RFR: 8252842: Extend jmap to support parallel heap dump [v13]

[Lin Zang]

On Thu, 18 Mar 2021 12:09:25 GMT, Ralf Schmelter <rschmelter at openjdk.org> wrote:

>> Lin Zang has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   reduce memory consumption and fix memory leak issue
>
> src/hotspot/share/services/heapDumper.cpp line 1741:
> 
>> 1739:   uint length() { return _length; }
>> 1740: 
>> 1741:   static const size_t LargeObjectSizeThreshold = 128 * (1 << 20); // 128 MB
> 
> I think this threshold is much too large. I've instrumented the code to track the maximum memory use of the buffers and tested the max memory use and the performance for 2, 4 and 8 parallel iteration threads and for a threshold of 128 MB and 1 MB (using my scenario with a 32 GB heap):
> 
> 128 MB threshold:
> 2 threads -> 57 MB max memory, 64.6 seconds duration
> 4 threads -> 88 MB max memory, 40.5 seconds duration
> 8 threads -> 170 MB max memory, 29.6 seconds duration
> 
> 1MB threshold:
> 2 threads -> 8 MB max memory, 64.6 seconds duration
> 4 threads -> 15 MB max memory, 39.5 seconds duration
> 8 threads -> 32 MB max memory, 28.9 seconds duration
> 
> As you can see, using a much lower threshold has no discernable performance impact, but uses a lot less memory. Also note that the memory consumption can be much higher in the worst case scenario.

I agree， Thanks for the evaluation!  I think 1MB is reasonable. Will made the change in next update.

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