Chapter 2. Building a Beowulf Cluster

A Beowulf cluster is nothing more than a bunch of computers interconnected by Ethernet and running with a Linux or BSD operating system. A key feature is the communication over IP (Internet Protocol) that distributes problems among the boards. The entity of the boards or computers is called a cluster and each board or computer is called a node.

In this chapter, we will first see what is really required for each board to run inside a cluster environment. You will see examples of how to build a cheap and scalable cluster housing and how to modify an ATX power supply in order to use it as a power source. I will then explain the network interconnection of the Beowulf cluster and have a look at its network topology. The chapter concludes with an introduction to the microSD card usage for installation images and additional swap space as well as external network storage.

The following topics will be covered:

• Describing the minimally required equipment

• Building...

Minimal configuration and optional equipment

BBB is a single-board computer that has all the components needed to run Linux distributions that support ARMhf platforms. Due to the very powerful network utilities that come with Linux operating systems, it is not necessary to install a mouse or keyboard. Even a monitor is not required in order to install and configure a new BBB. First, we will have a look at the minimal configuration required to use a single board over a network.

Minimal configuration

A very powerful interface of Linux operating systems is its standard support for SSH. SSH is the abbreviation of Secure Shell, and it enables users to establish an authenticated and encrypted network connection to a remote PC that provides a Shell. Its command line can then be utilized to make use of the PC without any local monitor or keyboard. SSH is the secure replacement for the telnet service. The following diagram shows you the typical configuration of a local area network using SSH for the...

Building a scalable board-mounting system

The following image shows you the finished board housing with its key components as well as some installed BBBs. Here, a indicates the threaded rod with the straw as the spacer, b indicates BeagleBone Black, c indicates the Ethernet cable, d indicates 3.5" hard disc cooling fans, e indicates the 5 V power cable, and f indicates the plate with drilled holes.

The finished casing with installed BBBs

One of the most important things that you have to consider before building a super computer is the space you require. It is not only important to provide stable and practical housing for some BBB boards, but also to keep in mind that you might want to upgrade the system to more boards in the future. This means that you require a scalable system that is easy to upgrade. Also, you need to keep in mind that every single board requires its own power and has to be accessible by hand (reset, boot-selection, and the power button as well as the memory card, and so...

Using a low-cost power source

I have seen a picture on the Web where somebody powered a dozen older Beagle Boards with a lot of single DC adapters and built everything into a portable case. The result was a huge mess of cables. You should always try to keep your cables well organized in order to save space and improve the cooling performance. Using an ATX power supply with a cable tree can save you a lot of money compared to buying several standalone power supplies. They are stable and can also provide some protection for hardware, which cheap DC adapters don't always do. In the following section, I will explain the power requirements and how to modify an ATX power supply to fit our needs.

Power requirements

If you do not use an additional keyboard and mouse and only onboard flash memory, one board needs around 500 mA at 5 V voltage, which gives you a total power of 2.5 Watts for one board. Depending on the installed memory card or other additional hardware, you might need more.

Note

Please...

Setting up the network backbone

To interconnect BBB boards via Ethernet, we need a switch or a hub. There is a difference in the functionality between a switch and a hub:

• With hubs, computers can communicate with each other. Every computer is connected to the hub with a separate Ethernet cable. The hub is nothing more than a multiport repeater. This means that it just repeats all the information it receives for all other ports, and every connected PC has to decide whether the data is for it or not. This produces a lot of network traffic and can slow down the speed.

• Switches in comparison can control the flow of network traffic based on the address information in each packet. It learns which traffic packets are received by which PC and then forwards them only to the proper port. This allows simultaneous communication across the switch and improves the bandwidth. This is the reason why switches are the preferred choice of network interconnection for our BBB Beowulf cluster.

The following table...

The network topology

The typical network topology is a star configuration. This means that every BBB board has its own connection to the switch, and the switch itself is connected to the local area network (LAN). On most Beowulf clusters, there is one special board called the master node. This master node is used to provide the bridge between the cluster and the rest of the LAN. All users (if there are more persons that use the cluster) log in to the master node, and it is only responsible for user management and starting the correct programs on specified nodes. It usually doesn't contribute to any calculation tasks.

However, as BBB only has one network connector, it is not possible to use it as a bridge, because a bridge requires two network ports:

• One connected to the LAN.

• The other connected to the switch of the cluster.

Because of this, we only define one node as the master node, providing some special software features but also contributing to the calculations of the cluster. This way...

The storage memory

One thing you might want to think of in the beginning is the amount of space you require for applications and data. The standard version of BBB has 2 GB flash memory onboard and newer ones have 4 GB. In this book, I describe my strategy of using one additional memory card for the master node and additional external network storage for shared libraries.

A critical feature of computational nodes is the amount of RAM they have installed. On BBB, this is only 512 MB. If you are of the opinion that this is not enough for your tasks, then you can extend the RAM by installing Linux on an external SD card and create a swap partition on it. However, you have to keep in mind that the external swap space is much slower than the DDR3 memory (MB/s compared to GB/s). If the software is nicely programmed, data can always be sufficiently distributed on the nodes, and each node does not need much RAM. However, with more complicated libraries and tasks, you might want to upgrade some day...

Summary

In this chapter, you were introduced to the whole cluster concept regarding its hardware and interconnection. You were shown a working system configuration using only the minimally required amount of equipment and also some optional possibilities. A description of very basic housing including a cooling system was given as an example for a cheap yet nicely scalable possibility to mount the boards. You also learned how to build a cost-efficient power supply using a widely available ATX supply, and you were shown how to modify it to power several BBBs. Finally, you were introduced to the network topology and the purpose of network switches. A short description about the used storage system used throughout this book ended this chapter.

If you interconnect everything as described in this chapter, it means that you have created the hardware basis of a super computer cluster.

In the next chapter, you will be learning in detail how to install the operating system and how to configure master...