Sorta kinda all of the above. C4 the algo has no pauses, but individual threads stop to crawl their own stacks. i.e., threads stop doing mutator work, but only 1-by-1, and only for a very short partial self-stack crawl. C4 the impl I believe now has no pauses also. HotSpot the JVM has pauses, and yes much Zing was on forcing these pause times down. Cliff