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

[Hamlin Li]

On Fri, 19 Feb 2021 20:23:01 GMT, Chris Plummer <cjplummer at openjdk.org> wrote:

> 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.

Sure, will hold off.
Summarize the help messages as below:
"Number of parallel threads to use for heap inspection. "
"0: let the VM determine the number of threads to use. "
"1: use one thread, i.e. disable parallelism. "
"n: Request that threads be used. n must be positive. The VM will try to use n threads, but might use fewer threads than n."

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