BBN Pluribus at Wharton School, by Tony Patti, April 1979

The Pluribus''Pluribus'' is the ablative plural of the Latin word for "more" or "above."

multiprocessor Multiprocessing is the use of two or more central processing units (CPUs) within a single computer system. The term also refers to the ability of a system to support more than one processor or the ability to allocate tasks between them. There ar ...

was an early multi-processor computer designed by BBN for use as a

packet switch In telecommunications, packet switching is a method of grouping data into '' packets'' that are transmitted over a digital network. Packets are made of a header and a payload. Data in the header is used by networking hardware to direct the pack ...

in the

ARPANET The Advanced Research Projects Agency Network (ARPANET) was the first wide-area packet-switched network with distributed control and one of the first networks to implement the TCP/IP protocol suite. Both technologies became the technical fou ...

. Its design later influenced the

BBN Butterfly The BBN Butterfly was a massively parallel computer built by Bolt, Beranek and Newman in the 1980s. It was named for the "butterfly" multi-stage switching network around which it was built. Each machine had up to 512 CPUs, each with local memory, ...

computer. The Pluribus had its beginnings in 1972 when the need for a second-generation

interface message processor The Interface Message Processor (IMP) was the packet switching node used to interconnect participant networks to the ARPANET from the late 1960s to 1989. It was the first generation of gateways, which are known today as routers. An IMP was a r ...

(IMP) became apparent. At that time, the BBN had already installed IMPs at more than thirty-five

sites. These IMPs were

Honeywell 316 The Honeywell 316 was a popular 16-bit minicomputer built by Honeywell starting in 1969. It is part of the Series 16, which includes the Models 116 (1965, discrete), 316 (1969), 416 (1966), 516 (1966) and DDP-716 (1969). They were commonly used fo ...

and 516 minicomputers. The network was growing rapidly in several dimensions: number of nodes, hosts, and terminals; volume of traffic; and geographic coverage (including plans, now realized, for satellite extensions to

Europe Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a Continent#Subcontinents, subcontinent of Eurasia ...

and Hawaii). A goal was established to design a modular machine which, at its lower end, would be smaller and less expensive than the 316's and 516's while being expandable in capacity to provide ten times the bandwidth of, and capable of servicing five times as many input-output (I/O) devices as, the 516. Related goals included greater memory addressing capability and increased reliability. The designers decided on a multiprocessor approach because of its promising potential for modularity, for cost per performance advantages, for reliability, and because the IMP packet switch algorithms were clearly suitable for parallel processing by independent processors.

Hardware

A Pluribus consisted of two or more standard 19" electronic equipment racks, each divided into four bays. Each bay contained a backplane bus and an independent power supply. A bay might contain a processor bus, a shared memory bus, or an I/O bus. Custom-built bus couplers connected the bays to one another so that the processors could reach the shared memory and the I/O devices. A 6-processor Pluribus was used as a network switch to interconnect BBN's five Tenex/"Twenex" timesharing systems along with 378 terminals on direct serial and dial-in modem lines. The Pluribus used the

Lockheed SUE Lockheed (originally spelled Loughead) may refer to: Brands and enterprises * Lockheed Corporation The Lockheed Corporation was an American aerospace manufacturer. Lockheed was founded in 1926 and later merged with Martin Marietta to form L ...

as its processor. The SUE was similar to DEC's

PDP-11 The PDP-11 is a series of 16-bit minicomputers sold by Digital Equipment Corporation (DEC) from 1970 into the 1990s, one of a set of products in the Programmed Data Processor (PDP) series. In total, around 600,000 PDP-11s of all models were sold, ...

Software

The Pluribus software implemented

MIMD In computing, multiple instruction, multiple data (MIMD) is a technique employed to achieve parallelism. Machines using MIMD have a number of processors that function asynchronously and independently. At any time, different processors may be exe ...

symmetric multiprocessing. Software processes were implemented using non-preemptive multiprogramming. Process scheduling used a hardware device, called the ''pseudo-interrupt device'' or PID, that was accessible to both programs and to I/O devices. Each processor ran its own copy of the process scheduler, which would read an integer value from the PID. The value was used to select the process to run. If a program or device needed to signal another process to run, it would write that process' number into the PID. The PID would emit the highest priority process that anyone had requested, and served them out to all processors. An important aspect of the Pluribus software was the "STAGE" system, which detected system errors and took steps to recover from them. The processor clocks had interrupt handlers which implemented watchdog timers on all processors. If a processor stopped running, another processor would detect it and initiate a recovery. The recovery process would unlock any

locks Lock(s) may refer to: Common meanings *Lock and key, a mechanical device used to secure items of importance *Lock (water navigation), a device for boats to transit between different levels of water, as in a canal Arts and entertainment * ''Lock ...

placed on shared resources, release allocated storage, and restart all processing on all processors. This was acceptable on an ARPANET routing node, since any lost packets would eventually be retransmitted.

Hardware

Software

References

Further reading