Java8中用sun.misc.Contended避免僞共享(false sharing)

轉自：http://budairenqin.iteye.com/blog/2048257

關於僞共享這個概念，請先參照http://ifeve.com/falsesharing/ 

僞共享的樣子： 

Java view:
public class VolatileLong {
        volatile long v = 0L;
}

Memory view:
...–––-)(––––HV––––HV–––)(–––...

我們看到，兩個VolatileLong對象被load到了同一個緩存行裏面，如果一個線程要修改對象1，另一個線程同時要修改對象2，此時就要面對僞共享這個無形的性能殺手了 

jdk6中的解決辦法： 

Java view:
public class VolatileLong {
        volatile long p0, p1, p2, p3, p4, p5, p6;
        volatile long v = 0L;
        volatile long q0, q1, q2, q3, q4, q5, q6;
}

Memory view:
...–––-)(ppppppHVqqqqqq)(–––...

很多大神的代碼都是通過上面的方式也就是long padding來避免僞共享 
例如： 
1.Doug Lea的jsr166中早期的LinkedTransferQueue版本http://gee.cs.oswego.edu/cgi-bin/viewcvs.cgi/jsr166/src/jsr166y/LinkedTransferQueue.java?revision=1.1&view=markup 
2.還是Doug Lea的ConcurrentHashMapV8http://gee.cs.oswego.edu/cgi-bin/viewcvs.cgi/jsr166/src/jsr166e/ConcurrentHashMapV8.java?revision=1.121&view=markup（在java8中的版本已經用sun.misc.Contended替換long padding） 
3.大名鼎鼎的無鎖併發框架Disruptor https://github.com/LMAX-Exchange/disruptor 
4.等等等... 

long padding的解決辦法不怎麼優雅，並且在jdk7某個版本以後能會優化掉long padding，尼瑪java程序員是有多難啊，想盡了一切辦法對付jdk7中的這個優化，詳情點擊http://ifeve.com/false-sharing-java-7/ 

但是但是...開始使用jdk8的同學注意了，java8已經給出了官方的解決辦法... 
就是sun.misc.Contended註解http://mail.openjdk.java.net/pipermail/hotspot-dev/2012-November/007309.html 

Java view:
// jdk8新特性，Contended註解避免false sharing
// Restricted on user classpath
// Unlock: -XX:-RestrictContended
@sun.misc.Contended
public class VolatileLong {
        volatile long v = 0L;
}


Memory view:
...–––-)(******HV******)(–––...

要注意的是user classpath使用此註解默認是無效的，需要在jvm啓動時設置-XX:-RestrictContended 

jdk8中已經使用sun.misc.Contended的地方： 

src/share/classes/java/util/concurrent/ConcurrentHashMap.java
2458: @sun.misc.Contended static final class CounterCell {

src/share/classes/java/util/concurrent/Exchanger.java
310: * bookkeeping. Padded via @sun.misc.Contended to reduce m
313: @sun.misc.Contended static final class Node {

src/share/classes/java/util/concurrent/ForkJoinPool.java 161:@sun.misc.Contended
643: @sun.misc.Contended

src/share/classes/java/util/concurrent/atomic/Striped64.java
55: * (via @sun.misc.Contended) to reduce cache contention. Pa
119: @sun.misc.Contended static final class Cell {

src/share/classes/java/lang/Thread.java
2004: @sun.misc.Contended("tlr")
2008: @sun.misc.Contended("tlr")
2012: @sun.misc.Contended("tlr")

最後，貼上測試代碼，感興趣的各自測試吧 

public class FalseSharing implements Runnable {

    public final static int NUM_THREADS = 4; // change
    public final static long ITERATIONS = 500L * 1000L * 1000L;
    private final int arrayIndex;

    private static VolatileLong3[] longs = new VolatileLong3[NUM_THREADS];
    static {
        for (int i = 0; i < longs.length; i++) {
            longs[i] = new VolatileLong3();
        }
    }

    public FalseSharing(final int arrayIndex) {
        this.arrayIndex = arrayIndex;
    }

    public static void main(final String[] args) throws Exception {
        long start = System.nanoTime();
        runTest();
        System.out.println("duration = " + (System.nanoTime() - start));
    }

    private static void runTest() throws InterruptedException {
        Thread[] threads = new Thread[NUM_THREADS];

        for (int i = 0; i < threads.length; i++) {
            threads[i] = new Thread(new FalseSharing(i));
        }

        for (Thread t : threads) {
            t.start();
        }

        for (Thread t : threads) {
            t.join();
        }
    }

    public void run() {
        long i = ITERATIONS + 1;
        while (0 != --i) {
            longs[arrayIndex].value = i;
        }
    }

    public final static class VolatileLong {
        public volatile long value = 0L;
    }

    // long padding避免false sharing
    // 按理說jdk7以後long padding應該被優化掉了，但是從測試結果看padding仍然起作用
    public final static class VolatileLong2 {
        volatile long p0, p1, p2, p3, p4, p5, p6;
        public volatile long value = 0L;
        volatile long q0, q1, q2, q3, q4, q5, q6;
    }

    // jdk8新特性，Contended註解避免false sharing
    // Restricted on user classpath
    // Unlock: -XX:-RestrictContended
    @sun.misc.Contended
    public final static class VolatileLong3 {
        public volatile long value = 0L;
    }
}

我機器上的測試結果： 

VolatileLong： duration = 29594765000
VolatileLong2：duration = 7234555000
VolatileLong3：duration = 7167475000

測試代碼來自http://ifeve.com/falsesharing/，稍有改動