> Some of those systems come from vendors who have very competent teams and invest heavily in their systems, and some have tried fancy, expensive JVMs with supposedly better real-time performance and have failed to get them to work.

This is very important. There's far too much No True Scotsman in typical discussions around this. The fact remains that really good teams try and fail to meet needed performance criteria, even though their app/service meets all other requirements, is well architected, well coded, and the team has experience in GC tuning, pitfalls, etc.

> Maybe it can be done, but that doesn't mean that you're going to be able to do it, especially if you don't design all parts of your application from the beginning to operate within the constraints of real-time GC implementations.

Because... unless your solution is one where you can simply avoid doing a few stupid things that make the GC unhappy, you have to wade deep into managing memory, except you can't do it directly. You have to understand/infer how seemingly innocent code impacts GC, and then suggest/cajole/plead with the GC to do what you want. It required the kind of intimate knowledge that GC is supposed to save you from, but doesn't have the direct manipulation tools.

There are large classes of problems where GC is perfectly fine. Great even. But there are other problem domains where sluggishness or pauses are not acceptable, and not just realtime or soft realtime.