Mirage+: A Kernel Implementation of Distributed Shared Memory on a Network of Personal Computers

We describe the evolution of a distributed shared memory (DSM) system, Mirage, and the di culties encountered when moving the system from a Unix-based kernel on the VAX to a UNIX-based kernel on personal computers. Mirage provides a network transparent form of shared memory for a loosely coupled environment. The system hides network boundaries for processes that are accessing shared memory and is upward compatible with the Unix System V Interface De nition. This paper addresses the architectural dependencies in the design of the system and evaluates performance of the implementation. The new version, Mirage+, performs well compared to Mirage even though eight times the amount of data is sent on each page fault because of the larger page size used in the implementation. We show that performance of systems with a large page size to network packet size can be dramatically improved on conventional hardware by applying three well-known techniques: packet blasting, compression, and running at interrupt level. The measured time for a page fault in Mirage+ has been reduced 37% by sending a page using packet blasting instead of using a handshake for each portion of the page. When compression was added to Mirage+, the time to fault a page across the network was further improved by 47% when the page was compressed into one network packet. Our measured performance compares favorably with the amount of time it takes to fault a page from disk. Lastly, running at interrupt level may improve performance 16% when faulting pages without compression. 1 Unix is a Registered Trademark of Unix International.