Luckily as it happens I am an expert in JVM Garbage Collectors :-). All of Sun Java's Garbage Collectors have a stop-the-world phase, the concurrent ones minimi

Luckily as it happens I am an expert in JVM Garbage Collectors :-). All of Sun Java's Garbage Collectors have a stop-the-world phase, the concurrent ones minimize the length of that stop-the-world period but it still has one. If you read the section on the concurrent collector on the link you posted you can see it says "During each major collection cycle, the concurrent collector will pause all the application threads for a brief period at the beginning of the collection and again toward the middle of the collection. The second pause tends to be the longer of the two pauses and multiple threads are used to do the collection work during that pause"