> Azul sells a pauseless collector. AIUI the downside is that you need enough memory headroom and spare CPU cores for the collector to keep up with the mutator threads, which presumably is a significant overhead on large heaps.

I think that one of these vendors tried Azul and couldn't get it to work. (I don't know the details. If I understood correctly, the problem wasn't the GC so much as that Azul couldn't run their code correctly or would have required extensive modifications or something along those lines. In any event, they decided not to use Azul's product.)