RFR: 8252842: Extend jmap to support parallel heap dump

[Chris Plummer]

On Thu, 4 Feb 2021 03:17:28 GMT, Lin Zang <lzang at openjdk.org> wrote:

>  IMO, there can be two ways which may help solve the compatibility issue of introducing "parallel" option:
> 
>     * unblock socket with timeout. In this way, old Jmap could work with new JDK, but it maybe complicated because of different implementation for different OS.
> 
>     * Two times communication as you mentioned,  it sounds a reasonable solution, I will work out a prototype and then discuss with you .
> 
A 3rd would be to create a new `dumpheap` command for the Attach API, maybe `dumpheap_parallel`. If you try using it with an old JVM, it should immediately return an error:

        st.print("Operation %s not recognized!", op->name());
        res = JNI_ERR;
If it does, you can fallback to the regular `dumpheap` command. If an old JVM attaches and uses `dumpheap`, you can choose to make it parallel by default if you wish. A slightly different variant of this idea is have something like `dumpheap_extraargs` or `dumpheap_ext`, leaving room to add even more args in the future (possibly a single argument with a list of args as you suggested before)

BTW, one thing to keep in mind that whatever we do with `dumpheap`, the jcmd version will support fine control over parallelism. So for that reason it wouldn't be so bad to just have `dumpheap` and `inspectheap` always use some default parallelism, and allow tuning it with the jcmd versions. But since the `inspectheap` parallel support already went into 16, that might be hard to undo.

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PR: https://git.openjdk.java.net/jdk/pull/2261