Datapaths; Potential Bandwidth-Related Problems - Red Hat ENTERPRISE LINUX 4 - INTRODUCTION TO SYSTEM ADMINISTRATION Administration Manual

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3.1.1.1. Examples of Buses
No matter where in a computer system you look, there are buses. Here are a few of the more common
ones:
Mass storage buses (ATA and SCSI)
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Networks 1 (Ethernet and Token Ring)
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Memory buses (PC133 and Rambus®)
•
Expansion buses (PCI, ISA, USB)
•

3.1.2. Datapaths

Datapaths can be harder to identify but, like buses, they are everywhere. Also like buses, datapaths
enable point-to-point communication. However, unlike buses, datapaths:
Use a simpler protocol (if any)
•
Have little (if any) mechanical standardization
•
The reason for these differences is that datapaths are normally internal to some system component
and are not used to facilitate the ad-hoc interconnection of different components. As such, datapaths
are highly optimized for a particular situation, where speed and low cost are preferred over slower and
more expensive general-purpose flexibility.
3.1.2.1. Examples of Datapaths
Here are some typical datapaths:
CPU to on-chip cache datapath
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Graphics processor to video memory datapath
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3.1.3. Potential Bandwidth-Related Problems

There are two ways in which bandwidth-related problems may occur (for either buses or datapaths):
1. The bus or datapath may represent a shared resource. In this situation, high levels of contention
for the bus reduces the effective bandwidth available for all devices on the bus.
A SCSI bus with several highly-active disk drives would be a good example of this. The highly-
active disk drives saturate the SCSI bus, leaving little bandwidth available for any other device
on the same bus. The end result is that all I/O to any of the devices on this bus is slow, even if
each device on the bus is not overly active.
2. The bus or datapath may be a dedicated resource with a fixed number of devices attached to it.
In this case, the electrical characteristics of the bus (and to some extent the nature of the protocol
being used) limit the available bandwidth. This is usually more the case with datapaths than with
buses. This is one reason why graphics adapters tend to perform more slowly when operating at
higher resolutions and/or color depths — for every screen refresh, there is more data that must
be passed along the datapath connecting video memory and the graphics processor.
1. Instead of an intra-system bus, networks can be thought of as an inter-system bus.
Chapter 3. Bandwidth and Processing Power