多线程中的锁系统(四)-谈谈自旋锁
阅读目录：
基础
内核锁：基于内核对象构造的锁机制，就是通常说的内核构造模式。用户模式构造和内核模式构造
优点：cpu利用最大化。它发现资源被锁住，请求就排队等候。线程切换到别处干活，直到接受到可用信号，线程再切回来继续处理请求。
缺点：托管代码->用户模式代码->内核代码损耗、线程上下文切换损耗。
在锁的时间比较短时，系统频繁忙于休眠、切换，是个很大的性能损耗。
自旋锁：原子操作+自循环。通常说的用户构造模式。 线程不休眠，一直循环尝试对资源访问，直到可用。
优点：完美解决内核锁的缺点。
缺点：长时间一直循环会导致cpu的白白浪费，高并发竞争下、CPU的消耗特别严重。
混合锁：内核锁+自旋锁。 混合锁是先自旋锁一段时间或自旋多少次，再转成内核锁。
优点：内核锁和自旋锁的折中方案，利用前二者优点，避免出现极端情况（自旋时间过长，内核锁时间过短）。
缺点： 自旋多少时间、自旋多少次，这些策略很难把控。
在操作系统及net框架层，这块算法策略做的已经非常优了，有些API函数也提供了时间及次数可配置项，让使用者根据需求自行判断。
自旋锁示例
来看下我们自己简单实现的自旋锁：
int signal = 0; var li = new List<int>(); Parallel.For(0, 1000 * 10000, r => { while (Interlocked.Exchange(ref signal, 1) != 0)//加自旋锁 { //黑魔法 } li.Add(r); Interlocked.Exchange(ref signal, 0); //释放锁 }); Console.WriteLine(li.Count); //输出：10000000
上面就是自旋锁：Interlocked.Exchange+while
1：定义signal 0可用，1不可用。
2：Parallel模拟并发竞争，原子更改signal状态。 后续线程自旋访问signal，是否可用。
3：A线程使用完后，更改signal为0。 剩余线程竞争访问资源，B线程胜利后，更改signal为1，失败线程继续自旋，直到可用。
SpinLock
SpinLock是net4.0后Net提供的自旋锁类库，内部做了优化。
简单看下实例：
var li = new List<int>(); var sl = new SpinLock(); Parallel.For(0, 1000 * 10000, r => { bool gotLock = false; //释放成功 sl.Enter(ref gotLock); //进入锁 li.Add(r); if (gotLock) sl.Exit(); //释放 }); Console.WriteLine(li.Count); //输出：10000000
继续SpinLock
new SpinLock(false) 这个构造函数主要用来检查死锁用，true是开启。
在开启状态下，一旦发生死锁会直接抛异常的。
SpinLock实现的部分源码：
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(); } }
从代码中发现SpinLock并不是简单的实现那样一直自旋，其内部做了很多优化。
1：内部使用了Interlocked.CompareExchange保持原子操作， m_owner 0可用，1不可用。
2：第一次获得锁失败后，继续调用ContinueTryEnter，ContinueTryEnter有三种获得锁的情况。
3：ContinueTryEnter函数第一种获得锁的方式，使用了while+SpinWait。
4：第一种方式达到最大等待者数量后，命中走第二种。 继续自旋 turn * 100次。100这个值是处理器核数(4, 8 ,16)下最好的。
5：第二种如果还不能获得锁，走第三种。这种就带有混合构造的意思了，如下：
if (yieldsoFar % 40 == 0) Thread.Sleep(1); else if (yieldsoFar % 10 == 0) Thread.Sleep(0); else Thread.Yield();
Thread.Sleep(1) ： 终止当前线程，放弃剩下时间片 休眠1毫秒， 退出跟其他线程抢占cpu。当然这个一般会更多，系统无法保证这么细的时间粒度。
Thread.Sleep(0)： 终止当前线程，放弃剩下时间片。 但立马还会跟其他线程抢cpu，能不能抢到跟线程优先级有关。
Thread.Yeild()： 结束当前线程，让出CPU给其他准备好的线程。其他线程ok后或没有还没有准备好，继续执行当前，Thread.Yeild()会返回个bool值，表示CPU是否让出成功。
从源码中可以学到不少编程技巧，比如可以借鉴自旋+Thread.Yeild() 或 while+Thread.Yeild()等组合使用方式。
总结
本章介绍了自旋锁的基础及楼主的经验。 关于SpinLock类源码这块，只简单理解了下并没有深究。
测试了下SpinLock和自己实现的自旋锁性能对比（并行添加1000w List<int>()），SpinLock是单纯的自旋锁性能2倍以上。
另外测试了lock的性能，是系统SpinLock性能的3倍以上，可见lock内部自旋的效率更高，CLR暂没开源，看不到CLR具体实现的代码。
参考http://www.projky.com/dotnet/4.0/System/Threading/SpinLock.cs.html