LOCUS (operating system)
Encyclopedia
LOCUS was a distributed operating system
developed at UCLA during the 1980s. It was notable for providing an early implementation of the single-system image
idea, where a cluster
of machines appeared to be one larger machine.
A desire to commercialize the technologies developed for LOCUS inspired the creation of the Locus Computing Corporation
which went on to include ideas from LOCUS in various products, including OSF/1 AD and, finally, the SCO–Tandem
UnixWare NonStop Clusters
product.
/45s using 1 and 10 megabit ring networks, by 1983 the system was running on 17 VAX-11/750
s using a 10 megabit Ethernet
. The system was Unix
compatible and provided both a single root view of the file system and a unified process space across all nodes.
The development of LOCUS was supported by an ARPA research contract, DSS-MDA-903-82-C-0189.
, the data of files could be stored on more than one node and LOCUS would keep the various copies up to date. This provided particularly good access times for files that were read more often than they were written, the normal case for directories for example.
In order to ensure that all access was made to the most recent version of any file LOCUS would nominate one node as the "current synchronization site" (CSS) for a particular file system. All accesses to files a file system would need to be coordinated with the appropriate CSS.
systems LOCUS sometimes found it necessary to break the illusion of a single system, notably to allow some files to be different on a per-node basis. For example it was possible to build a LOCUS cluster containing both PDP-11/45 and VAX 750 machines, but instruction sets used were not identical, so two versions of each object program would be neededRather like Apple Fat binary
files
The solution was to replace the files that needed to be different on a per node basis by special hidden directories. These directories would then contain the different versions of the file. When a user accessed one of these hidden directories the system would check the users context and open the appropriate file.
For example, if the user was running on one of the PDP-11/45's and typed the command
and exec
calls would examine an advice list which determined on which node the process would be run. LOCUS was designed to work with heterogeneous nodes, (e.g. a mix of VAX 750s and PDP 11/45s) and could decide to execute a process on a different node if it needed a particular instruction set. As an optimization a run call was added which was equivalent to a combined fork and exec, thus avoiding the overhead of copying the process memory image to another node before overwriting it by the new image.run is the same operation as spawn
on Windows
systems.
for inter node communication, including named pipe
s,
the disconnected nodes could continue to access files. When the nodes were reconnected any files modified by the disconnected nodes would be merged back into the system. For some file types (for example mailboxes) the system would perform the merge automatically, for others the user would be informed (by mail) and tools were provided to allow access to the different versions of the file.
Distributed operating system
A distributed operating system is the logical aggregation of operating system software over a collection of independent, networked, communicating, and spatially disseminated computational nodes. Individual system nodes each hold a discrete software subset of the global aggregate operating system...
developed at UCLA during the 1980s. It was notable for providing an early implementation of the single-system image
Single-system image
In distributed computing, a single system image cluster is a cluster of machines that appears to be one single system. The concept is often considered synonymous with that of a distributed operating system, but a single image may be presented for more limited purposes, just job scheduling for...
idea, where a cluster
Cluster (computing)
A computer cluster is a group of linked computers, working together closely thus in many respects forming a single computer. The components of a cluster are commonly, but not always, connected to each other through fast local area networks...
of machines appeared to be one larger machine.
A desire to commercialize the technologies developed for LOCUS inspired the creation of the Locus Computing Corporation
Locus Computing Corporation
Locus Computing Corporation was formed in 1982 by Gerald J. Popekto commercialize the technologies developed for the LOCUS distributed operating system at UCLA...
which went on to include ideas from LOCUS in various products, including OSF/1 AD and, finally, the SCO–Tandem
Tandem Computers
Tandem Computers, Inc. was the dominant manufacturer of fault-tolerant computer systems for ATM networks, banks, stock exchanges, telephone switching centers, and other similar commercial transaction processing applications requiring maximum uptime and zero data loss. The company was founded in...
UnixWare NonStop Clusters
UnixWare NonStop Clusters
NonStop Clusters was an add-on package for SCO UnixWare that allowed creation of fault-tolerant single-system image clusters of machines running UnixWare...
product.
Description
The LOCUS system was created at UCLA between 1980 and 1983, initial implementation was on a cluster of PDP-11PDP-11
The PDP-11 was a series of 16-bit minicomputers sold by Digital Equipment Corporation from 1970 into the 1990s, one of a succession of products in the PDP series. The PDP-11 replaced the PDP-8 in many real-time applications, although both product lines lived in parallel for more than 10 years...
