UseCompressedOops 在 jdk-13 和 jdk-15 中使用 CompressedClassPointers
UseCompressedOops UseCompressedClassPointers in jdk-13 and jdk-15
我无意中发现了 jdk-15
中的一个我不知道的变化。假设我有一个非常简单的问题:3 个整数数组的大小是多少?为此,我使用 JOL。代码相当简单:
import org.openjdk.jol.info.ClassLayout;
import org.openjdk.jol.vm.VM;
public class Array {
public static void main(String [] args){
int [] array = new int[3];
System.out.println(ClassLayout.parseInstance(array).toPrintable());
}
}
我运行这个用jdk-13
:
java -Djdk.attach.allowAttachSelf -Djol.tryWithSudo=true -cp jol-cli.jar Array.java
我得到输出:
[I object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 01 00 00 00 (00000001 00000000 00000000 00000000) (1)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) 18 0e 07 00 (00011000 00001110 00000111 00000000) (462360)
12 4 (object header) 03 00 00 00 (00000011 00000000 00000000 00000000) (3)
16 12 int [I.<elements> N/A
28 4 (loss due to the next object alignment)
Instance size: 32 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
这很明显:
12 bytes --> Object headers
4 bytes --> size of array
12 bytes --> elements of array themselves
4 bytes --> padding to align by 8 bytes
----
32 bytes total
运行 这个带有 jdk-15
的例子产生相同的输出,相同的 32 bytes
。预计...
对于第二部分,我想禁用 JVM 优化:-XX:-UseCompressedOops
。我 运行 这个 jdk-13
:
java -Djdk.attach.allowAttachSelf -Djol.tryWithSudo=true -cp jol-cli.jar -XX:-UseCompressedOops Array.java
[I object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 11 00 00 00 (00010001 00000000 00000000 00000000) (17)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) 40 0c f0 33 (01000000 00001100 11110000 00110011) (871369792)
12 4 (object header) 02 00 00 00 (00000010 00000000 00000000 00000000) (2)
16 4 (object header) 03 00 00 00 (00000011 00000000 00000000 00000000) (3)
20 4 (alignment/padding gap)
24 12 int [I.<elements> N/A
36 4 (loss due to the next object alignment)
Instance size: 40 bytes
Space losses: 4 bytes internal + 4 bytes external = 8 bytes total
好吧,有点意料之中:
16 bytes --> object headers (I did -XX:-UseCompressedOops after all)
4 bytes --> array size
4 bytes --> alignment for array headers (AFAIK this is only done for arrays)
12 bytes --> array elements themselves
4 bytes --> 4 bytes padding
----
40 bytes total
现在让我们运行与jdk-15
相同的例子:
[I object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 01 00 00 00 (00000001 00000000 00000000 00000000) (1)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) 0e 09 00 00 (00001110 00001001 00000000 00000000) (2318)
12 4 (object header) 03 00 00 00 (00000011 00000000 00000000 00000000) (3)
16 12 int [I.<elements> N/A
28 4 (loss due to the next object alignment)
Instance size: 32 bytes
为什么现在是 32 bytes
?为什么不是 40
,就像 jdk-13
?
在 jdk-13
和 jdk-15
中,这两个选项都默认启用:
java -XX:+PrintFlagsFinal -version | grep Compressed
bool UseCompressedClassPointers = true
bool UseCompressedOops = true
当-XX:-UseCompressedOops
被禁用时,意味着UseCompressedClassPointers
也被禁用。这就是为什么当 UseCompressedOops
关闭时,header 大小会增加 4 bytes
,因为 UseCompressedOops
会关闭 UseCompressedClassPointers
。至少在 jdk-13
:
中是这样
java -XX:+PrintFlagsFinal -XX:-UseCompressedOops -version | grep Compressed
bool UseCompressedClassPointers = false
bool UseCompressedOops = false
jdk-15
发生了变化:
bool UseCompressedClassPointers = true
bool UseCompressedOops = false
所以禁用 UseCompressedOops
并不意味着 UseCompressedClassPointers
也被禁用,所以它停留在 4 bytes
.
