Hello Lambda experts, Since we upgraded JDK from 11 to 17, we’re experiencing metaspace pressure, largely due to Lambda class implementation changes.
There’s a scenario (see attached test case), that is especially puzzled me,
hopefully, you can share some insights.
In this test case, there is only one Lambda class is created inside the loop,
but each one for the same functions outside loop.
Example output:
0: Func = LambdaFunc$$Lambda/0x00001f80000c4a20@4de8b406
testMethod() called
1: Func = LambdaFunc$$Lambda/0x00001f80000c4a20@4de8b406
testMethod() called
2: Func = LambdaFunc$$Lambda/0x00001f80000c4a20@4de8b406
testMethod() called
3: Func = LambdaFunc$$Lambda/0x00001f80000c4a20@4de8b406
testMethod() called
4: Func = LambdaFunc$$Lambda/0x00001f80000c4a20@4de8b406
testMethod() called
….
Outside loop1, Func = LambdaFunc$$Lambda/0x00001f80000c4c58@402f32ff
testMethod() called
Outside loop2 Func = LambdaFunc$$Lambda/0x00001f80000d1000@5ae9a829
testMethod() called
Outside loop3 Func = LambdaFunc$$Lambda/0x00001f80000d1238@548b7f67
testMethod() called
And jcmd also confirmed there were 4 Lambda classes created:
49: CLD 0x000060000134cb50: "app" instance of
jdk.internal.loader.ClassLoaders$AppClassLoader
Loaded classes:
1: LambdaFunc$$Lambda/0x00001f80000d1238
2: LambdaFunc$$Lambda/0x00001f80000d1000
3: LambdaFunc$$Lambda/0x00001f80000c4c58
4: LambdaFunc$$Lambda/0x00001f80000c4a20
5: LambdaFunc
Looking into bytecode, all four call sites have the same invokedynamic bytecode
(invokedynamic #7, 0 // InvokeDynamic
#0:run:()Ljava/lang/Runnable; ) and the first invokedynamic bytecode is inside
the loop.
But when I ran the program with -XX:+TraceBytecodes, it seems that the first
invokedynamic was hoisted and result was used in the subsequence loop.
Can anyone explain where this magic happens? If the magic can apply to the
instances outside the loop, so that only one Lambda class is created?
Thank you for your time and expertise,
-Zhengyu
LambdaFunc.java
Description: LambdaFunc.java
