8346239: Improve memory efficiency of JimageDiffGenerator

[jerboaa]

Please review this fairly simple change to improve how the JimageDiffGenerator works. The original implementation is pretty naive and read all bytes into memory and then compared them. This improved version only reads bytes on a bound buffer into memory compares those bytes and if equal continues on to reading the next bytes (2k at most) at a time. Previously it was 2*N (where N is the file size of a file in bytes) part of the JDK. Ouch.

There is still the off-chance of reading a full file into memory when the generator detects a change, but this shouldn't happen for large files since the total diff should be around 600K as of today.

This also reverts changes from JDK-8344036 other than the /timeout change to the test, which is preserved as I think this bump is no longer needed.

Testing:

• GHA
• jlink tests on a fastdebug build with --with-native-debug-symbols=internal (so as to have a large libjvm.so).
• Manual reproducer from the bug which fails with OOM before the patch and passes after

Thoughts?

---

Progress

• Change must be properly reviewed (1 review required, with at least 1 Reviewer)
• Change must not contain extraneous whitespace
• Commit message must refer to an issue

Issue

• JDK-8346239: Improve memory efficiency of JimageDiffGenerator (Bug - P4)

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk.git pull/22835/head:pull/22835
$ git checkout pull/22835

Update a local copy of the PR:
$ git checkout pull/22835
$ git pull https://git.openjdk.org/jdk.git pull/22835/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 22835

View PR using the GUI difftool:
$ git pr show -t 22835

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Download this PR as a diff file:
https://git.openjdk.org/jdk/pull/22835.diff

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Link to Webrev Comment

[bridgekeeper]

👋 Welcome back sgehwolf! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

[openjdk]

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[openjdk]

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[jerboaa]

@MBaesken @RealFYang Could you please test this PR since you originally ran into JDK-8346239 (this bug) and JDK-8344036. Thanks!

[mlbridge]

Webrevs

• 00: Full (c4c614c5)

[RealFYang]

@MBaesken @RealFYang Could you please test this PR since you originally ran into JDK-8346239 (this bug) and JDK-8344036. Thanks!

Yes, I can confirm that it works on my linux-aarch64 platform. Great! Thanks!

[MBaesken]

Hi Severin, I added it to our build/test queue.

[jerboaa]

Yes, I can confirm that it works on my linux-aarch64 platform. Great! Thanks!

Thank you!

[jerboaa]

Hi Severin, I added it to our build/test queue.

Thanks, Matthias!