RFR: JDK-8306441: Two phase segmented heap dump [v17]

[Kevin Walls]

On Wed, 12 Jul 2023 07:58:47 GMT, Yi Yang <yyang at openjdk.org> wrote:

>> ### Motivation and proposal
>> Hi, heap dump brings about pauses for application's execution(STW), this is a well-known pain. JDK-8252842 have added parallel support to heapdump in an attempt to alleviate this issue. However, all concurrent threads competitively write heap data to the same file, and more memory is required to maintain the concurrent buffer queue. In experiments, we did not feel a significant performance improvement from that.
>> 
>> The minor-pause solution, which is presented in this PR, is a two-phase segmented heap dump:
>> 
>> - Phase 1(STW): Concurrent threads directly write data to multiple heap files.
>> - Phase 2(Non-STW): Merge multiple heap files into one complete heap dump file. This process can happen outside safepoint.
>> 
>> Now concurrent worker threads are not required to maintain a buffer queue, which would result in more memory overhead, nor do they need to compete for locks. The changes in the overall design are as follows:
>> 
>> ![image](https://github.com/openjdk/jdk/assets/5010047/77e4764a-62b5-4336-8b45-fc880ba14c4a)
>> <p align="center">Fig1. Before</p>
>> 
>> ![image](https://github.com/openjdk/jdk/assets/5010047/931ab874-64d1-4337-ae32-3066eed809fc)
>> <p align="center">Fig2. After this patch</p>
>> 
>> ### Performance evaluation
>> | memory | numOfThread | CompressionMode | STW | Total |
>> | -------| ----------- | --------------- | --- | ---- |
>> | 8g | 1 T | N | 15.612 | 15.612 |
>> | 8g | 32 T | N | 2.561725 | 14.498 |
>> | 8g | 32 T | C1 | 2.3084878 | 14.198 |
>> | 8g | 32 T | C2 | 10.9355128 | 21.882 |
>> | 8g | 96 T | N | 2.6790452 | 14.012 |
>> | 8g | 96 T | C1 | 2.3044796 | 3.589 |
>> | 8g | 96 T | C2 | 9.7585151 | 20.219 |
>> | 16g | 1 T | N | 26.278 | 26.278 |
>> | 16g | 32 T | N | 5.231374 | 26.417 |
>> | 16g | 32 T | C1 | 5.6946983 | 6.538 |
>> | 16g | 32 T | C2 | 21.8211105 | 41.133 |
>> | 16g | 96 T | N | 6.2445556 | 27.141 |
>> | 16g | 96 T | C1 | 4.6007096 | 6.259 |
>> | 16g | 96 T | C2 | 19.2965783 | 39.007 |
>> | 32g | 1 T | N | 48.149 | 48.149 |
>> | 32g | 32 T | N | 10.7734677 | 61.643 |
>> | 32g | 32 T | C1 | 10.1642097 | 10.903 |
>> | 32g | 32 T | C2 | 43.8407607 | 88.152 |
>> | 32g | 96 T | N | 13.1522042 | 61.432 |
>> | 32g | 96 T | C1 | 9.0954641 | 9.885 |
>> | 32g | 96 T | C2 | 38.9900931 | 80.574 |
>> | 64g | 1 T | N | 100.583 | 100.583 |
>> | 64g | 32 T | N | 20.9233744 | 134.701 |
>> | 64g | 32 T | C1 | 18.5023784 | 19.358 |
>> | 64g | 32 T | C2 | 86.4748377 | 172.707 |
>> | 64g | 96 T | N | 26.7374116 | 126.08 |
>> | 64g | ...
>
> Yi Yang has updated the pull request incrementally with one additional commit since the last revision:
> 
>   fix test compilation failure

I think this is a good feature, although I have a few other questions that I didn't yet have time to understand:
if we specify a large number for -parallel=, is it limited to the number of safepoint_workers?
Should we feedback to the user how many threads were really used?
Should the user really have to always say how many threads?  Do they just want to say "use all the worker threads"?
Does UseDynamicNumberOfGCThreads affect this?
Does this work with Serial GC? Maybe it doesn't maybe that's unfortunate (Serial may be appropriate for some large heaps, and you still want a faster heap dump?)

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PR Comment: https://git.openjdk.org/jdk/pull/13667#issuecomment-1636728200