2.5 Motherboard

The motherboard is a printed circuit board that contains most of the active electronic components of the PC node and their interconnection. The motherboard provides the logical and physical infrastructure for integrating the subsystems of a cluster node and determines the set of components that may be used. The motherboard defines the functionality of the node, the range of performance that can be exploited, the maximum capacities of its storage, and the number of subsystems that can be interconnected. With the exception of the microprocessor itself, the selection of the motherboard is the most important decision in determining the qualities of the PC node to be used as the building block of the system. It is certainly the most obvious piece of a node other than the case or packaging in which it is enclosed.

The motherboard integrates all of the electronics of the node in a robust and configurable package. Sockets and connectors on the motherboard include the following:

Microprocessor(s)
Memory
Peripheral controllers on the PCI-X bus
AGP port
Floppy disk cables
ATA or SCSI cables for hard disk and CD-ROM
Power
Front panel LEDs, speakers, switches, and so forth.
External I/O for mouse, keyboard, joystick, serial line, sound, USB, and so forth.

Other chips on the motherboard provide

the system bus that links the processor(s) to memory,
the interface between the peripheral buses and the system bus, and
programmable read-only memory (PROM) containing the BIOS software.

As the preceding lists show, motherboards are an amalgamation of all of the buses and many peripherals in a cluster node. The memory bus is contained within the motherboard. All I/O buses a system supports are also included here. As data movement is the most serious impediment to achieving peak processor performance, the motherboard is one of the single most important components in a system.

We note that the motherboard restricts as well as enables functionality. In selecting a motherboard as the basis for a cluster node, one should consider several requirements including

processor family,
processor clock speed,
number of processors,
memory capacity,
memory type,
disk interface,
required I/O slots
number and types of I/O buses

2.5.1 Chipsets

Chipsets are a combination of all of the logic on a motherboard. Typically included are the memory bus, PCI, PCI-X and AGP bridges. In many cases, integrated peripherals are also part of the chipset. This may include disk controllers and USB controllers. Because the chipset combines all of these components, performance properties of single components are often attributed to the chipset itself.

The chipset is split into two logical portions. The north bridge connects the front side bus, which connects the processor, the memory bus, and AGP. AGP is located on the north bridge so as to have special access to main memory. The south bridge contains I/O bus bridges and any integrated peripherals that may be included, like disk and USB controllers. This provides controllers for all of the simple devices mentioned later in the peripherals section.

2.5.2 BIOS

The BIOS is the software that initializes all system hardware into a state such that the operating system can boot. BIOSes are not universal; that is, the BIOS included with a motherboard is specifically tailored to that motherboard. The BIOS is the first software that runs after the system is powered up. The BIOS will start by running a power on self test (POST) that includes this ubiquitous memory test. POST also checks other major systems. The BIOS runs initialization code present on peripherals, including controller-specific code that initializes SCSI or IDE buses. Once these steps are completed, the BIOS locates a drive to boot from, and does so.

Figure 2.1: Block diagram of a motherboard chipset. The chipset consists of the entire diagram excluding the processor and memory.

PXE (Pre-execution environment) is a system by which nodes can boot based on a network-provided configuration and boot image. The system is implemented as a combination of two common network services. First, a node will DHCP for an address. The DHCP server will return an offer and lease with extra PXE data. This extra data contains an IP address of a tftp server, a boot image filename (that is served from the server), and an extra configuration string that is passed to the boot image. Most new machines support this, and accordingly many cluster management software systems use this feature for installations. This feature is implemented by the BIOS in motherboards with integrated ethernet controllers, and in the on-card device initialization code on add-on ethernet controllers.

LinuxBIOS is a BIOS implementation based on the Linux kernel. It can perform all important tasks needed for an operating system to boot. These tasks are largely the same as proprietary BIOSes, but some of these steps have been streamlined in such a way that all operating systems do not function properly when booted from LinuxBIOS. At this point, Linux and Windows 2000 are supported. Work is under way to supply all BIOS features necessary to run other operating systems as well. This approach offers several benefits. Since source code is available for LinuxBIOS, the potential exists for users to fix BIOS bugs. LinuxBIOS is also performs far better than proprietary BIOSes in terms of boot time. This reduction has yielded boot times under five seconds. This speed is far better than times in the ten to ninety second range seen with proprietary BIOSes. This performance increase doesn't affect user applications, as most user applications don't require node reboots.