RFR: 8252842: Extend jmap to support parallel heap dump

[Lin Zang]

On Fri, 12 Feb 2021 00:56:47 GMT, Lin Zang <lzang at openjdk.org> wrote:

>>> BTW, one thing to keep in mind that whatever we do with `dumpheap`, the jcmd version will support fine control over parallelism. So for that reason it wouldn't be so bad to just have `dumpheap` and `inspectheap` always use some default parallelism, and allow tuning it with the jcmd versions. But since the `inspectheap` parallel support already went into 16, that might be hard to undo.
>> 
>> Hi Chris, Lin,
>> IMHO, 16 is not an LTS, maybe it's better for us to adjust the behavior of jmap -histo in 17 which is LTS? I just filed https://bugs.openjdk.java.net/browse/JDK-8261482 to address the issue if you both think it's OK to undo it.
>> Thanks.
>
> Hi Chris, 
> I have update the PR to add a new dumpheapext command that could handle more arguments passed from JMap.java. 
> And I have tested with "jmap -dump:file=dump.bin,noparallel [pid]", with this change, new jmap targeting on old hotspot would print: 
> 
> Exception in thread "main" com.sun.tools.attach.AttachOperationFailedException: Operation dumpheapext not recognized!
> 	at jdk.attach/sun.tools.attach.VirtualMachineImpl.execute(VirtualMachineImpl.java:227)
> 	at jdk.attach/sun.tools.attach.HotSpotVirtualMachine.executeCommand(HotSpotVirtualMachine.java:309)
> 	at jdk.jcmd/sun.tools.jmap.JMap.executeCommandForPid(JMap.java:133)
> 	at jdk.jcmd/sun.tools.jmap.JMap.dump(JMap.java:262)
> 	at jdk.jcmd/sun.tools.jmap.JMap.main(JMap.java:114)
> 
> IMH, one problem is that it doesn't give out the reason clearly that "dumpheapext" is actually caused by new option "noparallel". But I think having this kind of error message is acceptable. Otherwise it seems the choice we left is remove the "parallel/noparallel" for Jmap. What do you think?

Dear All, 
     I am going to create a CSR for the new internal command "dumpheapext", one problem is that seem if a new CSR is created, this PR will depends on 2 CSRs, I am not sure is this reasonable?  Thanks!

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PR: https://git.openjdk.java.net/jdk/pull/2261