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

[Lin Zang]

On Sat, 13 Feb 2021 05:46:32 GMT, Chris Plummer <cjplummer at openjdk.org> wrote:

> > Yes, It is not known at present, but the heapInspect and heapDumper accept an outputStream, and the actual parallel thread number can be got by gang->active_workers() after WithUpdatedActiveWorkers. Then maybe some code can be added there to record the actual number and finally print to outputStream at the end.
> 
> So you are proposing having the attach "heapdumpext" and "inspectheap" commands always print the number of threads used? I'm not so sure that is a good idea since there are probably existing tools and tests that parse the output, and changing its format could break them.

Correct， if there is parsing tools (which I think there alreay are.), they will be broken.
So I am afraid user can only tuning the thread number by expriments and some common sense such as the core number of their server or the multi-processing scenario.

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