Java8的伪共享和缓存行填充--@Contended注释

在我的前一篇文章<伪共享和缓存行填充,从Java 6, Java 7 到Java 8>中, 我们演示了在Java 8中,可以采用@Contended在类级别上的注释,来进行缓存行填充。这样,多线程情况下的伪共享冲突问题。 感兴趣的同学可以查看该文。

 

其实,@Contended注释还可以应用于字段级别(Field-Level),当应用于字段级别时,被注释的字段将和其他字段隔离开来,会被加载在独立的缓存行上。在字段级别上,@Contended还支持一个“contention group”属性(Class-Level不支持),同一group的字段们在内存上将是连续,但和其他他字段隔离开来。

 

上面只是泛泛的介绍一下。关于@Contended应用于Field-Level特别是contention group的相关的资料很少,源代码中的注释中有一些,还有关于JEP-142(即关于增加@Contended的提议)的邮件讨论组中的描述(http://mail.openjdk.java.net/pipermail/hotspot-dev/2012-November/007309.html),其中的讲解是非常详细的(由于该讨论发生在@Contended实现的最初阶段,不能保证和现在的实现完全一致), 我摘录和翻译如下:

 

@Contended注释的行为如下所示:

A,在类上应用Contended:

@Contended
    public static class ContendedTest2 {
        private Object plainField1;
        private Object plainField2;
        private Object plainField3;
        private Object plainField4;
    }

 

将使整个字段块的两端都被填充:(以下是使用 –XX:+PrintFieldLayout的输出)(翻译注:注意前面的@140表示字段在类中的地址偏移)

TestContended$ContendedTest2: field layout
    Entire class is marked contended
     @140 --- instance fields start ---
     @140 "plainField1" Ljava.lang.Object;
     @144 "plainField2" Ljava.lang.Object;
     @148 "plainField3" Ljava.lang.Object;
     @152 "plainField4" Ljava.lang.Object;
     @288 --- instance fields end ---
     @288 --- instance ends ---

 

注意,我们使用了128bytes的填充 -- 2倍于大多数硬件缓存行的大小 -- 来避免相邻扇区预取导致的伪共享冲突。

 

B,在字段上应用Contended:

public static class ContendedTest1 {
        @Contended
        private Object contendedField1;
        private Object plainField1;
        private Object plainField2;
        private Object plainField3;
        private Object plainField4;
    }

 

将导致该字段从连续的字段块中分离开来并高效的添加填充:

TestContended$ContendedTest1: field layout
     @ 12 --- instance fields start ---
     @ 12 "plainField1" Ljava.lang.Object;
     @ 16 "plainField2" Ljava.lang.Object;
     @ 20 "plainField3" Ljava.lang.Object;
     @ 24 "plainField4" Ljava.lang.Object;
     @156 "contendedField1" Ljava.lang.Object; (contended, group = 0)
     @288 --- instance fields end ---
     @288 --- instance ends ---

 

C, 注解多个字段使他们分别被填充:

public static class ContendedTest4 {
        @Contended
        private Object contendedField1;

        @Contended
        private Object contendedField2;

        private Object plainField3;
        private Object plainField4;
    }

 

被注解的2个字段都被独立地填充:

TestContended$ContendedTest4: field layout
     @ 12 --- instance fields start ---
     @ 12 "plainField3" Ljava.lang.Object;
     @ 16 "plainField4" Ljava.lang.Object;
     @148 "contendedField1" Ljava.lang.Object; (contended, group = 0)
     @280 "contendedField2" Ljava.lang.Object; (contended, group = 0)
     @416 --- instance fields end ---
     @416 --- instance ends ---

 

在有些cases中,你会想对字段进行分组,同一组的字段会和其他字段有访问冲突,但是和同一组的没有。例如,(同一个线程的)代码同时更新2个字段是很常见的情况。如果同时把2个字段都添加@Contended注解是足够的(翻译注:但是太足够了),但我们可以通过去掉他们之间的填充,来优化它们的内存空间占用。为了区分组,我们有一个参数“contention group”来描述:

