Tempest and Typhoon: user-level shared memory

Future parallel computers must efficiently execute not only hand-coded applications but also programs written in high-level, parallel programming languages. Today´s machines limit these programs to a single communication paradigm, either message-passing or shared-memory, which results in uneven performance. The authors address this problem by defining an interface, Tempest, that exposes low-level communication and memory-system mechanisms so programmers and compilers can customize policies for a given application. Typhoon is a proposed hardware platform that implements these mechanisms with a fully-programmable, user-level processor in the network interface. The authors demonstrate the utility of Tempest with two examples. First, the Stache protocol uses Tempest´s fine-grain access control mechanisms to manage part of a processor´s local memory as a large, fully-associative cache for remote data. The authors simulated Typhoon on the Wisconsin Wind Tunnel and found that Stache running on Typhoon performs comparably (±30%) to an all-hardware Dir_NNB cache-coherence protocol for five shared-memory programs. Second, they illustrate how programmers or compilers can use Tempest´s flexibility to exploit an application´s sharing patterns with a custom protocol. For the EM3D application, the custom protocol improves performance up to 35% over the all-hardware protocol