多线程中的锁系统(四)-谈谈自旋锁

阅读目录：

1. 基础
2. 自旋锁示例
3. SpinLock
4. 继续SpinLock
5. 总结

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++; 
            } 
        }
 
        /// 

        /// decrements the waiters, in case of the timeout is expired
        /// 
        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

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()），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