所以:

public static class ContendedTest5 {
        @Contended("updater1")
        private Object contendedField1;

        @Contended("updater1")
        private Object contendedField2;

        @Contended("updater2")
        private Object contendedField3;

        private Object plainField5;
        private Object plainField6;
    }

 

内存布局是:

TestContended$ContendedTest5: field layout
     @ 12 --- instance fields start ---
     @ 12 "plainField5" Ljava.lang.Object;
     @ 16 "plainField6" Ljava.lang.Object;
     @148 "contendedField1" Ljava.lang.Object; (contended, group = 12)
     @152 "contendedField2" Ljava.lang.Object; (contended, group = 12)
     @284 "contendedField3" Ljava.lang.Object; (contended, group = 15)
     @416 --- instance fields end ---
     @416 --- instance ends ---

 

注意$contendedField1 和$contendedField2和其他字段之间有填充,但是它们之间是紧挨着的。
 
 
以上是对邮件组中大牛们原始实现解释的翻译。
 
下面我们来做一个测试,看@Contended在字段级别,并且带分组的情况下,是否能解决伪缓存问题。
import sun.misc.Contended;

public class VolatileLong {
    @Contended("group0")
    public volatile long value1 = 0L;  
    @Contended("group0")
    public volatile long value2 = 0L;  
    
    @Contended("group1")
    public volatile long value3 = 0L;  
    @Contended("group1")
    public volatile long value4 = 0L;  
}

 

我们用2个线程来修改字段--

测试1:线程0修改value1和value2;线程1修改value3和value4;他们都在同一组中。

测试2:线程0修改value1和value3;线程1修改value2和value4;他们在不同组中。

 
测试1:
public final class FalseSharing implements Runnable {
    public final static long ITERATIONS = 500L * 1000L * 1000L;
    private static VolatileLong volatileLong;
    private String groupId;

    public FalseSharing(String groupId) {
        this.groupId = groupId;

    }

    public static void main(final String[] args) throws Exception {
        // Thread.sleep(10000);
        System.out.println("starting....");

        volatileLong = new VolatileLong();
        final long start = System.nanoTime();
        runTest();
        System.out.println("duration = " + (System.nanoTime() - start));
    }

    private static void runTest() throws InterruptedException {
        Thread t0 = new Thread(new FalseSharing("t0"));
        Thread t1 = new Thread(new FalseSharing("t1"));
        t0.start();
        t1.start();
        t0.join();
        t1.join();
    }

    public void run() {
        long i = ITERATIONS + 1;
        if (groupId.equals("t0")) {
            while (0 != --i) {
                volatileLong.value1 = i;
                volatileLong.value2 = i;
            }
        } else if (groupId.equals("t1")) {
            while (0 != --i) {
                volatileLong.value3 = i;
                volatileLong.value4 = i;
            }
        }
    }
}
 
测试2:(基于以上代码修改下面的部分)
public void run() {
        long i = ITERATIONS + 1;
        if (groupId.equals("t0")) {
            while (0 != --i) {
                volatileLong.value1 = i;
                volatileLong.value3 = i;
            }
        } else if (groupId.equals("t1")) {
            while (0 != --i) {
                volatileLong.value2 = i;
                volatileLong.value4 = i;
            }
        }
    }

 

测试1:

starting....
duration = 16821484056

测试2:

starting....
duration = 39191867777

 

可以看出,如果同一线程修改的是同一“contention group”中的字段,没有伪共享冲突,比有伪共享冲突的情况要快1倍多。

 

后记:

测试3:不使用@Contended

public class VolatileLong {
    public volatile long value1 = 0L;  
    public volatile long value2 = 0L;  
    public volatile long value3 = 0L;  
    public volatile long value4 = 0L;  
}

结果:

starting....
duration = 38096777198

 

 

参考:

http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/8-b132/sun/misc/Contended.java

http://openjdk.java.net/jeps/142

http://mail.openjdk.java.net/pipermail/hotspot-dev/2012-November/007309.html

posted @ 2016-06-28 14:04 Binhua Liu 阅读(...) 评论(...) 编辑 收藏