/45s using 1 and 10 megabit ring networks, by 1983 the system was running on 17 VAX-11/750
VAX-11
The VAX-11 was a family of minicomputers developed and manufactured by Digital Equipment Corporation using processors implementing the VAX instruction set architecture . The VAX-11/780 was the first VAX computer.- VAX-11/780 :...
s using a 10 megabit Ethernet
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
. The system was Unix
Unix
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
compatible and provided both a single root view of the file system and a unified process space across all nodes.
The development of LOCUS was supported by an ARPA research contract, DSS-MDA-903-82-C-0189.
File system
In order to allow reliable and rapid access to the cluster wide filesystem LOCUS used replicationReplication (computer science)
Replication is the process of sharing information so as to ensure consistency between redundant resources, such as software or hardware components, to improve reliability, fault-tolerance, or accessibility. It could be data replication if the same data is stored on multiple storage devices, or...
, the data of files could be stored on more than one node and LOCUS would keep the various copies up to date. This provided particularly good access times for files that were read more often than they were written, the normal case for directories for example.
In order to ensure that all access was made to the most recent version of any file LOCUS would nominate one node as the "current synchronization site" (CSS) for a particular file system. All accesses to files a file system would need to be coordinated with the appropriate CSS.
Node dependent files
As with other SSISingle-system image
In distributed computing, a single system image cluster is a cluster of machines that appears to be one single system. The concept is often considered synonymous with that of a distributed operating system, but a single image may be presented for more limited purposes, just job scheduling for...
systems LOCUS sometimes found it necessary to break the illusion of a single system, notably to allow some files to be different on a per-node basis. For example it was possible to build a LOCUS cluster containing both PDP-11/45 and VAX 750 machines, but instruction sets used were not identical, so two versions of each object program would be neededRather like Apple Fat binary
Fat binary
A fat binary is a computer program with code native to multiple Instruction sets which can consequently be run on multiple processor types. The usual method of implementation is to include a version of the machine code for each instruction set, preceded by code compatible with all operating...
files
The solution was to replace the files that needed to be different on a per node basis by special hidden directories. These directories would then contain the different versions of the file. When a user accessed one of these hidden directories the system would check the users context and open the appropriate file.
For example, if the user was running on one of the PDP-11/45's and typed the command
/bin/who then the system would find that /bin/who was actually a hidden directory and run the command /bin/who/45. Another user on a VAX node who typed /bin/who would run the command /bin/who/vax.Processes
LOCUS provided a single process space. Processes could be created on any node on the system. Both the Unix forkFork (operating system)
In computing, when a process forks, it creates a copy of itself. More generally, a fork in a multithreading environment means that a thread of execution is duplicated, creating a child thread from the parent thread....
and exec
Exec (operating system)
The exec collection of functions of Unix-like operating systems cause the running process to be completely replaced by the program passed as an argument to the function...
calls would examine an advice list which determined on which node the process would be run. LOCUS was designed to work with heterogeneous nodes, (e.g. a mix of VAX 750s and PDP 11/45s) and could decide to execute a process on a different node if it needed a particular instruction set. As an optimization a run call was added which was equivalent to a combined fork and exec, thus avoiding the overhead of copying the process memory image to another node before overwriting it by the new image.run is the same operation as spawn
Spawn (computing)
Spawn in computing refers to a function that loads and executes a new child process.The current process may or may not continue to execute asynchronously...
on Windows
Microsoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
systems.
Pipes
Processes could use pipesPipeline (Unix)
In Unix-like computer operating systems , a pipeline is the original software pipeline: a set of processes chained by their standard streams, so that the output of each process feeds directly as input to the next one. Each connection is implemented by an anonymous pipe...
for inter node communication, including named pipe
Named pipe
In computing, a named pipe is an extension to the traditional pipe concept on Unix and Unix-like systems, and is one of the methods of inter-process communication. The concept is also found in Microsoft Windows, although the semantics differ substantially...
s,
Partitioning
The LOCUS system was designed to be able to cope with network partitioning - one or more nodes becoming disconnected from the rest of the system. As the file system was replicatedReplication (computer science)
Replication is the process of sharing information so as to ensure consistency between redundant resources, such as software or hardware components, to improve reliability, fault-tolerance, or accessibility. It could be data replication if the same data is stored on multiple storage devices, or...
the disconnected nodes could continue to access files. When the nodes were reconnected any files modified by the disconnected nodes would be merged back into the system. For some file types (for example mailboxes) the system would perform the merge automatically, for others the user would be informed (by mail) and tools were provided to allow access to the different versions of the file.