C# 并行编程 之 并发集合 (.Net Framework 4.0)
此文为个人学习《C#并行编程高级教程》的笔记，总结并调试了一些文章中的代码示例。 在以后开发过程中可以加以运用。
对于并行任务，与其相关紧密的就是对一些共享资源，数据结构的并行访问。经常要做的就是对一些队列进行加锁-解锁，然后执行类似插入，删除等等互斥操作。 .NetFramework 4.0 中提供了一些封装好的支持并行操作数据容器，可以减少并行编程的复杂程度。
基本信息
.NetFramework中并行集合的名字空间： System.Collections.Concurrent
并行容器：
- ConcurrentQueue
- ConcurrentStack
- ConcurrentBag ： 一个无序的数据结构集，当不需要考虑顺序时非常有用。
- BlockingCollection ： 与经典的阻塞队列数据结构类似
- ConcurrentDictionary
这些集合在某种程度上使用了无锁技术(CAS Compare-and-Swap和内存屏障 Memory Barrier)，与加互斥锁相比获得了性能的提升。但在串行程序中，最好不用这些集合，它们必然会影响性能。
关于CAS:
- http://www.tuicool.com/articles/zuui6z
- http://www.360doc.com/content/11/0914/16/7656248_148221200.shtml
关于内存屏障
- http://en.wikipedia.org/wiki/Memory_barrier
用法与示例
ConcurrentQueue
其完全无锁，但当CAS面临资源竞争失败时可能会陷入自旋并重试操作。
- Enqueue：在队尾插入元素
- TryDequeue：尝试删除队头元素，并通过out参数返回
- TryPeek：尝试将对头元素通过out参数返回，但不删除该元素。
程序示例：
using System;
using System.Text;
using System.Threading.Tasks;
using System.Collections.Concurrent;
namespace Sample4_1_concurrent_queue
{
class Program
{
internal static ConcurrentQueue _TestQueue;
class ThreadWork1 // producer
{
public ThreadWork1()
{ }
public void run()
{
System.Console.WriteLine("ThreadWork1 run { ");
for (int i = 0; i < 100; i++)
{
System.Console.WriteLine("ThreadWork1 producer: " + i);
_TestQueue.Enqueue(i);
}
System.Console.WriteLine("ThreadWork1 run } ");
}
}
class ThreadWork2 // consumer
{
public ThreadWork2()
{ }
public void run()
{
int i = 0;
bool IsDequeuue = false;
System.Console.WriteLine("ThreadWork2 run { ");
for (; ; )
{
IsDequeuue = _TestQueue.TryDequeue(out i);
if (IsDequeuue)
System.Console.WriteLine("ThreadWork2 consumer: " + i * i + " =====");
if (i == 99)
break;
}
System.Console.WriteLine("ThreadWork2 run } ");
}
}
static void StartT1()
{
ThreadWork1 work1 = new ThreadWork1();
work1.run();
}
static void StartT2()
{
ThreadWork2 work2 = new ThreadWork2();
work2.run();
}
static void Main(string[] args)
{
Task t1 = new Task(() => StartT1());
Task t2 = new Task(() => StartT2());
_TestQueue = new ConcurrentQueue();
Console.WriteLine("Sample 3-1 Main {");
Console.WriteLine("Main t1 t2 started {");
t1.Start();
t2.Start();
Console.WriteLine("Main t1 t2 started }");
Console.WriteLine("Main wait t1 t2 end {");
Task.WaitAll(t1, t2);
Console.WriteLine("Main wait t1 t2 end }");
Console.WriteLine("Sample 3-1 Main }");
Console.ReadKey();
}
}
}
ConcurrentStack
其完全无锁，但当CAS面临资源竞争失败时可能会陷入自旋并重试操作。
- Push：向栈顶插入元素
- TryPop：从栈顶弹出元素，并且通过out 参数返回
- TryPeek：返回栈顶元素，但不弹出。
程序示例：
using System;
using System.Text;
using System.Threading.Tasks;
using System.Collections.Concurrent;
namespace Sample4_2_concurrent_stack
{
class Program
{
internal static ConcurrentStack _TestStack;
class ThreadWork1 // producer
{
public ThreadWork1()
{ }
public void run()
{
System.Console.WriteLine("ThreadWork1 run { ");
