Not every GC pass. Some modern GCs (like OpenJDK's default GC, G1) track mutation and maintain "remembered sets." They only need to scan objects that have chang

Not every GC pass. Some modern GCs (like OpenJDK's default GC, G1) track mutation and maintain "remembered sets." They only need to scan objects that have changed since the last scan (they're actually even more efficient than that, but this is a simplification). In addition, when this scanning is required, it is performed concurrently to the application, so the technique of moving the work to a separate thread is done automatically.