Intel Misses the Bus
An initiative to increase the performance of the venerable PCI bus lacks vital support from Intel Corp.
As its processor and memory performance metrics have increased over the years, the PC server has been constrained by the bottleneck of its PCI bus, which even today still clocks in at 33 MHz. To increase server performance and extend the life of the venerable PCI bus, Compaq Computer Corp., Hewlett-Packard Co. and IBM Corp. announced an initiative to ratchet up PCI bus performance by a factor of 400 percent, to 133 MHz.
While the newly proposed PCI specification, dubbed PCI-X, has garnered industry support from a number of OEMs, PCI pioneer Intel Corp. is noticeably absent from the gang. PCI-X proponents fear that Intel’s absence could cast aspersions over the ability of the new technology to succeed in a market dominated by all things Intel.
The PCI bus runs at 33 MHz, a speed half that of the 66-MHz backside bus of a Pentium-, Pentium Pro- or Pentium II-class motherboard. During 1998, the industry saw the release of Intel’s 440BX chipset and a subsequent increase in the performance of the x86 architecture’s frontside bus from 66 to 100 MHz. The frontside bus is the portion of the motherboard topology that governs the clock frequency and timings of memory resources. However, with the 440BX chipset, the backside bus was left at the existing 66-MHz performance threshold, leaving the PCI bus behind at the 33-MHz half-speed.
If Compaq, IBM and HP have their collective way, however, all of this is about to change. The three computing giants hope to establish PCI-X as a new technology standard simply by virtue of their dominating positions in the PC server marketplace.
This is not the first time that a vendor has attempted to advance a new PCI-related technology standard, however. In September 1997, Micron Electronics Inc. (Nampa, Idaho, www.micron.com) released the Powerdigm XSU workstation, a Windows NT-based workstation that leverages Micron’s unique Samurai chipset. Rather than featuring Intel’s just-released Accelerated Graphics Port (AGP), a highly touted successor to PCI that provides for a high-speed 66-MHz graphics bus, the Samurai chipset jacks up PCI bus performance to 66 MHz, making a theoretical memory bandwidth of 400 MBps available to a wide range of peripherals, not simply graphics accelerator cards. Micron's Samurai chipset has failed to find broad acceptance because most major OEMs have chosen to support Intel's AGP bus instead.
The 64-bit, 133-MHz PCI-X will extend performance in a similar manner. PCI-X proponents say that the new specification will provide the critical I/O bandwidth required by PC servers running high-speed I/O technologies such as Gigabit Ethernet, Fibre Channel, Ultra3 SCSI and Cluster Interconnects.
Given the support of the three largest PC server vendors, PCI-X probably has a much better chance of success than Micron’s Samurai, says Rob Enderle, a senior analyst with market research and consultancy firm Giga Information Group (Santa Clara, Calif.).
"Our collaboration with HP and IBM will foster the extension of existing PCI bus standards well into the future to meet the demanding computing requirements of enterprise customers," maintains Mary McDowell, vice president and general manager of Compaq’s industry standard server division.
Because the 64-bit PCI-X bus can operate at speeds up to 133 MHz, it theoretically provides burst transfer rates above 1 GBps, which is approximately eight times the rate of the most common PCI implementations today. In addition to providing backward compatibility with the current PCI bus specification, PCI-X also features an enhanced protocol that its proponents contend increases the efficiency of data transfers and ensures the reliability of the complicated electrical timing sequences necessitated by PCI-X’s 133-MHz clock speed.
PCI-X’s impact will likely be felt most palpably in the server and high-speed interconnect marketplaces.
"We are working on PCI-X because we think it represents an important, mainstream extension of PCI technology," explains Robert Selinger, vice president of the advanced technology group with Adaptec Inc. (Sunnyvale, Calif., www.adaptec.com). "Adaptec is committed to faster I/O, such as PCI-X, due to its fundamental impact on the future of servers and workstations." Adaptec and other companies are working with IBM, HP and Compaq to develop the PCI-X specification. Because Adaptec manufactures high-performance SCSI cards, the company has a vested interest in making sure that an initiative such as PCI-X -- which would increase data-throughput on the PCI bus by several orders of magnitude -- succeeds.
According to Giga’s Enderle, Intel’s absence from the list of PCI-X boosters can most likely be attributed to an issue of inertia on Intel’s part. "Intel had another project in place to do the same thing [as PCI-X], and these folks [Compaq, IBM, HP, and others] just got tired of waiting for them," Enderle explains. "While Intel is looking at technologies like 1394 and AGP, it’s really not involved with the new high-speed bus."
Questions of involvement aside, Enderle believes that the new PCI-X specification will garner enough industry momentum to compel Intel to embrace and adopt it. "I think Intel will have to accept the new specification, because it really does matter for workstations and servers. [It matters] primarily for servers, which have a bus bottleneck that is kind of rare," Enderle observes. "It really doesn't matter to desktops yet, but could in a few years."