for (int i = 0; i < 100; i++)
{
System.Console.WriteLine("ThreadWork1 producer: " + i);
_TestStack.Push(i);
}
System.Console.WriteLine("ThreadWork1 run } ");
}
}
class ThreadWork2 // consumer
{
public ThreadWork2()
{ }
public void run()
{
int i = 0;
bool IsDequeuue = false;
System.Console.WriteLine("ThreadWork2 run { ");
for (; ; )
{
IsDequeuue = _TestStack.TryPop(out i);
if (IsDequeuue)
System.Console.WriteLine("ThreadWork2 consumer: " + i * i + " =====" + i);
if (i == 99)
break;
}
System.Console.WriteLine("ThreadWork2 run } ");
}
}
static void StartT1()
{
ThreadWork1 work1 = new ThreadWork1();
work1.run();
}
static void StartT2()
{
ThreadWork2 work2 = new ThreadWork2();
work2.run();
}
static void Main(string[] args)
{
Task t1 = new Task(() => StartT1());
Task t2 = new Task(() => StartT2());
_TestStack = new ConcurrentStack();
Console.WriteLine("Sample 4-1 Main {");
Console.WriteLine("Main t1 t2 started {");
t1.Start();
t2.Start();
Console.WriteLine("Main t1 t2 started }");
Console.WriteLine("Main wait t1 t2 end {");
Task.WaitAll(t1, t2);
Console.WriteLine("Main wait t1 t2 end }");
Console.WriteLine("Sample 4-1 Main }");
Console.ReadKey();
}
}
}
测试中一个有趣的现象：
虽然生产者已经在栈中插入值已经到了25，但消费者第一个出栈的居然是4，而不是25。很像是出错了。但仔细想想入栈，出栈和打印语句是两个部分，而且并不是原子操作，出现这种现象应该也算正常。
Main t1 t2 started {
Main t1 t2 started }
Main wait t1 t2 end {
ThreadWork1 run {
ThreadWork1 producer: 0
ThreadWork2 run {
ThreadWork1 producer: 1
ThreadWork1 producer: 2
ThreadWork1 producer: 3
ThreadWork1 producer: 4
ThreadWork1 producer: 5
ThreadWork1 producer: 6
ThreadWork1 producer: 7
ThreadWork1 producer: 8
ThreadWork1 producer: 9
ThreadWork1 producer: 10
ThreadWork1 producer: 11
ThreadWork1 producer: 12
ThreadWork1 producer: 13
ThreadWork1 producer: 14
ThreadWork1 producer: 15
ThreadWork1 producer: 16
ThreadWork1 producer: 17
ThreadWork1 producer: 18
ThreadWork1 producer: 19
ThreadWork1 producer: 20
ThreadWork1 producer: 21
ThreadWork1 producer: 22
ThreadWork1 producer: 23
ThreadWork1 producer: 24
ThreadWork1 producer: 25
ThreadWork2 consumer: 16 =====4
ThreadWork2 consumer: 625 =====25
ThreadWork2 consumer: 576 =====24
ThreadWork2 consumer: 529 =====23
ThreadWork1 producer: 26
ThreadWork1 producer: 27
ThreadWork1 producer: 28
ConcurrentBag
一个无序的集合，程序可以向其中插入元素，或删除元素。
在同一个线程中向集合插入，删除元素的效率很高。
Add：向集合中插入元素 TryTake：从集合中取出元素并删除 TryPeek：从集合中取出元素，但不删除该元素。
程序示例：
using System;
using System.Text;
using System.Threading.Tasks;
using System.Collections.Concurrent;
namespace Sample4_3_concurrent_bag
{
class Program
{
internal static ConcurrentBag _TestBag;
class ThreadWork1 // producer
{
public ThreadWork1()
{ }
public void run()
{
System.Console.WriteLine("ThreadWork1 run { ");
for (int i = 0; i < 100; i++)
{
System.Console.WriteLine("ThreadWork1 producer: " + i);
