英文:
Heap allocation source code in in jdk(G1GC)
问题
我想了解 Java 如何在堆中分配对象。我想知道在使用 "new" 关键字时，JDK 代码的哪些部分会被触发。
我该如何定义一个函数，用户可以从 Java 代码中调用，并且该函数将实现 JDK 源代码中的功能？
我知道 jdk14 默认使用 G1GC 作为垃圾收集器，G1GC 代码位于 jdk14/src/hotspot/share/GC/G1 文件夹，但我无法理解 G1Allocator 如何为用户线程分配内存（如果它确实这样做的话）。
英文:
I want to understand how java allocates objects in heap. I want to know which parts of the JDK code are triggered when the "new" keyword is used.
How can I define a function that a user can call from the java code and that would implement functionality in JDK source code?
I am aware of the fact that jdk14 uses G1GC as a default garbage collector and G1GC code is present in jdk14/src/hotspot/share/GC/G1 folder but I am unable to follow G1Allocator allocates memory to the user threads(if it does).
答案1
得分: 1
已知的任何实现在分配内存时默认都会使用“TLAB”（线程本地分配缓冲区）。如果不使用它，分配速度会慢得多。尽管我没有深入研究关于这个主题的代码，但你可以从这里，例如开始。
在源代码中有一个非常好的关于请求“new”时会发生什么的注释，在这里可以找到。基本上，如果可以使用“TLAB”（例如，对象不比它大），就会使用它；否则，将为每个“new”执行原始的“malloc”。
关于“G1”，以下是它所做的主要要点。一般解释再次在注释中，有一句话：
> 所有非-TLAB分配请求都应该发送到mem_allocate()
“mem_allocate”的操作可以从这里开始。
英文:
Any known implementation will use TLAB (thread local allocation buffer) by default when allocating memory. Without it - the allocation would be much more slower. Though I have not dived into the code too much about this subject, you can start from here, for example.
There is a very good comment in the source code about what happens when a new is requested here. Basically if TLAB can be used (an Object is not bigger than that for example), it will be; otherwise raw malloc for every new will be done.
Regarding G1 here are the mains points of what it does. A general explanation is again in the comments, with a phrase :
> All non-TLAB allocation requests should go to mem_allocate()
What mem_allocate does can be started from here.
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