DIPC: The Linux Way of Distributed Programming

This article discusses the main characteristics of Distributed Inter-Process Communication (DIPC), a relatively simple system software that provides users of the Linux operating system with both the distributed shared memory and the message passing paradigms of distributed programming.

Before Linux, powerful UNIX operating systems were considered a luxury. Linux made it possible for ordinary people to have access to an affordable and reliable computing platform. The only problem is that Linux was originally based on decades-old designs (see Resources 7), making it less attractive for more technically minded users. Linux's answer to this problem is either port and adaptation or introduction of newer concepts.

Building multi-computers (see Resources 1) and programming them are among the more popular research subjects and demand for them is rapidly rising. Any solution to distributed programming under Linux should keep up with one of Linux's more important features: availability to ordinary users.

Motivation

Linux already had symmetric multi-processing capabilities. However, it did not provide enough standard facilities for distributed software development. Programmers and users had to resort to different add-on packages and various programming models to write or use distributed software. The mechanisms provided by these packages usually differed greatly from one another, each requiring users to learn some new material which was not of any use to them when migrating to other methods. Many also required detailed involvement of the programmer in the process of transferring data over the network; an example is the PVM (Parallel Virtual Machine) software (see Resources 8). The need for a simpler distributed programming model, usable by more programmers, was obvious.

What is Distributed Programming?

The DIPC Software

DIPC (Distributed Inter-Process Communication) is a software-only solution for enabling people to build and program multiple computers easily. Each node can be an ordinary personal computer. These nodes must be connected to each other by a TCP/IP (see Resources 3) network. It does not use network broadcasting, which helps it work in networks without such capabilities. Also, no assumption of a synchronized clock is made. These features mean that DIPC could be used in a wide area network (WAN).

Right from the start, it was decided that ease of application programming and the simplicity of the DIPC itself should be among the most important factors in the system design, even if it were to mean some loss in performance. This decision was backed by the observation that computing and telecommunications equipment speeds are improving rapidly, while training and programming times for distributed applications are not.

In DIPC, UNIX System V IPC mechanisms (see Resources 4), consisting of semaphores, messages and shared memories, are modified to function in a network environment. This means that installing DIPC requires changing and recompiling the kernel. Here, the same system calls used to provide communication between processes running on the same computer can be used to allow the communication of processes running on different machines. There is no new system call for the application programmer's use. There is also no library to be linked to the application code, and no need for any modifications in compilers. DIPC could be used with any language that allows access to the operating system's system calls. It is completely camouflaged in the kernel.

The result is that DIPC supports both the message passing and the distributed shared memory paradigms of distributed programming, providing more options for the application programmer (see Resources 5). It also allows the processes to share only selected parts of their address space in order to reduce the problems of false sharing.

It was decided to implement DIPC in the user space as much as possible, with minimal changes to the kernel. This leads to a cleaner and simpler design, but in a monolithic operating system such as Linux it has the drawback of requiring frequent copy operations between kernel and user address spaces (see Resources 2). As UNIX does not allow user space processes to access and change kernel data structures at will, DIPC must have two parts. The more important part is a program named dipcd, which runs with superuser privileges; dipcd forks several processes to do its work. The other part is inside the kernel giving dipcd work and letting it see and manipulate kernel data. The two parts use a private system call to exchange data. This system call must not be used by other processes in the system.

DIPC provides easy data transfer over the network and assumes that the code to use these data already resides at the suitable places. This is justifiable when one considers that in most cases, the program's code changes much less frequently than the data.

About the Example Distributed Program

Listing 1. Example Distributed Program

DIPC is only concerned with providing mechanisms for distributed programming. The policies, e.g., how a program is parallelized, or where an application program's processes should run, are determined by the programmer or the end user.

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