_TestBag.Add(i);
}
System.Console.WriteLine("ThreadWork1 run } ");
}
}
class ThreadWork2 // consumer
{
public ThreadWork2()
{ }
public void run()
{
int i = 0;
int nCnt = 0;
bool IsDequeuue = false;
System.Console.WriteLine("ThreadWork2 run { ");
for (;;)
{
IsDequeuue = _TestBag.TryTake(out i);
if (IsDequeuue)
{
System.Console.WriteLine("ThreadWork2 consumer: " + i * i + " =====" + i);
nCnt++;
}
if (nCnt == 99)
break;
}
System.Console.WriteLine("ThreadWork2 run } ");
}
}
static void StartT1()
{
ThreadWork1 work1 = new ThreadWork1();
work1.run();
}
static void StartT2()
{
ThreadWork2 work2 = new ThreadWork2();
work2.run();
}
static void Main(string[] args)
{
Task t1 = new Task(() => StartT1());
Task t2 = new Task(() => StartT2());
_TestBag = new ConcurrentBag();
Console.WriteLine("Sample 4-3 Main {");
Console.WriteLine("Main t1 t2 started {");
t1.Start();
t2.Start();
Console.WriteLine("Main t1 t2 started }");
Console.WriteLine("Main wait t1 t2 end {");
Task.WaitAll(t1, t2);
Console.WriteLine("Main wait t1 t2 end }");
Console.WriteLine("Sample 4-3 Main }");
Console.ReadKey();
}
}
}
BlockingCollection
一个支持界限和阻塞的容器
- Add ：向容器中插入元素
- TryTake：从容器中取出元素并删除
- TryPeek：从容器中取出元素，但不删除。
- CompleteAdding：告诉容器，添加元素完成。此时如果还想继续添加会发生异常。
- IsCompleted：告诉消费线程，生产者线程还在继续运行中，任务还未完成。
示例程序：
程序中，消费者线程完全使用 while (!_TestBCollection.IsCompleted) 作为退出运行的判断条件。
在Worker1中，有两条语句被注释掉了，当i 为50时设置CompleteAdding，但当继续向其中插入元素时，系统抛出异常，提示无法再继续插入。
using System;
using System.Text;
using System.Threading.Tasks;
using System.Collections.Concurrent;
namespace Sample4_4_concurrent_bag
{
class Program
{
internal static BlockingCollection _TestBCollection;
class ThreadWork1 // producer
{
public ThreadWork1()
{ }
public void run()
{
System.Console.WriteLine("ThreadWork1 run { ");
for (int i = 0; i < 100; i++)
{
System.Console.WriteLine("ThreadWork1 producer: " + i);
_TestBCollection.Add(i);
//if (i == 50)
// _TestBCollection.CompleteAdding();
}
_TestBCollection.CompleteAdding();
System.Console.WriteLine("ThreadWork1 run } ");
}
}
class ThreadWork2 // consumer
{
public ThreadWork2()
{ }
public void run()
{
int i = 0;
int nCnt = 0;
bool IsDequeuue = false;
System.Console.WriteLine("ThreadWork2 run { ");
while (!_TestBCollection.IsCompleted)
{
IsDequeuue = _TestBCollection.TryTake(out i);
if (IsDequeuue)
{
System.Console.WriteLine("ThreadWork2 consumer: " + i * i + " =====" + i);
nCnt++;
}
}
System.Console.WriteLine("ThreadWork2 run } ");
}
}
static void StartT1()
{
ThreadWork1 work1 = new ThreadWork1();
work1.run();
}
static void StartT2()
{
ThreadWork2 work2 = new ThreadWork2();
work2.run();
}
static void Main(string[] args)
{
Task t1 = new Task(() => StartT1());
Task t2 = new Task(() => StartT2());
_TestBCollection = new BlockingCollection();
Console.WriteLine("Sample 4-4 Main {");
