PCI-Express Compliance: Does It Even Matter?

For a while now we’ve been under the impression that video card size and power consumption was ultimately capped by the PCI-Express specification. At present time the specification and its addendums specify normal (75W), 150W, 225W, and 300W PCIe card operation. In the case of 300W cards in particular this is achieved through 75W from the PCIe slot, 75W from a 6pin PCIe power connector, and 150W from an 8pin PCIe power connector. As the name implies, the PCIe specification also defines what the 6pin and 8pin power connectors are supposed to be capable of, which is where 75W and 150W come from respectively.

Altogether the biggest, most powerful card configuration in the PCIe specification allows for a 12.283” long, triple-wide card that consumes 300W. To date we’ve never seen a card exceed the physical specifications, but we’ve seen several cards exceed the electrical specifications. This includes cards such as the 5970 and some overclocking-oriented 5870s that were designed to handle more than 300W when overclocked, and even more exotic cards such as the Asus ARES 5870X2 that simply drew more than 300W from the get-go. We have yet to see a reference design from AMD/NVIDIA however that exceeds any part of the PCIe specification by default.

So it has been clear for some time now that cards can exceed the PCIe specifications without incurring the immediately wrath of an army of lawyers, but at the same time this doesn’t establish what the benefits or losses are of being or not being PCIe compliant. To have a reference design exceed the PCIe specifications is certainly a new mark for the GPU industry, so we decided to get right to the bottom of the matter and get an answer to the following question: does PCI-Express compliance matter?

To answer this question we went to two parties. The first of which was of course AMD, whose product is in question. AMD’s answer basically amounts to a polite deflection: it’s an ultra-enthusiast card that at default settings does not exceed the power available by the combination of the PCIe slot and PCIe power connectors. Furthermore, as they correctly note, the 6990 is not the first card to ship at over 300W, as the ARES and other cards were drawing more than 300W a year ago. It’s a polite answer that glosses over the fact that no, the 6990 isn’t technically PCIe compliant.

To get a second opinion on the matter we went straight to the source: The Peripheral Component Interconnect Special Interest Group (PCI-SIG), which is the industry group that defines the PCIe standard and runs the workshops that test for product compliance. The PCI-SIG’s member list is virtually everyone in the computing industry, including AMD, NVIDIA, and Intel, so everyone has some level of representation with the group.

So what does the PCI-SIG think about cards such as the 6990 which exceed the PCIe specification? In a nutshell, they don’t directly care. The group’s working philosophy is closer to approving cards that work than it is about strictly enforcing standards, so their direct interest in the matter is limited. The holy grail of the PCI-SIG is the PCI Express Integrators List, which lists all the motherboards and add-on cards that have passed compliance testing. The principal purpose of the list is to help OEMs and system integrators choose hardware, relying on the list and by extension PCI-SIG testing to confirm that the product meets the PCIe standards, so that they can be sure it will work in their systems.

The Integrators List is more or less exclusively OEM focused, which means it has little significance for niche products such as the 6990 which is split between end-user installation and highly customized OEM builds. The 6990 does not need to be on the list to be sold to its target market. Similarly the 5970 was never submitted/approved for listing, and we wouldn’t expect the 6990 to be submitted either.

It is worth noting however that while the PCI-SIG does have power specifications, they’re not a principal concern of the group and they want to avoid doing anything that would limit product innovation. While the 300W specification was laid out under the belief that a further specification would not be necessary, the PCI-SIG does not even test for power specification compliance under their current compliance testing procedures.  Conceivably the 6990 could be submitted and could pass the test, leading to it being labeled PCIe compliant. Of course it’s equally conceivable that the PCI-SIG could start doing power compliance testing if it became an issue…

At the end of the day as the PCI-SIG is a pro-compliance organization as opposed to being a standard-enforcement organization, there’s little to lose for AMD or their partners by not being compliant with the PCIe power specifications. By not having passed compliance testing the only “penalty” for AMD is that they cannot claim the 6990 is PCIe compliant; funny enough they can even use the PCIe logo (we’ve already seen a Sapphire 6990 box with it). So does PCIe compliance matter? For mainstream products PCIe compliance matters for the purposes of getting OEM sales; for everything else including niche products like the 6990, PCIe compliance does not matter.

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  • smookyolo - Tuesday, March 08, 2011 - link

    My 470 still beats this at compute tasks. Hehehe.

    And damn, this card is noisy.
    Reply
  • RussianSensation - Tuesday, March 08, 2011 - link

    Not even close, unless you are talking about outdated distributed computing projects like Folding@Home code. Try any of the modern DC projects like Collatz Conjecture, MilkyWay@home, etc. and a single HD4850 will smoke a GTX580. This is because Fermi cards are limited to 1/8th of their double-precision performance.

    In other words, an HD6990 which has 5,100 Gflops of single-precision performance will have 1,275 Glops double precision performance (since AMD allows for 1/4th of its SP). In comparison, the GTX470 has 1,089 Gflops of SP performance which only translates into 136 Gflops in DP. Therefore, a single HD6990 is 9.4x faster in modern computational GPGPU tasks.
    Reply
  • palladium - Tuesday, March 08, 2011 - link

    Those are just theoretical performance numbers. Not all programs *even newer ones* can effectively extract ILP from AMD's VLIW4 architecture. Those that can will no doubt with faster; others that can't would be slower. As far as I'm aware lots of programs still prefer nV's scalar arch but that might change with time. Reply
  • MrSpadge - Tuesday, March 08, 2011 - link

    Well.. if you can oly use 1 of 4 VLIW units in DP then you don't need any ILP. Just keep the threads in flight and it's almost like nVidias scalar architecture, just with everything else being different ;)

    MrS
    Reply
  • IanCutress - Tuesday, March 08, 2011 - link

    It all depends on the driver and compiler implementation, and the guy/gal coding it. If you code the same but the compilers are generations apart, then the compiler with the higher generation wins out. If you've had more experience with CUDA based OpenCL, then your NVIDIA OpenCL implementation will outperform your ATI Stream implementation. Pick your card for it's purpose. My homebrew stuff works great on NVIDIA, but I only code for NVIDIA - same thing for big league compute directions. Reply
  • stx53550 - Tuesday, March 15, 2011 - link

    off yourself idiot Reply
  • m.amitava - Tuesday, March 08, 2011 - link

    ".....Cayman’s better power management, leading to a TDP of 37W"

    - is it honestly THAT good? :P
    Reply
  • m.amitava - Tuesday, March 08, 2011 - link

    oops...re-read...that was idle TDP !! Reply
  • MamiyaOtaru - Tuesday, March 08, 2011 - link

    my old 7900gt used 48 at load
    D:

    Don't like the direction this is going. In GPUs it's hard to see any performance advances that don't come with equivalent increases in power usage, unlike what Core 2 was compared to Pentium4.
    Reply
  • Shadowmaster625 - Tuesday, March 08, 2011 - link

    Are you kidding? I have a 7900GTX I dont even use, because it fried my only spare large power supply. A 5670 is twice as fast and consumes next to nothing. Reply

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