DIPC: The Linux Way of Distributed Programming
Distributed Shared Memory (DSM) (see Resources 6) in DIPC uses a multiple-readers/single-writer protocol. DIPC replicates the contents of the shared memory in each computer with reader processes so they can work in parallel, but there can be only one computer with processes that write to a shared memory. The strict consistency model is used here, meaning that a read will return the most recently written value. It also means there is no need for the programmer to do any special synchronization activity when accessing a distributed shared memory segment. The major disadvantage with this scheme is a possible loss of performance in comparison to other DSM consistency models.
DIPC can be configured to provide a segment-based or page-based DSM. In the first case, DIPC transfers the entire contents of the shared memory from computer to computer, with no regard as to whether all data will be used. This could reduce the data transfer administration time. In the page-based mode, 4KB pages are transferred as needed, making possible multiple parallel writes to different pages.
In DIPC, each computer is allowed to access the shared memory for at least a configurable time quantum. This lessens the chance of the shared memory being transferred frequently over the network, which could result in bad performance.
DIPC assumes a fail-stop (see Resources 9) distributed environment, so it employs time-outs to find out about any problem. The at-most-once semantics (see Resources 1) is used here, meaning DIPC tries everything just once. In case of error, it simply informs the relevant processes, either by a system call return value or, for shared memory read/writes, by a signal. DIPC itself does not do anything to overcome the problem. The user processes should decide how to deal with the error. This is normal behavior in many other cases in UNIX .
It is important to provide some means to make sure that the data are accessed only by people with proper permissions. DIPC uses login names, not user IDs, to identify users. Remote operations are performed after assuming the identity of the person who executed the system call originally. For this to work, one login name on all computers in a DIPC cluster should denote the same person.
In order to prevent intrusion to DIPC clusters, addresses of the computers allowed to take part in a cluster should be put in a file for DIPC to consult.
DIPC is under development mainly in the Iran University of Science and Technology's (IUST) Department of Computer Engineering, but people from different parts of the world are currently working on it. A port to Linux for Motorola 680x0 processors has been completed. This made DIPC a heterogeneous system, as the two versions can communicate with each other. DIPC's sources and related documents can be found on the Internet via anonymous FTP at sunsite.unc.edu, in /pub/Linux/system/network/distrib/, or can be downloaded from DIPC's web page at http://wallybox.cei.net/dipc/.
DIPC belongs to the Linux users community, and the ultimate goal is to make it an integral part of the Linux operating system. Considering the inadequacy of computing and informational facilities in IUST, the only way to make sure this software will survive is for interested people to join in its development.
To subscribe to DIPC's mailing list, send e-mail to email@example.com with the body containing “subscribe linux-dipc”. Postings should go to firstname.lastname@example.org.
DIPC is a simple distributed system that works by bringing new functionality to an IPC system designed decades ago. Many of the DIPC's nicer features are the result of its being hidden inside the kernel. Considering its main characteristics, DIPC has the potential to introduce ordinary programmers to distributed programming, thus making Linux one of the first operating systems with usable and really used distributed programming facilities.
Several experimental distributed systems are available for use. Many of them have been implemented in universities running UNIX variants on workstations produced by different manufacturers. The fact that, in most cases, researchers did not have free access to the underlying operating system's source code has had a big influence on the design decisions. The availability of source code in Linux has provided new ways to deal with the problems of distributed programming. DIPC is an example of what can be done when one has access to the operating system sources. Some could mention the problems in porting DIPC to proprietary operating systems with no publicly available source code as a drawback. However, in our opinion, proprietary operating system vendors and their users are the ones at a loss here, as they cannot take advantage of more easy-to-use distributed systems developed by third parties. This statement does not mean DIPC could not be implemented in other UNIX variants supporting System V IPC, but implies that the port can only be attempted by people with access to kernel source code.