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

[Chris Plummer]

On Fri, 19 Feb 2021 20:20:19 GMT, Chris Plummer <cjplummer at openjdk.org> wrote:

>> Hi @plummercj @sspitsyn @linzang 
>> Thanks for discussion. I understood that currently the final decision is to still use "parallel=<N>" and do not introduce new option "noparallel".
>> Based on this decision, for jcmd GC.class_historgram, would you mind to double confirm whether currently implementation is fine? I think we only need to refine the words in help doc or CSR, am I right? please kindly correct me if I misunderstand it. :-)
>> If I'm understanding correctly, would you mind to suggest how should we describe <N>? IMHO, I don't think it's a good idea to expose to much details about the implementation, and it's hard to make its meaning precise. Following is my version:
>> 
>> "Degree of parallelism for heap iteration. "
>> "0 means let the VM determine the parallelism. "
>> "1 means use one thread, i.e. disable parallelism. "
>> "n means ask the VM try best to use n threads, but it's not guaranteed and dependant on the specific implementation and runtime environment. n must be positive."
>> 
>> How do you think about it?
>
>>"Degree of parallelism for heap iteration. "
> 
> How about "Number of parallel threads to use for heap inspection". I've noticed the term "heap iteration" in a few places, but I think "heap inspection" would be better. That would be inline with terminology used in the source:
> 
>  130   VM_GC_HeapInspection(outputStream* out, bool request_full_gc,
>  131                        uint parallel_thread_num = 1) :
> 
> For the rest of the help:
> 
> "0: let the VM determine the number of threads to use. "
> "1: use one thread, i.e. disable parallelism. "
> "<n>: Request that <n> threads be used. <n> must be positive. The VM will try to use <n> threads, but might use fewer threads than <n>."

BTW, we have three PRs related to `parallel=<n>`. I think it would be best to hold off pushing any of them until they have all been reviewed and approved. One reason is because the `parallel=<n>` output should be consistent for all of them, and if there is a minor suggestion for a change in one, it should be applied to all 3.

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