不过,我自己回答了这个问题,如果有人能找到相关的 bug/change 就好了?到目前为止,我还没有成功。
我无意中发现了 jdk-15
中的一个我不知道的变化。假设我有一个非常简单的问题:3 个整数数组的大小是多少?为此,我使用 JOL。代码相当简单:
import org.openjdk.jol.info.ClassLayout;
import org.openjdk.jol.vm.VM;
public class Array {
public static void main(String [] args){
int [] array = new int[3];
System.out.println(ClassLayout.parseInstance(array).toPrintable());
}
}
我运行这个用jdk-13
:
java -Djdk.attach.allowAttachSelf -Djol.tryWithSudo=true -cp jol-cli.jar Array.java
我得到输出:
[I object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 01 00 00 00 (00000001 00000000 00000000 00000000) (1)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) 18 0e 07 00 (00011000 00001110 00000111 00000000) (462360)
12 4 (object header) 03 00 00 00 (00000011 00000000 00000000 00000000) (3)
16 12 int [I.<elements> N/A
28 4 (loss due to the next object alignment)
Instance size: 32 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
这很明显:
12 bytes --> Object headers
4 bytes --> size of array
12 bytes --> elements of array themselves
4 bytes --> padding to align by 8 bytes
----
32 bytes total
运行 这个带有 jdk-15
的例子产生相同的输出,相同的 32 bytes
。预计...
对于第二部分,我想禁用 JVM 优化:-XX:-UseCompressedOops
。我 运行 这个 jdk-13
:
java -Djdk.attach.allowAttachSelf -Djol.tryWithSudo=true -cp jol-cli.jar -XX:-UseCompressedOops Array.java
[I object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 11 00 00 00 (00010001 00000000 00000000 00000000) (17)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) 40 0c f0 33 (01000000 00001100 11110000 00110011) (871369792)
12 4 (object header) 02 00 00 00 (00000010 00000000 00000000 00000000) (2)
16 4 (object header) 03 00 00 00 (00000011 00000000 00000000 00000000) (3)
20 4 (alignment/padding gap)
24 12 int [I.<elements> N/A
36 4 (loss due to the next object alignment)
Instance size: 40 bytes
Space losses: 4 bytes internal + 4 bytes external = 8 bytes total
好吧,有点意料之中:
16 bytes --> object headers (I did -XX:-UseCompressedOops after all)
4 bytes --> array size
4 bytes --> alignment for array headers (AFAIK this is only done for arrays)
12 bytes --> array elements themselves
4 bytes --> 4 bytes padding
----
40 bytes total
现在让我们运行与jdk-15
相同的例子:
[I object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 01 00 00 00 (00000001 00000000 00000000 00000000) (1)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) 0e 09 00 00 (00001110 00001001 00000000 00000000) (2318)
12 4 (object header) 03 00 00 00 (00000011 00000000 00000000 00000000) (3)
16 12 int [I.<elements> N/A
28 4 (loss due to the next object alignment)
Instance size: 32 bytes
为什么现在是 32 bytes
?为什么不是 40
,就像 jdk-13
?
在 jdk-13
和 jdk-15
中,这两个选项都默认启用:
java -XX:+PrintFlagsFinal -version | grep Compressed
bool UseCompressedClassPointers = true
bool UseCompressedOops = true
当-XX:-UseCompressedOops
被禁用时,意味着UseCompressedClassPointers
也被禁用。这就是为什么当 UseCompressedOops
关闭时,header 大小会增加 4 bytes
,因为 UseCompressedOops
会关闭 UseCompressedClassPointers
。至少在 jdk-13
:
java -XX:+PrintFlagsFinal -XX:-UseCompressedOops -version | grep Compressed
bool UseCompressedClassPointers = false
bool UseCompressedOops = false
jdk-15
发生了变化:
bool UseCompressedClassPointers = true
bool UseCompressedOops = false
所以禁用 UseCompressedOops
并不意味着 UseCompressedClassPointers
也被禁用,所以它停留在 4 bytes
.
不过,我自己回答了这个问题,如果有人能找到相关的 bug/change 就好了?到目前为止,我还没有成功。