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PerfMa 给 OpenJDK 社区提交的第一个 Patch

概述

前两天给openjdk gc-dev的email list提交了一个问题，主要是针对Full GC之后，GC日志里Metaspace的大小在GC前后都一直不变的问题，我在邮件里大概也提了下如何修复该问题，以及猜测了下为什么会有这个问题，内容比较简单，就不翻译啦，直接把邮件贴出来，PerfMa在后面会提交越来越多的patch给官方吧，虽然不一定每个patch都会被官方所接受，不过也希望为社区的发展贡献一份我们的力量。

邮件内容

Hi, all

We use metaspace instead of perm since JDK8, I found that after the Full GC, the size of the metaspace has always been the same from the GC log. I have looked at the source code of hotspot roughly. It has not bean fixed in the latest version. The problem which may have some impact on us to troubleshoot some metaspace problems.

I wrote a test case, and simply analyzed the hotspot code, and showed the output before and after the modification. I hope this problem can be fixed as soon as possible.

Demo：

PerfMa 给 OpenJDK 社区提交的第一个 Patch

My demo is very simple, just constantly create a new class loader to load a specific class, after loading, execute a System GC, so that we can see the relevant GC log output, the JVM parameters we run are

PerfMa 给 OpenJDK 社区提交的第一个 Patch

The GC log is

PerfMa 给 OpenJDK 社区提交的第一个 Patch

Let’s look at the source code of hotspot,

GenCollectedHeap::do_collection:

PerfMa 给 OpenJDK 社区提交的第一个 Patch

We see that metaspace’s gc log output is executed before ClassLoaderDataGraph::purge(); , that means the size of the metaspace is printed without reclaiming the memory of the metaspace, so the GC log seems no change before and after the GC metaspace.

After knowing the specific problem, we can make an adjustment to the code to move the logic of the output of metaspace size after to ClassLoaderDataGraph::purge()，just like this:

PerfMa 给 OpenJDK 社区提交的第一个 Patch

At this point we recompile hotspot, execute the above demo again, you can see the correct output as follows

PerfMa 给 OpenJDK 社区提交的第一个 Patch

I also probably thought about why this problem is caused. This is the version before JDK8, such as JDK7, you can see the same code location:

PerfMa 给 OpenJDK 社区提交的第一个 Patch

It can be seen that the GC engineer at that time simply replaced print_perm_heap_change(perm_prev_used); with MetaspaceAux::print_metaspace_change(metadata_prev_used); but forgot the difference between Perm and Metaspace.