引入
在JMH的例子中，JMHSample_37_CacheAccess这个例子很有意思，测试一个二维数组通过row、col两种不同的方式遍历数据的平均时间。1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51@BenchmarkMode(Mode.AverageTime)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
@Warmup(iterations = 5, time = 1, timeUnit = TimeUnit.SECONDS)
@Measurement(iterations = 5, time = 1, timeUnit = TimeUnit.SECONDS)
@Fork(5)
@State(Scope.Benchmark)
public class JMHSample_37_CacheAccess {
/*
* This sample serves as a warning against subtle differences in cache access patterns.
*
* Many performance differences may be explained by the way tests are accessing memory.
* In the example below, we walk the matrix either row-first, or col-first:
*/
private final static int COUNT = 4096;
private final static int MATRIX_SIZE = COUNT * COUNT;
private int[][] matrix;
@Setup
public void setup() {
matrix = new int[COUNT][COUNT];
Random random = new Random(1234);
for (int i = 0; i < COUNT; i++) {
for (int j = 0; j < COUNT; j++) {
matrix[i][j] = random.nextInt();
}
}
}
@Benchmark
@OperationsPerInvocation(MATRIX_SIZE)
public void colFirst(Blackhole bh) {
for (int c = 0; c < COUNT; c++) {
for (int r = 0; r < COUNT; r++) {
bh.consume(matrix[r][c]);
}
}
}
@Benchmark
@OperationsPerInvocation(MATRIX_SIZE)
public void rowFirst(Blackhole bh) {
for (int r = 0; r < COUNT; r++) {
for (int c = 0; c < COUNT; c++) {
bh.consume(matrix[r][c]);
}
}
}
}
如上述代码所示，两种遍历的次数相同，差异是一个按行读取，一个按列读取。那么，最终两种方式的性能会有差异吗？