RFR: 8252842: Extend jmap to support parallel heap dump

[Chris Plummer]

On Thu, 28 Jan 2021 21:55:41 GMT, Chris Plummer <cjplummer at openjdk.org> wrote:

>> Hi Lin,
>> It is also in my memory that you actually did not have 4 arguments.
>> The real incompatibility issue was that the order of arguments was swapped.
>> It is why it was relatively easy to fall back and just update the constants with 3 instead of 4 and swap the order of arguments back.
>> Thanks,
>> Serguei
>
>>  It is also in my memory that you actually did not have 4 arguments.
>> The real incompatibility issue was that the order of arguments was swapped.
> 
> Ah, now the fix makes a lot more sense. I was always wondering how changing the order allowed reducing the max args to 3, but apparently the two changes are not related. Also, it seems them that [JDK-8219721](https://bugs.openjdk.java.net/browse/JDK-8219721) does not have a correct analysis of the issue. It says:
> 
>> JDK-8215622 increased number of arguments of Attach API [1].
>> Thus AttachListener will be waiting the command from client infinitely. 
> 
> Perhaps it should be updated to avoid future confusion.

I think if you can avoid the 4th argument by just enabling parallel by default sounds like a good idea. Is there any reason not to use parallel heap dump? Also, I'm not familiar with the terms "compression backends" and "active workers". Can you explain them.

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