RFR: JDK-8261034: improve jcmd GC.class_histogram to support parallel [v3]

[Hamlin Li]

On Wed, 3 Feb 2021 21:28:40 GMT, Chris Plummer <cjplummer at openjdk.org> wrote:

>> Hamlin Li has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   JDK-8261034: improve jcmd GC.class_histogram to support parallel
>
> src/hotspot/share/services/diagnosticCommand.cpp line 559:
> 
>> 557:        "BOOLEAN", false, "false"),
>> 558:   _parallel_thread_num("-parallel", "parallel threads number for heap iteration",
>> 559:        "INT", false, "0")     {
> 
> Does "0" mean use a default number of parallel threads as it does for the jmap? It's unclear, but it seems that it doesn't, which I think leads to eventually hitting this assert in `update_active_workers(uint v)`:
>     assert(v != 0, "Trying to set active workers to 0");
> If that's not the case, please explain how 0 is handled. In any case, I think it should be made consistent with jmap and should also be documented in the above  help output:
>   parallel=<count> generate histogram using this many parallel threads, default 0
>                      0 use system determined number of threads
>                      1 use one thread, i.e., disable parallelism
>                      n use n threads, n must be positive

Thanks Chris for reviewing, I have updated the patch as you suggested, would you mind to have another look at it?
CSR is created at https://bugs.openjdk.java.net/browse/JDK-8261105

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PR: https://git.openjdk.java.net/jdk/pull/2379