Re: RFC: regarding metaspace(metadata?) dump

Thu Jan 12 08:29:00 UTC 2023

Thanks Ioi and Thomas for your valuable thoughts! I list some pros about metaspace dump.
1. Standardization. The format of metadata dump is standard and well-formed. It could be seamlessly integrated with DevOps/Diagnose/APM platforms, while SA is interactively and the output of jcmd is not well-formed and parser-unfriendly, and its content is subject to change. You can't expect DevOps platform to use SA/coredump/gdb conveniently and automatically in production environment.
2. Functionality. MetaspaceDumpBeforeFullGC could generate a small dump for further debugging, it works as well as heap dump.
3. JVM Metadata. Codecache dump, method counter, method data, they are unexplored scopes, reconstruct human-readable representation of compiled method.
4. Complexity. In my humble opinion, all of this stuff such as by-chunktype/show-loaders/VM.classloader_stats/VM.classloader_hierarchy/etc could be done in other place. VM is eligible to provides a standard and rich raw metadata output, third-party parser and UI render display them in their way instead of continuously adding new filter/grouping/hierarcy/VM.method_counter features when we do want to know them. Heap dump is a good example, it dumps all objects/symbols/etc to a binary file, and third-party tools orchestrate them to histogram/thread/classloader/domtree.
Basically, I know the content of metaspace dump file has overlap with some existing tools such as SA/jcmd/coredump/gdb/HeapDump, as Thomas commented inlinely, I don’t think metaspace dump can troubleshoot many problems that it is the only solution. I think metadata dump is more about providing a standardized and parser-friendly framework, so that users at all levels can inspect JVM metadata information they care about. In addition, something like M(etaspace)AT could orchestrate dump content with filter/grouping/hierarchy options.
> Also you mentioned that "Internally we implemented a metaspace dump that generates human-readable text". Can you share how this tool was implemented?
That's not surprising, it iterates CLD/Classes/etc and dumps basic information about metaspace, a demo metaspace dump can be found at https://gist.github.com/y1yang0/683d8a58dd946b3e9180682863df55ea <https://gist.github.com/y1yang0/683d8a58dd946b3e9180682863df55ea >
------------------------------------------------------------------
From:Thomas Stüfe <thomas.stuefe at gmail.com>
Send Time:2023 Jan. 12 (Thu.) 15:06
To:"YANG, Yi" <qingfeng.yy at alibaba-inc.com>
Cc:HotSpot Open Source Developers <hotspot-dev at openjdk.java.net>; hotspot-runtime-dev <hotspot-runtime-dev at openjdk.java.net>; hotspot-dev <hotspot-dev at openjdk.org>
Subject:Re: RFC: regarding metaspace(metadata?) dump
Hi Yi,
A lot of what you try to do already exists. For example, we also have `VM.metaspace`. This is a quite powerful command to analyze Metaspace-related issues, especially for fragmentation and other wastages. Generally speaking, it is the tool you use to look at the underpinnings of metaspace, the allocator, while tools like `VM.classloaders`, `VM.classloader_stats` and `VM.classes` look at things "from above", e.g. walk the CLDG. All these tools have already a bit of overlap.
For analyzing OOMEs, you need several tools, since it can be caused by multiple issues. E.g. tools that walk the CLDG don't see fragmentation, or unclaimed metaspace for dead loaders.
Please find more remarks inline.
On Wed, Jan 11, 2023 at 1:56 PM Yi Yang <qingfeng.yy at alibaba-inc.com <mailto:qingfeng.yy at alibaba-inc.com >> wrote:
Hi,
Internally, we often receive feedback from users and ask for help on metaspace-related issues, for example
1. Users are eager to know which GroovyClassLoader loads which classes, why they are not unloaded,
and why they are leading to Metaspace OOME.
There are several tools to do this, for example:
`VM.metaspace show-loaders show-classes` 
`VM.classloaders show-classes` 
both show you loaded classes by loader, and the former also shows you metaspace stats needed to understand OOMEs. None of these tools shows you why loaders are kept alive, but for that you need heap- and GC-root-analysis. This quickly enters the territory of Eclipse MAT and similar tools, where having a text-based tool alone gets cumbersome.
2. They want to know the class structure of dynamically generated classes in some scenarios such as 
deserialization
Interesting. This seems to be a very specific query; not sure how general the need for this is. `VM.classes -verbose` shows a part of the story. 
3. Finding memory leaking about duplicated classes
Again, 
`VM.metaspace show-loaders show-classes` 
`VM.classloaders show-classes` 
but also `VM.classes` and `VM.classloader_stats` are your friends here.
...
Internally we implemented a metaspace dump that generates human-readable text, it looks something like this:
[Basic Information]
Dump Reason : JCMD
MaxMetaspaceSize : 18446744073709547520 B
CompressedClassSpaceSize : 1073741824 B
Class Space Used : 309992 B
Class Space Capacity : 395264 B
...
