C# 原子操作
Interlocked.Increment 方法：让++成为原子操作；Interlocked.Decrement 方法让--成为原子操作。
什么叫原子操作呢。就是不会被别人打断，因为C#中的一个语句，编译成机器代码后会变成多个语句。
在多线程环境中，线程切换有可能会发生在这多个语句中间。使用Interlocked.Increment,Interlocked.Decrement 可以避免被打断,保证线程安全。
使用Interlocked.Increment 方法和Interlocked.Decrement 方法MSND例子：
using System;
using System.Threading;
class Test
{
static void Main()
{
Thread thread1 = new Thread(new ThreadStart(ThreadMethod));
Thread thread2 = new Thread(new ThreadStart(ThreadMethod));
thread1.Start();
thread2.Start();
thread1.Join();
thread2.Join();
// Have the garbage collector run the finalizer for each
// instance of CountClass and wait for it to finish.
GC.Collect();
GC.WaitForPendingFinalizers();
Console.WriteLine("UnsafeInstanceCount: {0}" +
"\nSafeCountInstances: {1}",
CountClass.UnsafeInstanceCount.ToString(),
CountClass.SafeInstanceCount.ToString());
}
static void ThreadMethod()
{
CountClass cClass;
// Create 100,000 instances of CountClass.
for(int i = 0; i < 100000; i++)
{
cClass = new CountClass();
}
}
}
class CountClass
{
static int unsafeInstanceCount = 0;//不使用原子操作
static int safeInstanceCount = 0;//使用原子操作
static public int UnsafeInstanceCount
{
get {return unsafeInstanceCount;}
}
static public int SafeInstanceCount
{
get {return safeInstanceCount;}
}
public CountClass()
{
unsafeInstanceCount++;
Interlocked.Increment(ref safeInstanceCount);
}
~CountClass()
{
unsafeInstanceCount--;
Interlocked.Decrement(ref safeInstanceCount);
}
}
不用原子操作例子
class Program
{
static void Main(string[] args)
{
for (int loop = 0; loop < 20; loop++)
{
sum = 0;
Thread t1 = new Thread(Thread1);
Thread t2 = new Thread(Thread2);
t1.Start();
t2.Start();
t1.Join();
t2.Join();
Console.WriteLine("sum = " + sum); // sum = 200000 ?
}
}
static int sum;
static void Thread1()
{
for (int i = 0; i < 100000; i++) sum++;
}
static void Thread2()
{
for (int i = 0; i < 100000; i++) sum++;
}
}
结果：
/*
sum = 200000
sum = 200000
sum = 200000
sum = 200000
sum = 200000
sum = 200000
sum = 200000
sum = 200000
sum = 192361
sum = 175155
sum = 200000
sum = 176024
sum = 200000
sum = 200000
sum = 200000
sum = 200000
sum = 200000
sum = 200000
sum = 200000
sum = 176322
*/
Why the sum is not always 200000?
The reason is that sum++ is not thread safe (see the possible problem).
That is the reason we need Interlocked.Increment(), which guarantees the sum is always 200000.
Thread1 (sum++) Thread2 (sum++)
--------------------------------------------------------------------
mov EAX, dword ptr sum .
inc EAX .
. mov EAX, dword ptr sum // load sum into a register
. inc EAX // increase it
. mov dword ptr sum, EAX // save back
mov dword ptr sum, EAX
--------------------------------------------------------------------
problem: two sum++ are called in different thread,
but the sum is incremented only once.
也就是说因为C#中的一个语句，编译成机器代码后会变成多个语句，线程不安全，sum++的第100次操作就被打断了，而在第200000次++操作结束后CPU才轮询到sum++的第100次操作，这时sum的值就是101，
