During the sweep phase, not only do references change, but objects move around in memory. This poses a new set of problems for application threads executing concurrently. Among these problems:
• Copying an object is not an atomic operation. This means that after part of the object has been copied, the original is still being modified by the application.
• Updating references to the object is not an atomic operation. This means that some parts of the application might be using the old object reference and some other parts might be using the new one.
Addressing these problems is possible (Azul Pauseless GC for JVM, http://www.azulsystems.com/zing/pgc, is one example), but has not been done in the CLR GC. It is simpler to declare that the sweep phase does not support application threads executing concurrently with the garbage collector.引自 Pausing Threads during the Sweep Phase
