Just an FYI to your last paragraph: The core indexing/ranking/storage components of Vespa are C++, and run in a separate process (no jni).
In my own attempt to compare the two, I found the memory consumption of Vespa was easier to predict and understand (there are formulas for it in the documentation).
In my own attempt to compare the two, I found the memory consumption of Vespa was easier to predict and understand (there are formulas for it in the documentation).