<div dir="ltr"><div dir="ltr"><div>Hi Yi,</div></div><div dir="ltr"><br></div><div>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.<br></div><div><br></div><div>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.<br></div><div><br></div><div>Please find more remarks inline.<br></div><div><br></div><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Wed, Jan 11, 2023 at 1:56 PM Yi Yang <<a href="mailto:qingfeng.yy@alibaba-inc.com">qingfeng.yy@alibaba-inc.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div><div style="line-height:1.7;font-family:tahoma;font-size:14px;color:rgb(0,0,0)"><div style="clear:both">Hi,<br></div><div style="clear:both"><br></div><div style="clear:both"><div style="clear:both">Internally, we often receive feedback from users and ask for help on metaspace-related issues, for example</div><div style="clear:both">1. Users are eager to know which GroovyClassLoader loads which classes, why they are not unloaded,</div><div style="clear:both">and why they are leading to Metaspace OOME.</div></div></div></div></blockquote><div><br></div><div>There are several tools to do this, for example:</div><div><br></div><div>`VM.metaspace show-loaders show-classes` <br></div><div>`VM.classloaders show-classes` </div><div><br></div><div>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.<br></div><div></div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div><div style="line-height:1.7;font-family:tahoma;font-size:14px;color:rgb(0,0,0)"><div style="clear:both"><div style="clear:both">2. They want to know the class structure of dynamically generated classes in some scenarios such as </div><div style="clear:both">deserialization</div></div></div></div></blockquote><div><br></div><div>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. <br></div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div><div style="line-height:1.7;font-family:tahoma;font-size:14px;color:rgb(0,0,0)"><div style="clear:both"><div style="clear:both">3. Finding memory leaking about duplicated classes</div></div></div></div></blockquote><div><br></div><div>Again, <br></div><div><br></div><div><div>`VM.metaspace show-loaders show-classes` <br></div><div>`VM.classloaders show-classes` <br></div><div><br></div><div>but also `VM.classes` and `VM.classloader_stats` are your friends here.<br></div><div><br></div></div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div><div style="line-height:1.7;font-family:tahoma;font-size:14px;color:rgb(0,0,0)"><div style="clear:both"><div style="clear:both">...</div><div style="clear:both">Internally we implemented a metaspace dump that generates human-readable text, it looks something like this:</div><div style="clear:both"><br></div><div style="clear:both">[Basic Information]</div><div style="clear:both">Dump Reason : JCMD</div><div style="clear:both">MaxMetaspaceSize : 18446744073709547520 B</div><div style="clear:both">CompressedClassSpaceSize : 1073741824 B</div><div style="clear:both">Class Space Used : 309992 B</div><div style="clear:both">Class Space Capacity : 395264 B</div><div style="clear:both">...</div><div style="clear:both">[Class Loader Data]</div><div style="clear:both">ClassLoaderData : loader = 0x000000008024f928, loader_klass = 0x0000000800010098, loader_klass_name = </div><div style="clear:both">sun/misc/Launcher$AppClassLoader, label = N/A</div><div style="clear:both"> Class Used Chunks :</div><div style="clear:both"> * Chunk : [0x0000000800060000, 0x0000000800060230, 0x0000000800060800)</div><div style="clear:both"> NonClass Used Chunks :</div><div style="clear:both"> * Chunk : [0x00007fd8379c1000, 0x00007fd8379c1350, 0x00007fd8379c2000)</div><div style="clear:both"> Klasses :</div><div style="clear:both"> Klass : 0x0000000800060028, name = Test, size = 520 B</div><div style="clear:both"> ConstantPool : 0x00007fd8379c1050, size = 296 B</div><div style="clear:both">...</div><div style="clear:both"><br></div></div></div></div></blockquote><div><br></div><div>`VM.metaspace` shows you the chunk composition of arenas if needed.