The J-Machine: A fine-grain parallel computer

Most modern computers, whether parallel or sequential, are coarse grained. They are composed of physically large nodes with tens of megabytes of memory. Only a small fraction of the silicon area in the machine is devoted to computation. By increasing the ratio of computation area to memory area, fine-grain computers offer the potential of improving cost/performance by several orders of magnitude. To efficiently operate at such a fine grain, however, a machine must provide mechanisms that permit rapid access to global data and fast interaction between nodes. The MIT J-Machine is a fine-grain concurrent computer that provides low-overhead mechanisms for parallel computing. Prototype J-Machines have been operational since July 1991. The J-Machine communication mechanism permits a node to send a message to any other node in the machine in < 2 μs. On message arrival, a task is created and dispatched in < 1 μs. A translation mechanism supports a global virtual address space. These mechanisms efficiently support most proposed models of concurrent computation and allow parallelism to be exploited at a grain size of 10 operations. The hardware is an ensemble of up to 65,536 nodes each containing a 36-bit processor, 4K 36-bit words of on-chip memory, 256K words of DRAM and a router. The nodes are connected by a high-speed three-dimensional mesh network.