RFR: 8252842: Extend jmap to support parallel heap dump

[Chris Plummer]

On Wed, 3 Feb 2021 02:19:43 GMT, Lin Zang <lzang at openjdk.org> wrote:

>> Hi All,
>> As discussed in this PR, it is reasonable to enable parallel heap dump by default rather than introducing a new "parallel" option. I would like to close the CSR at https://bugs.openjdk.java.net/browse/JDK-8260424.
>
> Dear All,
> I have updated the patch and remove the introduction of new "parallel" option. So there can be no backward-compatibility issue for jmap -dump command.  May I ask your help to remove the CSR label if agreed? 
> 
> BRs,
> Lin

Hi Lin, When I suggested that it would ok just to just always use the default number of parallel threads, I wasn't considering that we also have `jmap -histo`, which also supports parallel threads, but includes an argument to specify it. For example, `jmap -histo:parallel=<n>`. Now this PR adds parallel support to `jmap -dump`, but you are suggesting not to have a parallel option to specify it, and use a default number of threads, which would be the same as what you would get with `jmap -histo:parallel=0`.  So this is inconsistent. If `-histo` is going to allow you to control the number of parallel threads, then `-dump` should also. I guess an option would be to not allow `-histo` to control the number of threads, and have it work just like you are proposing for `-dump`.

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