> so what you mean is that, even after throwing facebook scale resources at java.. it is possible for a <10 people team to get 10X performance over java using the features that you mention.

That is not what I mean. Writing a userspace TCP stack isn't a feature of C++.

Explicit use of vector intrinsics are not the source of any 10x performance boost, nor anything to do with C++ in particular. Again, only a small (but probably not minuscule) portion of the difference has to do with Java vs. C++. The bulk of the difference is due to all sorts of optimizations, most of them could have been done in Java as well. But the ScyllaDB people are more experienced in C++ than in Java, and as they use sequential code anyway, there isn't a big downside for using C++ -- certainly not for them -- so it was the better choice. From what little I know, the reasons why such optimizations weren't done in Cassandra are because 1. the people working on it aren't low-level optimization experts, but more importantly, 2. because the performance was good enough.

You aren't comparing the same program written in two languages. Seastar stuff is written by C++ performance experts who are fanatical about tuning, and does all kinds of unusual far-out things that Cassandra doesn't do to get high performance.

> That's a huge loss of face for java IMHO

Because the JVM JIT, like every other compiler, sucks at developing vectorized algorithms ad-hoc; a task usually carried out by human experts in that?