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Comment by dc-programmer | Hacker News Reader

Ok thanks, that helped clear some things up. I didn’t realize generational GC reuses the same contiguous memory blocks for the young regions. Makes sense from a fragmentation and resident page locality perspective.

Do you know if the early generation region settings can be tweaked (alignment, size, number of threads/regions)? I’m wondering what happens if you “overflow” these areas by generating too many objects

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There are many parameters you can tweak, for sure. Overflowing an area probably triggers an immediate GC run, and if that doesn’t free up enough space, an additional “young” region is used. You only need to specify a high enough maximum value for the total memory consumption of all generations.

One situation you can run into is if you generate and quickly drop objects faster than the GC can collect them, or rather, the process spends most of its time with GC rather than with actual program execution, then after a certain while an exception will be raised. That situation almost always indicates a bug in your program, and the exception helps to see the cause of the program stalling. Of course that behavior is also configurable.

Excellent. I appreciate the information. I’m just getting into the JVM dev ecosystem so I have a lot to learn

The latest GCs are _fast_. I benchmarked ZGC and IIRC it ate up something like 4GB/s of garbage (24 threads doing nothing but allocating) with about 2 milliseconds of stop-the-world pause time over around 3 minutes of runtime.

Note that benchmarking GCs properly is really hard. Changes in size distribution, tree shape, and lifespan can lead to drastically different results (and to make matters worse, the type of code that is easiest to write as a benchmark tends to be types of code that GC can handle really well).

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