C4 never pauses, and that's impressive. But there's no free lunch. The work the GC would do when the app is paused is sometimes being simply done by the app threads instead.
Shenandoah uses a forwarding pointer in each object, adding overhead but limiting the problem only to write barriers. Here is Christine commenting on Azul vs Shenandoah [2]
From the talk: average pause is 6-7ms, max is 15ms, and the talk is one year old.
She hints at further developments in a version 2 which would make it entirely pauseless.
She has made another talk at RedHat's DevNation conference a few days ago, but they just won't put the video online arg!
Did you know Objective-C does locks and retain counting without allocating any extra fields in objects?
Don't want compacting? You'll pay for it in allocation.
Don't want pausing? You'll pay for it in application threads.
> Value types can result in more copying, reducing performance over pointer indirections through nursery allocations
Having value types means you can pass by copy, but it also means you can allocate on the stack and pass by reference--in other words, you get performant passing without involving the GC.
To my knowledge this is false. AFAIK while the C4 algorithm is pauseless the C4 implementation is not. It's just that the pauses are really short.