}
#endif
if (IsThreadOwnerTrackingEnabled)
{
// Slow path for enabled thread tracking mode
ContinueTryEnterWithThreadTracking(millisecondsTimeout, startTicks, ref lockTaken);
return;
}
// then thread tracking is disabled
// In this case there are three ways to acquire the lock
// 1- the first way the thread either tries to get the lock if it's free or updates the waiters, if the turn >= the processors count then go to 3 else go to 2
// 2- In this step the waiter threads spins and tries to acquire the lock, the number of spin iterations and spin count is dependent on the thread turn
// the late the thread arrives the more it spins and less frequent it check the lock avilability
// Also the spins count is increaes each iteration
// If the spins iterations finished and failed to acquire the lock, go to step 3
// 3- This is the yielding step, there are two ways of yielding Thread.Yield and Sleep(1)
// If the timeout is expired in after step 1, we need to decrement the waiters count before returning
int observedOwner;
//***Step 1, take the lock or update the waiters
// try to acquire the lock directly if possoble or update the waiters count
SpinWait spinner = new SpinWait();
while (true)
{
observedOwner = m_owner;
if ((observedOwner & LOCK_ANONYMOUS_OWNED) == LOCK_UNOWNED)
{
#if !FEATURE_CORECLR
Thread.BeginCriticalRegion();
#endif
#if PFX_LEGACY_3_5
if (Interlocked.CompareExchange(ref m_owner, observedOwner | 1, observedOwner) == observedOwner)
{
lockTaken = true;
return;
}
#else
if (Interlocked.CompareExchange(ref m_owner, observedOwner | 1, observedOwner, ref lockTaken) == observedOwner)
{
return;
}
#endif
#if !FEATURE_CORECLR
Thread.EndCriticalRegion();
#endif
}
else //failed to acquire the lock,then try to update the waiters. If the waiters count reached the maximum, jsut break the loop to avoid overflow
if ((observedOwner & WAITERS_MASK) == MAXIMUM_WAITERS || Interlocked.CompareExchange(ref m_owner, observedOwner + 2, observedOwner) == observedOwner)
break;
spinner.SpinOnce();
}
// Check the timeout.
if (millisecondsTimeout == 0 ||
(millisecondsTimeout != Timeout.Infinite &&
TimeoutExpired(startTicks, millisecondsTimeout)))
{
DecrementWaiters();
return;
}
//***Step 2. Spinning
//lock acquired failed and waiters updated
int turn = ((observedOwner + 2) & WAITERS_MASK) / 2;
int processorCount = PlatformHelper.ProcessorCount;
if (turn < processorCount)
{
int processFactor = 1;
for (int i = 1; i <= turn * SPINNING_FACTOR; i++)
{
Thread.SpinWait((turn + i) * SPINNING_FACTOR * processFactor);
if (processFactor < processorCount)
processFactor++;
observedOwner = m_owner;
if ((observedOwner & LOCK_ANONYMOUS_OWNED) == LOCK_UNOWNED)
{
#if !FEATURE_CORECLR
Thread.BeginCriticalRegion();
#endif
int newOwner = (observedOwner & WAITERS_MASK) == 0 ? // Gets the number of waiters, if zero
observedOwner | 1 // don't decrement it. just set the lock bit, it is zzero because a previous call of Exit(false) ehich corrupted the waiters
: (observedOwner - 2) | 1; // otherwise decrement the waiters and set the lock bit
Contract.Assert((newOwner & WAITERS_MASK) >= 0);
#if PFX_LEGACY_3_5
if (Interlocked.CompareExchange(ref m_owner, newOwner, observedOwner) == observedOwner)
{
lockTaken = true;
return;
}
#else
if (Interlocked.CompareExchange(ref m_owner, newOwner, observedOwner, ref lockTaken) == observedOwner)
{
return;
}
#endif
#if !FEATURE_CORECLR
Thread.EndCriticalRegion();
#endif
}
}
}
// Check the timeout.