[Class Loader Data]
ClassLoaderData : loader = 0x000000008024f928, loader_klass = 0x0000000800010098, loader_klass_name = 
sun/misc/Launcher$AppClassLoader, label = N/A
 Class Used Chunks :
 * Chunk : [0x0000000800060000, 0x0000000800060230, 0x0000000800060800)
 NonClass Used Chunks :
 * Chunk : [0x00007fd8379c1000, 0x00007fd8379c1350, 0x00007fd8379c2000)
 Klasses :
 Klass : 0x0000000800060028, name = Test, size = 520 B
 ConstantPool : 0x00007fd8379c1050, size = 296 B
...
`VM.metaspace` shows you the chunk composition of arenas if needed.
E.g. : `VM.metaspace by-chunktype show-loaders`
```
Usage per loader: 
 1: CLD 0x00007f72fc29b820: "app" instance of jdk.internal.loader.ClassLoaders$AppClassLoader 
 Loaded classes: 
 1: de.stuefe.repros.MiscUtils$$Lambda$1/0x0000000801001448 
 2: de.stuefe.repros.MiscUtils 
 3: de.stuefe.repros.Simple2 
 4: de.stuefe.repros.Simple 
 5: de.stuefe.repros.SimpleBase 
 6: de.stuefe.repros.I2 
 7: de.stuefe.repros.I1
 -total-: 7 classes
 Non-Class: 
 Usage by chunk level:
 4m chunks: (none)
 2m chunks: (none)
 1m chunks: (none)
 512k chunks: (none)
 256k chunks: (none)
 128k chunks: (none)
 64k chunks: (none)
 32k chunks: (none)
 16k chunks: (none)
 8k chunks: 1 chunk, 8,00 KB capacity, 8,00 KB (100%) committed, 8,00 KB (100%) used, 0 bytes ( 0%) free, 0 bytes ( 0%) waste 
 4k chunks: 1 chunk, 4,00 KB capacity, 4,00 KB (100%) committed, 256 bytes ( 6%) used, 3,75 KB ( 94%) free, 0 bytes ( 0%) waste 
 2k chunks: (none)
 1k chunks: (none)
 -total-: 2 chunks, 12,00 KB capacity, 12,00 KB (100%) committed, 8,25 KB ( 69%) used, 3,75 KB ( 31%) free, 0 bytes ( 0%) waste 
 deallocated: 1 blocks with 24 bytes
 Class: 
 Usage by chunk level:
 .... and so forth
```
but for analyzing potential fragmentation issues (which have been rare since JEP 387) the "Waste" section at the end of the printout is much more helpful, e.g.:
```
Waste (unused committed space):(percentages refer to total committed size 384,00 KB):
 Waste in chunks in use: 0 bytes ( 0%)
 Free in chunks in use: 85,01 KB ( 22%)
 In free chunks: 0 bytes ( 0%)
Deallocated from chunks in use: 928 bytes ( <1%) (3 blocks)
 -total-: 85,91 KB ( 22%)
```
It has been working effectively for several years and has helped many users solve metaspace-related problems.
But a more user-friendly way is that JDK can inherently support this capability. We hope that format of the metaspace
dump file can take both flexibility and compatibility into account, and the content of dump file should be detailed
enough to meet the needs of both application developers and lower-level developers.
Based on above considerations, I think using JSON as its file format is an appropriate solution(But XML or binary 
format are still not excluded as candidates). Specifically, in earlier thoughts, I thought the format of the metaspace
file could be as follows(pretty printed)
https://gist.github.com/y1yang0/ab3034b6381b8a9d215602c89af4e9c3 <https://gist.github.com/y1yang0/ab3034b6381b8a9d215602c89af4e9c3 >
Using the JSON format, we can flexibly add new fields without breaking compatibility. It is debatable as to which data
to write. We can reach a consensus that third-party parsers(Metaspace Analyzer Tool) can at least reconstruct Java
source code from the dump file. Based on this, we can write more useful information for low-level troubleshooting
or debugging. (e.g. the init_state of InstanceKlass).
 In addition, we can even output the native code and associated information with regard to Method, third-party parser
 can reconstruct the human-readable assembly representation of the compiled method based on dump file. To some extent,
we have implemented code cache dump by the way. For this reason, I'm not sure if the title of the RFC proposal should
be called metaspace dump, maybe metadata dump? It looks more like a metadata-dump framework.
Do you have any thoughts about metaspace/metadata dump? Looking forward to hearing your feedback, any comments are invaluable!
Best regards,
Yi Yang
Analyzing the structure of generated classes sounds interesting, and could help with analyzing issues with bytecode instrumentation tools.
For analyzing general metaspace OOMEs we are already covered quite well. Not perfect, but your proposal does intersect with existing tools a lot. To keep code complexity down, I'd rather avoid adding duplicate features.
Cheers, Thomas
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