JOL 28 Difference

本篇文章基于V0.16 JOLSample_28_Difference

本来用来演示如何使用GraphLayout差异来计算对象图之间的变更。

我们还是使用ConcurrentHashMap, 然后设定三个测量步骤

1. ConcurrentHashMap 对象初始测量为空

This is the example how would one use the GraphLayout differences to figure out the object graph changes.
Here, we have the ConcurrentHashMap, and three measurements:

1. The initial CHM that has no backing storage;
2. After adding the first KV pair, when both KV pair is allocated, and the backing storage is allocated;
3. After adding the second KV pair.

An API for subtracting the GraphLayouts helps to show the difference between the snapshots.
Note that differences are based on object addresses, so if GC moves under our feet, the difference is unreliable.
It is a good idea to keep the allocations at minimum between the snapshots.

public class JOLSample_28_Difference {

    public static void main(String[] args) {
        out.println(VM.current().details());

        Map<String, String> chm = new ConcurrentHashMap<>();

        GraphLayout gl1 = GraphLayout.parseInstance(chm);

        chm.put("Foo", "Bar");
        GraphLayout gl2 = GraphLayout.parseInstance(chm);

        chm.put("Foo2", "Bar2");
        GraphLayout gl3 = GraphLayout.parseInstance(chm);

        System.out.println(gl2.subtract(gl1).toPrintable());
        System.out.println(gl3.subtract(gl2).toPrintable());
    }

}

运行结果

# Running 64-bit HotSpot VM.
# Using compressed oop with 3-bit shift.
# Using compressed klass with 3-bit shift.
# WARNING | Compressed references base/shifts are guessed by the experiment!
# WARNING | Therefore, computed addresses are just guesses, and ARE NOT RELIABLE.
# WARNING | Make sure to attach Serviceability Agent to get the reliable addresses.
# Objects are 8 bytes aligned.
# Field sizes by type: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]
# Array element sizes: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]

 object externals:
          ADDRESS       SIZE TYPE                                           PATH                           VALUE
        76bdd9298         24 java.lang.String                               .table[7].key                  (object)
        76bdd92b0         24 [C                                             .table[7].key.value            [F, o, o]
        76bdd92c8         24 java.lang.String                               .table[7].val                  (object)
        76bdd92e0         24 [C                                             .table[7].val.value            [B, a, r]
        76bdd92f8         80 [Ljava.util.concurrent.ConcurrentHashMap$Node; .table                         [null, null, null, null, null, null, null, (object), null, null, null, null, null, (object), null, null]
        76bdd9348         32 java.util.concurrent.ConcurrentHashMap$Node    .table[7]                      (object)

Addresses are stable after 1 tries.


 object externals:
          ADDRESS       SIZE TYPE                                        PATH                           VALUE
        76bddac78         24 java.lang.String                            .table[13].key                 (object)
        76bddac90         24 [C                                          .table[13].key.value           [F, o, o, 2]
        76bddaca8         24 java.lang.String                            .table[13].val                 (object)
        76bddacc0         24 [C                                          .table[13].val.value           [B, a, r, 2]
        76bddacd8         32 java.util.concurrent.ConcurrentHashMap$Node .table[13]                     (object)

Addresses are stable after 1 tries.