Atomic Heap Transactions and Fine-grain Interrupts

Languages such as Java, ML, Scheme, and Haskell provide automatic storage management, that is; garbage collection. The two fundamental operations performed on a garbagecollected heap are "allocate" and "collect." Because the heap is in an inconsistent state during these operations, they must be performed atomically. Otherwise) a heap client might access the heap during a time when its fundamental invariants do not hold, corrupting the heap. Standard techniques for providing this atomicity guarantee have large latencies and other performance problems that impede their application in high-performance, interruptladen, thread-based systems applications. In particular, the standard techniques prevent thread schedulers from switching threads on VM page faults. We cast the space of possible implementations into a general taxonomy, and describe a new technique that provides a simple, low-overhead, low-latency interlock. We have implemented this technique in a version of SML/NJ, and, because of its applicability to thread-based systems, are currently implementing it in the scheduler of our raw-hardware SMLbased kernel, ML/OS. Our technique can be extended to provide other atomic sequences besides storage allocation.