讀寫鎖的概念很簡單,允許多個(gè)線程同時(shí)獲取讀鎖,但同一時(shí)間只允許一個(gè)線程獲得寫鎖,因此也稱作共享-獨(dú)占鎖。在C#中,推薦使用ReaderWriterLockSlim類來完成讀寫鎖的功能。
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某些場合下,對(duì)一個(gè)對(duì)象的讀取次數(shù)遠(yuǎn)遠(yuǎn)大于修改次數(shù),如果只是簡單的用lock方式加鎖,則會(huì)影響讀取的效率。而如果采用讀寫鎖,則多個(gè)線程可以同時(shí)讀取該對(duì)象,只有等到對(duì)象被寫入鎖占用的時(shí)候,才會(huì)阻塞。
簡單的說,當(dāng)某個(gè)線程進(jìn)入讀取模式時(shí),此時(shí)其他線程依然能進(jìn)入讀取模式,假設(shè)此時(shí)一個(gè)線程要進(jìn)入寫入模式,那么他不得不被阻塞。直到讀取模式退出為止。
同樣的,如果某個(gè)線程進(jìn)入了寫入模式,那么其他線程無論是要寫入還是讀取,都是會(huì)被阻塞的。
進(jìn)入寫入/讀取模式有2種方法:
EnterReadLock嘗試進(jìn)入寫入模式鎖定狀態(tài)。
TryEnterReadLock(Int32) 嘗試進(jìn)入讀取模式鎖定狀態(tài),可以選擇整數(shù)超時(shí)時(shí)間。
EnterWriteLock 嘗試進(jìn)入寫入模式鎖定狀態(tài)。
TryEnterWriteLock(Int32) 嘗試進(jìn)入寫入模式鎖定狀態(tài),可以選擇超時(shí)時(shí)間。
退出寫入/讀取模式有2種方法:
ExitReadLock 減少讀取模式的遞歸計(jì)數(shù),并在生成的計(jì)數(shù)為 0(零)時(shí)退出讀取模式。
ExitWriteLock 減少寫入模式的遞歸計(jì)數(shù),并在生成的計(jì)數(shù)為 0(零)時(shí)退出寫入模式。
下面演示一下用法:
public class Program { static private ReaderWriterLockSlim rwl = new ReaderWriterLockSlim(); static void Main(string[] args) { Thread t_read1 = new Thread(new ThreadStart(ReadSomething)); t_read1.Start(); Console.WriteLine("{0} Create Thread ID {1} , Start ReadSomething", DateTime.Now.ToString("hh:mm:ss fff"), t_read1.GetHashCode()); Thread t_read2 = new Thread(new ThreadStart(ReadSomething)); t_read2.Start(); Console.WriteLine("{0} Create Thread ID {1} , Start ReadSomething", DateTime.Now.ToString("hh:mm:ss fff"), t_read2.GetHashCode()); Thread t_write1 = new Thread(new ThreadStart(WriteSomething)); t_write1.Start(); Console.WriteLine("{0} Create Thread ID {1} , Start WriteSomething", DateTime.Now.ToString("hh:mm:ss fff"), t_write1.GetHashCode()); } static public void ReadSomething() { Console.WriteLine("{0} Thread ID {1} Begin EnterReadLock...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode()); rwl.EnterReadLock(); try { Console.WriteLine("{0} Thread ID {1} reading sth...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode()); Thread.Sleep(5000);//模擬讀取信息 Console.WriteLine("{0} Thread ID {1} reading end.", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode()); } finally { rwl.ExitReadLock(); Console.WriteLine("{0} Thread ID {1} ExitReadLock...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode()); } } static public void WriteSomething() { Console.WriteLine("{0} Thread ID {1} Begin EnterWriteLock...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode()); rwl.EnterWriteLock(); try { Console.WriteLine("{0} Thread ID {1} writing sth...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode()); Thread.Sleep(10000);//模擬寫入信息 Console.WriteLine("{0} Thread ID {1} writing end.", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode()); } finally { rwl.ExitWriteLock(); Console.WriteLine("{0} Thread ID {1} ExitWriteLock...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode()); } } }
