RFR: 8257234 : Add gz option to SA jmap to write a gzipped heap dump [v14]

[Lin Zang]

On Thu, 4 Feb 2021 23:23:41 GMT, Chris Plummer <cjplummer at openjdk.org> wrote:

>> Thanks for the thorough list, I have tested most of them but not all. I will cover them and update here later.
>> 
>>> So really there are two implementations of heap dumping, one in the VM and one in SA, but a total of 5 ways to get to them. I'm just starting to get my head wrapped around all this myself. We've created a very confusing situation for us, and unfortunately I think some of the changes that got us here was driven by a goal to simply things.
>> 
>> Yes, it makes me even curious about why there can be two different implementation of them (one by C++ and one by Java),  it seems we need double the work of adding new options for those tools, like parallal for jmap histo and dump, etc.
>
> One implementation is in the JVM itself, to be used when the JVM is still running well, and not just from command line tools. Heap dumping can also be triggered by the JVM itslef by setting flags like -XX:+HeapDumpBeforeFullGC. The other implementation is in SA, to be used on a core file or a hung JVM process, although it can also be used on a JVM that is still running well. BTW, there used to be a 3rd implementation. The old hprof profiler (a JVMTI agent) also was used to generate hprof files.

Dear @plummercj,

> `--dumpfile` and `--gz` can only be used with `--binaryheap`. That should be made clear in the help text.

I added indentation for these two sub-options.

> * SA's `jhsdb jmap --binaryheap` (which uses JMap.java with `-heap:format=b`)
> * SA's clhsdb `dumpheap` command (which also uses JMap.java with `-heap:format=b`)
> * executing  SA's `sun/jvm/hotspot/tools/JMap` class directly
> * `jmap -dump` command (Uses Attach API's `heapdump` command, which uses hotspot `HeapDump` class)
> * `GC.heap_dump` dcmd, which also uses the hotspot `HeapDumper` class
All 5 commands help message have been tested.

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PR: https://git.openjdk.java.net/jdk/pull/1712