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

[Chris Plummer]

On Sat, 13 Feb 2021 03:51:10 GMT, Serguei Spitsyn <sspitsyn at openjdk.org> wrote:

> I also think, it is better to stick with `parallel=<n>`.
> Then it has to be explained what is the default number of parallel threads for the option `-parallel=0`.
> It can be specified as some implementation-dependent number of thread executors or alike.
> Also, I'd prefer to define some upper limit for the parameter `<n>`.
> Would it be okay to specify it as 9? My assumption is that a real maximum would be the default number of parallel threads.
> If it is too high then it is possible to pick any number between 1 and 9.

I worry about having to define what the default number or parallel threads is, because it's a (somewhat) complex formula based on the number of processors, plus also has to take into account `-XX:ParallelGCThreads`. Cramming this all into the short help is probably counterproductive. I think basically if you choose something other than 0 or 1, you shouldn't expect to be able to pick a good value for <n> without doing some experimenting.

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