可以看到3號(hào)線程和4號(hào)線程能夠同時(shí)進(jìn)入讀模式,而5號(hào)線程過了5秒鐘后(即3,4號(hào)線程退出讀鎖后),才能進(jìn)入寫模式。
把上述代碼修改一下,先開啟2個(gè)寫模式的線程,然后在開啟讀模式線程,代碼如下:
static void Main(string[] args) { Thread t_write1 = new Thread(new ThreadStart(WriteSomething)); t_write1.Start(); Console.WriteLine("{0} Create Thread ID {1} , Start WriteSomething", DateTime.Now.ToString("hh:mm:ss fff"), t_write1.GetHashCode()); Thread t_write2 = new Thread(new ThreadStart(WriteSomething)); t_write2.Start(); Console.WriteLine("{0} Create Thread ID {1} , Start WriteSomething", DateTime.Now.ToString("hh:mm:ss fff"), t_write2.GetHashCode()); Thread t_read1 = new Thread(new ThreadStart(ReadSomething)); t_read1.Start(); Console.WriteLine("{0} Create Thread ID {1} , Start ReadSomething", DateTime.Now.ToString("hh:mm:ss fff"), t_read1.GetHashCode()); Thread t_read2 = new Thread(new ThreadStart(ReadSomething)); t_read2.Start(); Console.WriteLine("{0} Create Thread ID {1} , Start ReadSomething", DateTime.Now.ToString("hh:mm:ss fff"), t_read2.GetHashCode()); }
結(jié)果如下:
可以看到,3號(hào)線程和4號(hào)線程都要進(jìn)入寫模式,但是3號(hào)線程先占用寫入鎖,因此4號(hào)線程不得不等了10s后才進(jìn)入。5號(hào)線程和6號(hào)線程需要占用讀取鎖,因此等4號(hào)線程退出寫入鎖后才能繼續(xù)下去。
TryEnterReadLock和TryEnterWriteLock可以設(shè)置一個(gè)超時(shí)時(shí)間,運(yùn)行到這句話的時(shí)候,線程會(huì)阻塞在此,如果此時(shí)能占用鎖,那么返回true,如果到超時(shí)時(shí)間還未占用鎖,那么返回false,放棄鎖的占用,直接繼續(xù)執(zhí)行下面的代碼。
EnterUpgradeableReadLock
ReaderWriterLockSlim類提供了可升級(jí)讀模式,這種方式和讀模式的區(qū)別在于它還有通過調(diào)用 EnterWriteLock 或 TryEnterWriteLock 方法升級(jí)為寫入模式。 因?yàn)槊看沃荒苡幸粋€(gè)線程處于可升級(jí)模式。進(jìn)入可升級(jí)模式的線程,不會(huì)影響讀取模式的線程,即當(dāng)一個(gè)線程進(jìn)入可升級(jí)模式,任意數(shù)量線程可以同時(shí)進(jìn)入讀取模式,不會(huì)阻塞。如果有多個(gè)線程已經(jīng)在等待獲取寫入鎖,那么運(yùn)行EnterUpgradeableReadLock將會(huì)阻塞,直到那些線程超時(shí)或者退出寫入鎖。
下面代碼演示了如何在可升級(jí)讀模式下,升級(jí)到寫入鎖。
static public void UpgradeableRead() { Console.WriteLine("{0} Thread ID {1} Begin EnterUpgradeableReadLock...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode()); rwl.EnterUpgradeableReadLock(); try { Console.WriteLine("{0} Thread ID {1} doing sth...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode()); Console.WriteLine("{0} Thread ID {1} Begin EnterWriteLock...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode()); rwl.EnterWriteLock(); try { Console.WriteLine("{0} Thread ID {1} writing sth...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode()); Thread.Sleep(10000);//模擬寫入信息 Console.WriteLine("{0} Thread ID {1} writing end.", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode()); } finally { rwl.ExitWriteLock(); Console.WriteLine("{0} Thread ID {1} ExitWriteLock...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode()); } Thread.Sleep(10000);//模擬讀取信息 Console.WriteLine("{0} Thread ID {1} doing end.", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode()); } finally { rwl.ExitUpgradeableReadLock(); Console.WriteLine("{0} Thread ID {1} ExitUpgradeableReadLock...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode()); } }
