04. 缓存行与伪共享
Cache Line & False Sharing | 缓存行与伪共享
缓存系统中是以缓存行（Cache Line）为单位存储的，缓存行是
当多线程修改互相独立的变量时，如果这些变量共享同一个缓存行，就会无意中影响彼此的性能，这就是伪共享。
若两个变量放在同一个缓存行中，在多线程情况下，可能会相互影响彼此的性能。如上图所示，
下面的例子是测试利用缓存行的特性和不利用缓存行的特性的效果对比。
public class CacheLineEffect {
//考虑一般缓存行大小是64字节，一个 long 类型占8字节
static long[][] arr;
public static void main(String[] args) {
arr = new long[1024 * 1024][];
for (int i = 0; i < 1024 * 1024; i++) {
arr[i] = new long[8];
for (int j = 0; j < 8; j++) {
arr[i][j] = 0L;
}
}
long sum = 0L;
// 按行读取
long marked = System.currentTimeMillis();
for (int i = 0; i < 1024 * 1024; i+=1) {
for(int j =0; j< 8;j++){
sum = arr[i][j];
}
}
System.out.println("Loop times:" + (System.currentTimeMillis() - marked) + "ms");
// 按列读取
marked = System.currentTimeMillis();
for (int i = 0; i < 8; i+=1) {
for(int j =0; j< 1024 * 1024;j++){
sum = arr[j][i];
}
}
System.out.println("Loop times:" + (System.currentTimeMillis() - marked) + "ms");
}
}
避免伪共享
解决伪共享问题，可以在变量的前后都占据一定的填充位置，尽量让变量占用一个完整的缓存行。如上图中，
doubles (8) 和 longs (8)
ints (4) 和 floats (4)
shorts (2) 和 chars (2)
booleans (1) 和 bytes (1)
references (4/8)
<子类字段重复上述顺序>
一条缓存行有6*8=48 字节，让不同的
public final static class VolatileLong
{
public volatile long value = 0L;
public long p1, p2, p3, p4, p5, p6; // 添加该行，错开缓存行，避免伪共享
}
某些
public static long preventFromOptimization(VolatileLong v) {
return v.p1 + v.p2 + v.p3 + v.p4 + v.p5 + v.p6;
}
完整的示例如下：
public class FalseSharing implements Runnable{
public final static long ITERATIONS = 500L * 1000L * 100L;
private int arrayIndex = 0;
private static ValuePadding[] longs;
public FalseSharing(final int arrayIndex) {
this.arrayIndex = arrayIndex;
}
public static void main(final String[] args) throws Exception {
for(int i=1;i<10;i++){
System.gc();
final long start = System.currentTimeMillis();
runTest(i);
System.out.println("Thread num "+i+" duration = " + (System.currentTimeMillis() - start));
}
}
private static void runTest(int NUM_THREADS) throws InterruptedException {
Thread[] threads = new Thread[NUM_THREADS];
longs = new ValuePadding[NUM_THREADS];
for (int i = 0; i < longs.length; i++) {
longs[i] = new ValuePadding();
}
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 = 0L;
}
}
public final static class ValuePadding {
protected long p1, p2, p3, p4, p5, p6, p7;
protected volatile long value = 0L;
protected long p9, p10, p11, p12, p13, p14;
protected long p15;
}
public final static class ValueNoPadding {
// protected long p1, p2, p3, p4, p5, p6, p7;
protected volatile long value = 0L;
// protected long p9, p10, p11, p12, p13, p14, p15;
}
}
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