That's a realtime GC and yes, you can buy a JVM from IBM that has one, they call it the Metronome collector:

http://researcher.watson.ibm.com/researcher/view_group_subpa...

Metronome has pause time in the hundreds of microseconds range. The tradeoff it makes is that, amongst other things, it requires read barriers i.e. every time you read a pointer from the heap you have to execute some extra code to interact with the GC subsystem. If you think about how often you read pointers from the heap in a typical program, well, that turns into a lot of barriers and a corresponding execution performance hit.