if (millisecondsTimeout != Timeout.Infinite && TimeoutExpired(startTicks, millisecondsTimeout))
{
DecrementWaiters();
return;
}
//*** Step 3, Yielding
//Sleep(1) every 50 yields
int yieldsoFar = 0;
while (true)
{
observedOwner = m_owner;
if ((observedOwner & LOCK_ANONYMOUS_OWNED) == LOCK_UNOWNED)
{
#if !FEATURE_CORECLR
Thread.BeginCriticalRegion();
#endif
int newOwner = (observedOwner & WAITERS_MASK) == 0 ? // Gets the number of waiters, if zero
observedOwner | 1 // don't decrement it. just set the lock bit, it is zzero because a previous call of Exit(false) ehich corrupted the waiters
: (observedOwner - 2) | 1; // otherwise decrement the waiters and set the lock bit
Contract.Assert((newOwner & WAITERS_MASK) >= 0);
#if PFX_LEGACY_3_5
if (Interlocked.CompareExchange(ref m_owner, newOwner, observedOwner) == observedOwner)
{
lockTaken = true;
return;
}
#else
if (Interlocked.CompareExchange(ref m_owner, newOwner, observedOwner, ref lockTaken) == observedOwner)
{
return;
}
#endif
#if !FEATURE_CORECLR
Thread.EndCriticalRegion();
#endif
}
if (yieldsoFar % SLEEP_ONE_FREQUENCY == 0)
{
Thread.Sleep(1);
}
else if (yieldsoFar % SLEEP_ZERO_FREQUENCY == 0)
{
Thread.Sleep(0);
}
else
{
#if PFX_LEGACY_3_5
Platform.Yield();
#else
Thread.Yield();
#endif
}
if (yieldsoFar % TIMEOUT_CHECK_FREQUENCY == 0)
{
//Check the timeout.
if (millisecondsTimeout != Timeout.Infinite && TimeoutExpired(startTicks, millisecondsTimeout))
{
DecrementWaiters();
return;
}
}
yieldsoFar++;
}
}
/// <summary>
/// decrements the waiters, in case of the timeout is expired
/// </summary>
private void DecrementWaiters()
{
SpinWait spinner = new SpinWait();
while (true)
{
int observedOwner = m_owner;
if ((observedOwner & WAITERS_MASK) == 0) return; // don't decrement the waiters if it's corrupted by previous call of Exit(false)
if (Interlocked.CompareExchange(ref m_owner, observedOwner - 2, observedOwner) == observedOwner)
{
Contract.Assert(!IsThreadOwnerTrackingEnabled); // Make sure the waiters never be negative which will cause the thread tracking bit to be flipped
break;
}
spinner.SpinOnce();
}
}
從代碼中發明SpinLock其實不是我們簡略的完成那樣一向自旋，其外部做了許多優化。
1：外部應用了Interlocked.CompareExchange堅持原子操作， m_owner 0可用，1弗成用。
2：第一次取得鎖掉敗後，持續挪用ContinueTryEnter，ContinueTryEnter有三種取得鎖的情形。
3：ContinueTryEnter函數第一種取得鎖的方法。 應用了while+SpinWait，後續再講。
4：第一種方法到達最年夜期待者數目後，射中走第二種。 持續自旋 turn * 100次。100這個值是處置器核數(4, 8 ,16)下最好的。
5：第二種假如還不克不及取得鎖，走第三種。 這類就有點混雜結構的意味了，以下：
if (yieldsoFar % 40 == 0)
Thread.Sleep(1);
else if (yieldsoFar % 10 == 0)
Thread.Sleep(0);
else
Thread.Yield();
Thread.Sleep(1) ： 終止以後線程，廢棄剩下時光片 休眠1毫秒。 加入跟其他線程搶占cpu。固然這個普通會更多，體系沒法包管這麼細的時光粒度。
Thread.Sleep(0)： 終止以後線程，廢棄剩下時光片。 但立馬還會跟其他線程搶cpu，能不克不及搶到跟線程優先級有關。
Thread.Yeild()： 停止以後線程。讓出cpu給其他預備好的線程。其他線程ok後或沒有預備好的線程，持續履行。 跟優先級有關。
Thread.Yeild()還會前往個bool值，能否讓出勝利。
從源碼中，我們可以學到很多編程技能。 好比我們也能夠應用 自旋+Thread.Yeild() 或 while+Thread.Yeild() 等組合。
五：總結
本章談了自旋鎖的基本+樓主的經歷。 SpinLock類源碼這塊，只深刻懂得了下，並沒有深究。
測了下SpinLock和本身完成的自旋鎖機能比較（並行添加1000w List<int>()），SpinLock是純真的自旋鎖機能2倍以上。
還測了下lock的機能，是體系SpinLock機能的3倍以上。 可見lock外部自旋的效力更高，CLR暫沒開源，所以看不到CLR詳細完成的代碼。
