RFR: 8270321: Startup regressions in 18-b5 caused by JDK-8266310

[Rémi Forax]

On Fri, 23 Jul 2021 18:03:31 GMT, Sergey Chernyshev <github.com+6394632+sercher at openjdk.org> wrote:

> Dear colleagues,
> 
> Please review the patch that replaces the lambdas with anonymous classes which solves the startup time regression as shown below.
> 
> I attached the Bytestacks flamegraphs for both original (regression) and fixed versions. The flamegraphs clearly show the lambdas were causing the performance issue.
> 
> [bytestacks_flamegraphs.zip](https://github.com/openjdk/jdk/files/6870446/bytestacks_flamegraphs.zip)
> 
> Although the proposed JDK-8270321 patch fixes the startup time (it might appear even better than it was before the regression was introduced, i.e. before JDK-8266310), it increases MaxRSS slightly compared to the version before JDK-8266310, which is shown in the below graphs.
> 
> ![StartupTime](https://user-images.githubusercontent.com/6394632/126822380-5b01ce90-b249-4294-9a62-8269440f942d.png)
> 
> ![MaxRSS](https://user-images.githubusercontent.com/6394632/126822404-899ab904-efc1-4377-9e0d-d8cdb5c0e5d0.png)
> 
> I additionally include the heap objects histograms to show the change does not increase the total live objects size significantly with only 1000 bytes the total difference, namely 1116128 bytes in 25002 live objects after the proposed fix JDK-8270321 compared to 1115128 bytes in 24990 objects in the version with the original patch reverted (i.e. before JDK-8266310).
> 
> [histograms.zip](https://github.com/openjdk/jdk/files/6870457/histograms.zip)
> 
> The patch was tested w/hotspot/tier1/tier2 test groups.

I don't understand your analysis, you are testing the startup time with -Xint which disable JITs, but there is no mention of -Xint in the bug report.
It's obvious to me that there is a regression with -Xint given that the lambda creation is using invokedynamic which is only optimizable by JITs. 

I think you should first try to reproduce the issue with the correct flags and then follow the advice from Mandy on how to implement the fix. Using an anonymous class may introduce more allocation than using a lambda once the code is JITed.

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PR: https://git.openjdk.java.net/jdk/pull/4893