Console.WriteLine("Main t1 t2 started {");
t1.Start();
t2.Start();
Console.WriteLine("Main t1 t2 started }");
Console.WriteLine("Main wait t1 t2 end {");
Task.WaitAll(t1, t2);
Console.WriteLine("Main wait t1 t2 end }");
Console.WriteLine("Sample 4-4 Main }");
Console.ReadKey();
}
}
}
当然可以尝试在Work1中注释掉 CompleteAdding 语句，此时Work2陷入循环无法退出。
ConcurrentDictionary
对于读操作是完全无锁的，当很多线程要修改数据时，它会使用细粒度的锁。
- AddOrUpdate：如果键不存在，方法会在容器中添加新的键和值，如果存在，则更新现有的键和值。
- GetOrAdd：如果键不存在，方法会向容器中添加新的键和值，如果存在则返回现有的值，并不添加新值。
- TryAdd：尝试在容器中添加新的键和值。
- TryGetValue：尝试根据指定的键获得值。
- TryRemove：尝试删除指定的键。
- TryUpdate：有条件的更新当前键所对应的值。
- GetEnumerator：返回一个能够遍历整个容器的枚举器。
程序示例：
using System;
using System.Text;
using System.Threading.Tasks;
using System.Collections.Concurrent;
namespace Sample4_5_concurrent_dictionary
{
class Program
{
internal static ConcurrentDictionary _TestDictionary;
class ThreadWork1 // producer
{
public ThreadWork1()
{ }
public void run()
{
System.Console.WriteLine("ThreadWork1 run { ");
for (int i = 0; i < 100; i++)
{
System.Console.WriteLine("ThreadWork1 producer: " + i);
_TestDictionary.TryAdd(i, i);
}
System.Console.WriteLine("ThreadWork1 run } ");
}
}
class ThreadWork2 // consumer
{
public ThreadWork2()
{ }
public void run()
{
int i = 0, nCnt = 0;
int nValue = 0;
bool IsOk = false;
System.Console.WriteLine("ThreadWork2 run { ");
while (nCnt < 100)
{
IsOk = _TestDictionary.TryGetValue(i, out nValue);
if (IsOk)
{
System.Console.WriteLine("ThreadWork2 consumer: " + i * i + " =====" + i);
nValue = nValue * nValue;
_TestDictionary.AddOrUpdate(i, nValue, (key, value) => { return value = nValue; });
nCnt++;
i++;
}
}
System.Console.WriteLine("ThreadWork2 run } ");
}
}
static void StartT1()
{
ThreadWork1 work1 = new ThreadWork1();
work1.run();
}
static void StartT2()
{
ThreadWork2 work2 = new ThreadWork2();
work2.run();
}
static void Main(string[] args)
{
Task t1 = new Task(() => StartT1());
Task t2 = new Task(() => StartT2());
bool bIsNext = true;
int nValue = 0;
_TestDictionary = new ConcurrentDictionary();
Console.WriteLine("Sample 4-5 Main {");
Console.WriteLine("Main t1 t2 started {");
t1.Start();
t2.Start();
Console.WriteLine("Main t1 t2 started }");
Console.WriteLine("Main wait t1 t2 end {");
Task.WaitAll(t1, t2);
Console.WriteLine("Main wait t1 t2 end }");
foreach (var pair in _TestDictionary)
{
Console.WriteLine(pair.Key + " : " + pair.Value);
}
System.Collections.Generic.IEnumerator>
enumer = _TestDictionary.GetEnumerator();
while (bIsNext)
{
bIsNext = enumer.MoveNext();
Console.WriteLine("Key: " + enumer.Current.Key +
" Value: " + enumer.Current.Value);
_TestDictionary.TryRemove(enumer.Current.Key, out nValue);
}
Console.WriteLine("\n\nDictionary Count: " + _TestDictionary.Count);
Console.WriteLine("Sample 4-5 Main }");
Console.ReadKey();
}
}
}