RFR: JDK-8260332: ParallelGC: Cooperative pretouch for oldgen expansion [v2]

[Kim Barrett]

On Tue, 16 Mar 2021 08:32:09 GMT, Amit Pawar <github.com+71302734+amitdpawar at openjdk.org> wrote:

> [https://bugs.openjdk.java.net/browse/JDK-8254699](JDK-8254699) contains test results in XL file to show PreTouchParallelChunkSize was recently changed from 1GB to 4MB on Linux after testing various sizes. I have downloaded the same XL file and same is updated for Oldgen case during resize and it gives some rough idea about the improvement for this fix and follow up fix. Please check "PretouchOldgenDuringResize" sheet for "Co-operative Fix" and "Adaptive Resize Fix" columns.
> 
> [PreTouchParallelChunkSize_TestResults_UpdatedForOldGenCase.xlsx](https://github.com/openjdk/jdk/files/6147180/PreTouchParallelChunkSize_TestResults_UpdatedForOldGenCase.xlsx)

I'm not sure how to interpret this. In particular, it says "Test done using November 27rd Openjdk build". That predates a number of recent changes in this area that seem relevant. Is that correct? Or is that stale and the data has been updated for a newer baseline.

Also, can you provide details about how this information is generated and collected?  I would like to reproduce the results and do some other experiments.

I think that, as proposed, the change is making already messy code still more messy, and some refactoring is needed. I don't know what the details of that might look like yet; I'm still exploring and poking at the code. I also need to look at the corresponding part of G1 that you mentioned.

> The "Adaptive Resize Fix" column in the sheet is for next suggested fix and may possibly help to improve further. For server JVM, expansion size of 512KB, 2MB (hugepages) and 64MB looks good for first resize but later needs some attention I think. JVM flag "MinHeapDeltaBytes" needs to be known by the user and need to set it upfront. I think this can be consider for first resize in every GC and later dynamically go for higher size like double the previous size to adopt to application nature. This way it may help to reduce the GC pause time during the expansion. I thought to share my observation and my understanding could be wrong. So please check and suggest.

I think something like this has a serious risk of growing the oldgen a lot more than needed, which may have serious downsides.

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PR: https://git.openjdk.java.net/jdk/pull/2976