讀寫鎖對(duì)于性能的影響是明顯的。
下面測試代碼:
public class Program { static private ReaderWriterLockSlim rwl = new ReaderWriterLockSlim(); static void Main(string[] args) { Stopwatch sw = new Stopwatch(); sw.Start(); List<Task> lstTask = new List<Task>(); for (int i = 0; i < 500; i++) { if (i % 25 != 0) { var t = Task.Factory.StartNew(ReadSomething); lstTask.Add(t); } else { var t = Task.Factory.StartNew(WriteSomething); lstTask.Add(t); } } Task.WaitAll(lstTask.ToArray()); sw.Stop(); Console.WriteLine("使用ReaderWriterLockSlim方式,耗時(shí):" + sw.Elapsed); sw.Restart(); lstTask = new List<Task>(); for (int i = 0; i < 500; i++) { if (i % 25 != 0) { var t = Task.Factory.StartNew(ReadSomething_lock); lstTask.Add(t); } else { var t = Task.Factory.StartNew(WriteSomething_lock); lstTask.Add(t); } } Task.WaitAll(lstTask.ToArray()); sw.Stop(); Console.WriteLine("使用lock方式,耗時(shí):" + sw.Elapsed); } static private object _lock1 = new object(); static public void ReadSomething_lock() { lock (_lock1) { //Console.WriteLine("{0} Thread ID {1} reading sth...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode()); Thread.Sleep(10);//模擬讀取信息 //Console.WriteLine("{0} Thread ID {1} reading end.", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode()); } } static public void WriteSomething_lock() { lock (_lock1) { //Console.WriteLine("{0} Thread ID {1} writing sth...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode()); Thread.Sleep(100);//模擬寫入信息 //Console.WriteLine("{0} Thread ID {1} writing end.", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode()); } } static public void ReadSomething() { rwl.EnterReadLock(); try { //Console.WriteLine("{0} Thread ID {1} reading sth...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode()); Thread.Sleep(10);//模擬讀取信息 //Console.WriteLine("{0} Thread ID {1} reading end.", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode()); } finally { rwl.ExitReadLock(); } } static public void WriteSomething() { rwl.EnterWriteLock(); try { //Console.WriteLine("{0} Thread ID {1} writing sth...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode()); Thread.Sleep(100);//模擬寫入信息 //Console.WriteLine("{0} Thread ID {1} writing end.", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode()); } finally { rwl.ExitWriteLock(); } } }
上述代碼,就500個(gè)Task,每個(gè)Task占用一個(gè)線程池線程,其中20個(gè)寫入線程和480個(gè)讀取線程,模擬操作。其中讀取數(shù)據(jù)花10ms,寫入操作花100ms,分別測試了對(duì)于lock方式和ReaderWriterLockSlim方式??梢宰鲆粋€(gè)估算,對(duì)于ReaderWriterLockSlim,假設(shè)480個(gè)線程同時(shí)讀取,那么消耗10ms,20個(gè)寫入操作占用2000ms,因此所消耗時(shí)間2010ms,而對(duì)于普通的lock方式,由于都是獨(dú)占性的,因此480個(gè)讀取操作占時(shí)間4800ms+20個(gè)寫入操作2000ms=6800ms。運(yùn)行結(jié)果顯示了性能提升明顯。
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