Mether-NFS: A Modified NFS Which Supports Virtual Shared Memory

Mether-NFS (MNFS) is a modified NFS which supports virtual shared memory. The model it supports is based on the Mether model. It is compatible in most ways with standard NFS. The major difference is in the way mapped files are handled. Mapped files, rather than adhering to standard NFS memory consistency semantics instead obey rules such that writes are globally ordered. This allows MNFS to be used to support shared memory programs. MNFS has been used on a 16-node SPARCStation ELC cluster to support shared memory programs. On appropriate applications we have realized supercomputer-level performance, which given the modest cost of the cluster implies a correspondingly higher price-performance. MNFS performance is superior to NFS performance in every case, but particularly in the case of write performance. Writes in MNFS are at least twice as fast as NFS. MNFS provides an extended memory operator, called Conditional Fetch-and-op (CFOP). CFOP is used to support memory synchronization. Programs using CFOP can atomically test and set a variable in 1.3 milliseconds. Our target performance goal for CFOP is 500 microseconds. In order to achieve this performance we have had to highly optimize the path through the SunOS kernel on both the client and server sides.