C#多线程编程中的锁系统（四）：自旋锁

2020-01-24 02:00:42

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目录
一：基础

二：自旋锁示例

三：SpinLock

四：继续SpinLock

五：总结

一：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。

ps：操作系统或net框架，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，让开发者根据需求自行判断。

二:自旋锁示例

来看下我们自己简单实现的自旋锁：

复制代码代码如下:

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。

三：SpinLock

SpinLock是net4.0后系统帮我们实现的自旋锁，内部做了优化。

简单看下实例：

复制代码代码如下:

 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
 

四：继续SpinLock

new SpinLock(false) 这个构造函数主要用来帮我们检查死锁用，true是开启。

开启状态下，如果发生死锁会直接抛异常的。

贴了一部分源码(已折叠)，我们来看下：

复制代码代码如下:

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();
            }

        }

从代码中发现SpinLock并不是我们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，后续再讲。

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。这种就有点混合构造的意味了，如下：

复制代码代码如下:

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后或没有准备好的线程，继续执行。跟优先级无关。

Thread.Yeild()还会返回个bool值，是否让出成功。

从源码中，我们可以学到不少编程技巧。比如我们也可以使用自旋+Thread.Yeild() 或 while+Thread.Yeild() 等组合。

五：总结

本章谈了自旋锁的基础+楼主的经验。 SpinLock类源码这块，只粗浅理解了下，并没有深究。

测了下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。

还测了下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，所以看不到CLR具体实现的代码。

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