ownership of the bus, permitting among other things direct memory access transfers without processor intervention. Arbitration among the pending PCI masters for the next transfer action can be overlapped with the current PCI bus operation, thereby hiding the arbitration latency and ensuring high sustained bus throughput.
High throughput is enabled by a process called linear burst transfers. A block of data being sent from one device to another on the PCI bus is moved without having to send the address of each word of data. Instead, the length of the block is specified along with the initial address of the location where the block is to be moved. Each time a word is received, the accepting unit increments a local address register in preparation for the next word of the block. PCI bus transfers can be conducted concurrently with operation of the processor and its system bus to avoid processor delays caused by PCI operation.
Although bus loading limits the number of PCI sockets to three or four, each connected board can logically represent as many as eight separate PCI functions for a total of 32. Up to 256 PCI buses can be incorporated into one system, although one rarely sees more than two.
The PCI standard includes complete bit-level specification of configuration registers. This makes possible the automatic configuration of connected peripheral devices for plug-n-play reconfigurability and elimination of jumpers.
3.8 Examples of a Beowulf Node
The following are examples of two classes of nodes derived from experience with two Beowulf systems in use today. There are many other choices and the products of other vendors in many cases are as worthy of consideration as the ones listed here. But these two systems provide useful starting points as you derive your own parts list to build or purchase the next generation of Beowulfs.
There are a few points of interest concerning the two configurations presented. In both cases, Fast Ethernet is employed with 100 Mbps peak throughput. Networking is discussed in Chapter 5.
As discussed earlier, a CD-ROM is not included because it is assumed that system installation will be performed over the network. A floppy drive is included to facilitate initial installation and crash recovery. Finally, the BIOS requires a video card to boot, so a very inexpensive one is included on every system.
