Is there a GC in Java that does not introduce latency (stop-the-world) by perhaps running 100% concurrently?

You might be interested in something like the Zing JVM.

Azul systems have put a lot of effort into low-latency JVM tuning. I believe there are also some interesting technical papers that explain how this is done.

It sounds like you're looking for the CMS GC — concurrent mark sweep.

^{(source: Memory Management in the JavaHotSpot™ Virtual Machine, p.12)}

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December 2015 update: you might be interested in Azul's "pauseless" GC. See Explanation of Azul's "pauseless" garbage collector for more.

I don't think there are any zero-pause garbage collectors. But generally speaking a low-pause collector like CMS will do the job.

The problems with pauses and pause reduction are as follows:

• A zero-pause (or almost zero-pause) collection regime significantly increases overheads for normal program execution. This is particularly true with multithreaded languages like Java.

• Schemes that use a dedicated thread or threads to do garbage collection can get swamped if the application generates too much garbage.

• Any GC scheme will give you poor performance if the application's memory usage patterns are too "lumpy" and/or you don't have enough physical and virtual memory.