Horizontal scaling vs vertical scaling of JVMs.

What is the best approach for scaling a JVM ?

Horizontal scaling : Stacking more memory (java heap) to existing JVM.
Vertical scaling : Adding more JVMs.

One of the fundamental scaling questions we hear is whether to add more heap to existing JVM/s or add more instances with existing heap sizing.

The answer is "it depends". Adding more heap to exisiting JVM/s would be better interms of performance (TPS), but this will hold true only to a certain limit.

Once the JVMs approach 2GB+ heaps, adding more heap will start to degrade
performance because of factors like GC. So, after some point adding more instances would make sense.

Based on simple webapp tests, horizontal scaling is better until 2GB heaps and there after vertical scaling of JVMs would be a better option.