多線程中的鎖系統(四)-談談自旋鎖，多線程自旋
目錄
一：基礎
二：自旋鎖示例
三：SpinLock
四：繼續SpinLock
五：總結
一：基礎
內核鎖：基於內核對象構造的鎖機制，就是通常說的內核構造模式。用戶模式構造和內核模式構造
優點：cpu利用最大化。它發現資源被鎖住，請求就排隊等候。線程切換到別處干活，直到接受到可用信號，線程再切回來繼續處理請求。
缺點：托管代碼->用戶模式代碼->內核代碼損耗、線程上下文切換損耗。
在鎖的時間比較短時，系統頻繁忙於休眠、切換，是個很大的性能損耗。
自旋鎖：原子操作+自循環。通常說的用戶構造模式。 線程不休眠，一直循環嘗試對資源訪問，直到可用。
優點：完美解決內核鎖的缺點。
缺點：長時間一直循環會導致cpu的白白浪費，高並發競爭下、CPU的消耗特別嚴重。
混合鎖：內核鎖+自旋鎖。 混合鎖是先自旋鎖一段時間或自旋多少次，再轉成內核鎖。
優點：內核鎖和自旋鎖的折中方案，利用前二者優點，避免出現極端情況（自旋時間過長，內核鎖時間過短）。
缺點： 自旋多少時間、自旋多少次，這些策略很難把控。
ps：操作系統或net框架，這塊算法策略做的已經非常優了，有些API函數也提供了時間及次數可配置項，讓開發者根據需求自行判斷。
二:自旋鎖示例
來看下我們自己簡單實現的自旋鎖：
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);
//輸出：10000000
上面就是自旋鎖：Interlocked.Exchange+while
1：定義signal 0可用，1不可用。
2：Parallel模擬並發競爭，原子更改signal狀態。 後續線程自旋訪問signal，是否可用。
3：A線程使用完後，更改signal為0。 剩余線程競爭訪問資源，B線程勝利後，更改signal為1，失敗線程繼續自旋，直到可用。
三：SpinLock
SpinLock是net4.0後系統幫我們實現的自旋鎖，內部做了優化。
簡單看下實例：
var li = new List<int>();
var sl = new SpinLock();
Parallel.For(0, 1000 * 10000, r =>
{
bool gotLock = false; //釋放成功
sl.Enter(ref gotLock); //進入鎖
li.Add(r);
if (gotLock) sl.Exit(); //釋放
});
Console.WriteLine(li.Count);
//輸出：10000000
四：繼續SpinLock
new SpinLock(false) 這個構造函數主要用來幫我們檢查死鎖用，true是開啟。
開啟狀態下，如果發生死鎖會直接拋異常的。
貼了一部分源碼(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();
}
}
View Code
從代碼中發現SpinLock並不是我們簡單的實現那樣一直自旋，其內部做了很多優化。
1：內部使用了Interlocked.CompareExchange保持原子操作， m_owner 0可用，1不可用。
2：第一次獲得鎖失敗後，繼續調用ContinueTryEnter，ContinueTryEnter有三種獲得鎖的情況。
3：ContinueTryEnter函數第一種獲得鎖的方式。 使用了while+SpinWait，後續再講。
4：第一種方式達到最大等待者數量後，命中走第二種。 繼續自旋 turn * 100次。100這個值是處理器核數(4, 8 ,16)下最好的。
5：第二種如果還不能獲得鎖，走第三種。 這種就有點混合構造的意味了，如下：
if (yieldsoFar % 40 == 0)
Thread.Sleep(1);
else if (yieldsoFar % 10 == 0)
Thread.Sleep(0);
else
Thread.Yield();
Thread.Sleep(1) ： 終止當前線程，放棄剩下時間片 休眠1毫秒。 退出跟其他線程搶占cpu。當然這個一般會更多，系統無法保證這麼細的時間粒度。
Thread.Sleep(0)： 終止當前線程，放棄剩下時間片。 但立馬還會跟其他線程搶cpu，能不能搶到跟線程優先級有關。
Thread.Yeild()： 結束當前線程。讓出cpu給其他准備好的線程。其他線程ok後或沒有准備好的線程，繼續執行。 跟優先級無關。
Thread.Yeild()還會返回個bool值，是否讓出成功。
從源碼中，我們可以學到不少編程技巧。 比如我們也可以使用 自旋+Thread.Yeild() 或 while+Thread.Yeild() 等組合。
五：總結
本章談了自旋鎖的基礎+樓主的經驗。 SpinLock類源碼這塊，只粗淺理解了下，並沒有深究。
測了下SpinLock和自己實現的自旋鎖性能對比（並行添加1000w List<int>()），SpinLock是單純的自旋鎖性能2倍以上。
還測了下lock的性能，是系統SpinLock性能的3倍以上。 可見lock內部自旋的效率更高，可惜看不到monitor.enter CLR實現的代碼。
參考資源
http://www.projky.com/dotnet/4.0/System/Threading/SpinLock.cs.html
作者：蘑菇先生 出處：http://www.cnblogs.com/mushroom/p/4245529.html
