alt Hacker NewsI read through it. Not bad. In fact, pretty good. It is about something that they actually did.
tl;dr: In JDk 25, filling a large array of references with objects living in a different heap region is extremely slow when using G1 GC as opposed to parallel GC. Solution: Move to Java 26. Or increase G1HeapRegionSize.
Details:
They used Amazon Corretto as JDK. When Arrays.fill() was called with UseG1GC and UseParallelGC, the former was really, really slow.
Then they go on to give an ARM64 primer (because they ran it on an Apple M4 Max).
The part where they mention g1BarrierSetAssembler_x86.cpp and g1BarrierSetAssembler_aarch64.cpp is where at least some readers get lost. They are essentially saying that the logic (for both x86 and ARM64) is the same. The reason why only those two files matter for this test, I believe, comes down to JEP 304[1]. Just accept it and continue reading. (And validate it later if you want to.)
They list around twelve lines of ARM instructions that matter for this test - cheat sheet, essentially.
The three lines of actually generated ARM instructions are shown then for parallel GC (the much faster one in this case).
They explain what a write barrier is (basically GC bookkeeping). So as a result, there are three instructions, but they are neat and tight. Also, we see that eight elements are filled in each iteration of the loop which fills the arrays.
Then, they show what was generated for G1 GC. And it is indeed too long. Around 20 instructions. However, some of them shouldn't have been executed because of three exit conditions. They explain why none of them fired.
They prove that by subtracting eight from the humongous threshold size, it is allocated to an Eden/young region and the problem vanishes because one of the three exit conditions is triggered thus.
Then, they show that it is a problem that increases almost linearly (my guess, based on the numbers) with the array size- so the larger the array size is, the more the time delay!!
Finally, the solution: JDK 26 took care of this issue. They also point to JEP 522 (I didn't go through it yet). If you have an older version, use the G1HeapRegionSize flag if you can do so.
That is it, essentially. Definitely long, but well-written, based on actual testing.