Workloads that use >> 100,000 classes

[Kennke, Roman]

Hello all,

Do we have any knowledge about workloads that would blow addressable class-limits with Lilliput? My current plan, with the improved encoding as implemented by Thomas, but would be very similar with any sort of indirection-table, Klass-ID, looks like this:

Lilliput as planned to upstream in JEP 450 (hopefully JDK24): 22 bits = 2^22 ~= 4,000,000 Klasses
Lilliput2 (4-byte-headers): 19 bits = 2^19 ~= 500,000 Klasses

While we are investigating further any solutions to extend those limits (e.g. the near-/far-Klasses discussion, which I very much enjoy), I am trying to find out any actual use-cases that might need as many classes. Realistically, this can only be any sort of class-generator, but I’d like to know more specific, and not rely on hear-say and possible myths. Workloads that I encountered in real world rarely seem to blow 10,000, I don’t think I’ve ever seen 100,000(s).

From what I remember people saying:
- Lambdas. I’m not sure this is an actual problem. AFAICT, each Lambda will generate one class and one instance of that class. That’s a 1:1 relationship between written code and generated Lambdas. It doesn’t look like it could possibly be order(s) of magnitudes worse than ordinary classes.
- JRuby: Generates one class per Ruby function. Depending on the workload, this could be 100,000s, I suppose. OTOH, if I understand correctly, those would not get instantiated, could thus be abstract (if we can get JRuby to generate them that way) and we could simply not allocate them in Class-Space to begin with (just like abstract classes and interfaces).
- I have heard from one customer who is worried about hitting the limit. I don’t have much information from them, but sounded like current compressed class-pointer limit of ~3,000,000 is ok, but ~500,000 might not be. I don’t yet know what exactly they are doing and whether or not there’s other ways to solve the problem (maybe they never get instantiated either?). I will try to find out more ASAP.

Are there any other use-cases that any of you know about that might blow the 500K limit?

Thanks & cheers,
Roman




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