<br></div><div><br></div><div>E.g. : `VM.metaspace by-chunktype show-loaders`<br></div><div><br></div><div>```</div><div>Usage per loader: <br> <br> 1: CLD 0x00007f72fc29b820: "app" instance of jdk.internal.loader.ClassLoaders$AppClassLoader <br> Loaded classes: <br> 1: de.stuefe.repros.MiscUtils$$Lambda$1/0x0000000801001448 <br> 2: de.stuefe.repros.MiscUtils <br> 3: de.stuefe.repros.Simple2 <br> 4: de.stuefe.repros.Simple <br> 5: de.stuefe.repros.SimpleBase <br> 6: de.stuefe.repros.I2 <br> 7: de.stuefe.repros.I1<br> -total-: 7 classes<br> Non-Class: <br> Usage by chunk level:<br> 4m chunks: (none)<br> 2m chunks: (none)<br> 1m chunks: (none)<br> 512k chunks: (none)<br> 256k chunks: (none)<br> 128k chunks: (none)<br> 64k chunks: (none)<br> 32k chunks: (none)<br> 16k chunks: (none)<br> 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 <br> 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 <br> 2k chunks: (none)<br> 1k chunks: (none)<br> -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 <br> deallocated: 1 blocks with 24 bytes<br><br> Class: <br> Usage by chunk level:</div><div> .... and so forth<br></div><div>```</div><div><br></div><div>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.:</div><div><br></div><div>```</div><div>Waste (unused committed space):(percentages refer to total committed size 384,00 KB):<br> Waste in chunks in use: 0 bytes ( 0%)<br> Free in chunks in use: 85,01 KB ( 22%)<br> In free chunks: 0 bytes ( 0%)<br>Deallocated from chunks in use: 928 bytes ( <1%) (3 blocks)<br> -total-: 85,91 KB ( 22%)<br></div><div>```<br></div><div></div><div></div><div><br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div><div style="line-height:1.7;font-family:tahoma;font-size:14px;color:rgb(0,0,0)"><div style="clear:both"><div style="clear:both"></div><div style="clear:both">It has been working effectively for several years and has helped many users solve metaspace-related problems.</div><div style="clear:both">But a more user-friendly way is that JDK can inherently support this capability. We hope that format of the metaspace</div><div style="clear:both">dump file can take both flexibility and compatibility into account, and the content of dump file should be detailed</div><div style="clear:both">enough to meet the needs of both application developers and lower-level developers.</div><div style="clear:both"><br></div><div style="clear:both">Based on above considerations, I think using JSON as its file format is an appropriate solution(But XML or binary </div><div style="clear:both">format are still not excluded as candidates). Specifically, in earlier thoughts, I thought the format of the metaspace</div><div style="clear:both">file could be as follows(pretty printed)</div><div style="clear:both"><br></div><div style="clear:both"><a href="https://gist.github.com/y1yang0/ab3034b6381b8a9d215602c89af4e9c3" target="_blank">https://gist.github.com/y1yang0/ab3034b6381b8a9d215602c89af4e9c3</a></div><div style="clear:both"><br></div><div style="clear:both">Using the JSON format, we can flexibly add new fields without breaking compatibility. It is debatable as to which data</div><div style="clear:both">to write. We can reach a consensus that third-party parsers(Metaspace Analyzer Tool) can at least reconstruct Java</div><div style="clear:both">source code from the dump file. Based on this, we can write more useful information for low-level troubleshooting</div><div style="clear:both">or debugging. (e.g. the init_state of InstanceKlass).</div><div style="clear:both"> In addition, we can even output the native code and associated information with regard to Method, third-party parser</div><div style="clear:both"> can reconstruct the human-readable assembly representation of the compiled method based on dump file. To some extent,</div><div style="clear:both">we have implemented code cache dump by the way. For this reason, I'm not sure if the title of the RFC proposal should</div><div style="clear:both">be called metaspace dump, maybe metadata dump? It looks more like a metadata-dump framework.</div><div style="clear:both"><br></div><div style="clear:both">Do you have any thoughts about metaspace/metadata dump? Looking forward to hearing your feedback, any comments are invaluable!</div><div style="clear:both"><br></div><div style="clear:both">Best regards,</div><span>Yi Yang</span></div></div></div></blockquote><div><br></div><div><br></div><div>Analyzing the structure of generated classes sounds interesting, and could help with analyzing issues with bytecode instrumentation tools.</div><div><br></div><div>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.</div><br><div>Cheers, Thomas<br></div><div><br></div><div> </div></div></div>