Accent

Abstract

Accent is a communication oriented operating system kernel being built at Carnegie-Mellon University to support the distributed personal computing project, Spice, and the development of a fault-tolerant distributed sensor network (DSN). Accent is. built around a single, powerful abstraction of communication between processes, with all kernel functions, such as device access and virtual memory management accessible through messages and distributable throughout a network. In this paper, specific attention is given to system supplied facilities which support transparent network access and fault-tolerant behavior.Many of these facilities are already being provided under a modified version of VAX/UNIX. The Accent system itself is currently being implemented on the Three Rivers Corp. PERQ.© 1981 A C M 0 -8 9 7 9 1 -0 6 2 -1 -1 2 / 8 1 -0 0 6 4 $00.75 -supporting the notion of multiple, independent virtual address spaces and a virtual machine specification which can accommodate diverse interpretations of process state;-supplying two kinds of protection: 1) address space protection, to insure that no process can affect another except through the use of the inter-process communication facility, and 2) access protection in the communication facility itself to prevent unauthorized communication between processes;defining inter-process communication in a way that allows transparent debugging, monitoring and fault recovery;taking advantage of these same mechanisms to allow transparent network extension independent of network hardware or protocols; allowing all services except the basic communication primitives to be viewed by processes as being provided through a communication interface; and structuring message communication to allow intermediary processes such as debuggers, protocol converters or network communication servers to better interpret the contents and purpose of messages.Accent stands out as a relatively pure example of a communication oriented operating system --i.e. an operating system which uses the abstraction of communication between processes as its basic organizing principle. The integration of virtual memory support, file access and inter-process communication in Accent makes possible performance improvements over previous communication oriented systems (e,g. [1]) as well as a more "transparent" network operating system design. System overviewPhysically, the distributed system built around Accent can be viewed as a loosely-connected collection of host machines, each with its own primary and secondary storage. The notion of a host machine is flexible in that it can include either uni-processors or tightly coupled multi-processors. In addition, no specific assumptions are made about the technology or topology of the underlying communication network which links together these host machines. The initial host machine chosen for Spice/DSN, the Three Rivers Corporation PERQ, is a uni-processor which currently possesses a 3MHz Ethernet [5] and will eventually possess a 10MHz Ethernet.Each host machine on the netwo...