问题描述
我写了一个程序来模拟MetaSpace OOM.但是我发现MetaSpace Size几乎总是Used MetaSpace的两倍.为什么?
I wrote a program to simulate MetaSpace OOM. But I found that MetaSpace Size is almost always twice as big as Used MetaSpace. Why?
我用标志-XX:MaxMetaspaceSize=50m运行程序，当Used MetaSpace达到25M而不是50M时，程序抛出OOM，为什么?
I run my program with flag -XX:MaxMetaspaceSize=50m, the program throw OOM when Used MetaSpace reached about 25M rather than 50M, Why?
推荐答案
我认为以下两个实验将解释 MetaSpace Size 和 Used MetaSpace 之间的差距是什么意思:
I think the following two experiment will explain what does the gap between MetaSpace Size and Used MetaSpace mean:
EXP-1 :每个ClassLoader加载一个类，我得到了:
EXP-1: load one class per ClassLoader, I got this:
EXP-2 :每个ClassLoader加载五个类，我得到了:
EXP-2: load five classes per ClassLoader, I got this:
为 Java 8文档 说:
As Java 8 document says:
我认为这意味着一个块通常可以包含一个以上的类，因此，当您每个ClassLoader加载更多的类时，MetaSpace Size的利用率就会提高.
I think it means a chunk can contains usually more than one class, so utilization rate of MetaSpace Size grows as you load more classes per ClassLoader.
您可以通过运行 此处的代码 ，使用命令:
You can reappear the results by running code here using command:
java -XX:MaxMetaspaceSize=50m MetadataOOMSimulator 100000000
注意:确保所有加载的类均不应出现在CLASSPATH中(通常包括当前目录)，否则这些类将由APPClassLoader而不是自定义的ClassLoader加载.
Attention: make sure that all classes be loaded should not appear in CLASSPATH (usually include current directory), or those classes will be loaded by APPClassLoader rather than customized ClassLoader.
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