Opinion: Why Counting ‘Platform’ PCIe Lanes (and using it in Marketing) Is Absurd

It’s at this point that I’d like to take a detour and discuss something I’m not particularly happy with: counting PCIe lanes.

The number of PCIe lanes on a processor, for as long as I can remember, has always been about which lanes come directly from the PCIe root, offering full bandwidth and with the lowest possible latency. In modern systems this is the processor itself, or in earlier, less integrated systems, the Northbridge. By this metric, a standard Intel mainstream processor has 16 lanes, an AMD Ryzen has 16 or 20, an Intel HEDT processor has 28 or 44 depending on the model, and an AMD Ryzen Threadripper has 60.

In Intel’s documentation, it explicitly lists what is available from the processor via the PCIe root complexes: here 44 lanes come from two lots of sixteen and one twelve lane complex. The DMI3 link to the chipset is in all but name a PCIe 3.0 x4 link, but is not included in this total.

The number of PCIe lanes on a chipset is a little different. Chipsets are for all practical purposes PCIe switches: using a limited bandwidth uplink, it is designed to carry traffic from low bandwidth controllers, such as SATA, Ethernet, and USB. AMD is limited in this regard, due to spending more time re-entering the pure CPU performance race over the last few years and outsource their designs to ASMedia. Intel has been increasing its PCIe 3.0 lane support on its chipsets for at least three generations, now supporting up to 24 PCIe 3.0 lanes. There are some caveats on what lanes can support which controllers, but in general we consider this 24.

Due to the shared uplink, PCIe lanes coming from the chipset (on both the AMD and Intel side) can be bottlenecked very easily, as well as being limited to PCIe 3.0 x4. The chipset introduces additional latency compared to having a controller directly attached to the processor, which is why we rarely see important hardware (GPUs, RAID controllers, FPGAs) connected to them.

The combination of the two lends itself to a variety of platform functionality and configurations. For example, for AMD's X399 platform that has 60 lanes from the processor, the following combinations are 'recommended':

X399 Potential Configurations
  Use PCIe Lanes Total
Content Creator 2 x Pro GPUs
2 x M.2 Cache Drives
10G Ethernet
1 x U.2 Storage
1 x M.2 OS/Apps
6 x SATA Local Backup
x16/x16 from CPU
x4 + x4 from CPU
x4 from CPU
x4 from CPU
x4 from CPU
From Chipset
52 Lanes
Extreme PC 2 x Gaming GPUs
1 x HDMI Capture Card
2 x M.2 for Games/Stream
10G Ethernet
1 x M.2 OS/Apps
6 x SATA Local Backup
x16/x16 from CPU
x8 from CPU
x4 + x4 from CPU
x4 from CPU
x4 from CPU
From Chipset
56 Lanes
Streamer 1 x Gaming GPU
1 x HDMI Capture Card
2 x M.2 Stream/Transcode
10G Ethernet
1 x U.2 Storage
1 x M.2 OS/Apps
6 x SATA Local Backup
x16 from CPU
x4 from CPU
x4 + x4 from CPU
x4 from CPU
x4 from CPU
x4 from CPU
From Chipset
40 Lanes
Render Farm 4 x Vega FE Pro GPUs
2 x M.2 Cache Drives
1 x M.2 OS/Apps
6 x SATA Local Backup
x16/x8/x8/x8
x4 + x4 from CPU
x4 from CPU
From Chipset
52 Lanes

What has started to happen is that these companies are combining both the CPU and chipset PCIe lane counts, in order to promote the biggest number. This is despite the fact that not all PCIe lanes are equal, they do not seem to care. As a result, Intel is cautiously promoting these new Skylake-X processors as having ’68 Platform PCIe lanes’, and has similar metrics in place for other upcoming hardware.

I want to nip this in the bud before it gets out of hand: this metric is misleading at best, and disingenuous at worst, especially given the history of how this metric has been provided in the past (and everyone will ignore the ‘Platform’ qualifier). Just because a number is bigger/smaller than a vendor expected does not give them the right to redefine it and mislead consumers.

To cite precedent: in the smartphone space, around 4-5 years ago, vendors were counting almost anything in the main processor as a core to provide a ‘full core count’. This meant that GPU segments became ‘cores’, special IP blocks for signal and image processing became ‘cores’, security IP blocks became ‘cores’. It was absurd to hear that a smartphone processor had fifteen cores, when the main general purpose cores were a quartet of ARM Cortex A7 designs. Users who follow the smartphone industry will notice that this nonsense stopped pretty quickly, partly due to anything being called a core, but some hints towards artificial cores potentially being placed in the system. If allowed to continue, this would have been a pointless metric.

The same thing is going to happen if the notion of ‘Platform PCIe Lanes’ is allowed to continue.

Explaining the Jump to Using HCC Silicon Test Bed and Setup
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  • mmrezaie - Monday, September 25, 2017 - link

    unofficially threadripper supports ECC. Do you have the plan to look into it?

    p.s. I sent an email to Anandtech support about abusive ads directing to some questionable websites. I am in EU and I see these ads for a long time now.
    Reply
  • Ryan Smith - Monday, September 25, 2017 - link

    "p.s. I sent an email to Anandtech support about abusive ads directing to some questionable websites. I am in EU and I see these ads for a long time now."

    Er, we don't have a support email address. So I'm not sure who you sent that to.

    Anyhow, we're always trying to squash malvertising. It comes in on programmatic ads, which does make the process tricky. But if you can get it to reliably and repeatedly trigger, please contact me. If we can get network logs collected, then we can isolate the source and get said ads pulled.
    Reply
  • mmrezaie - Monday, September 25, 2017 - link

    I sent it to advertisement link I found in the "contact us" page. Sorry by saying support. That's what we call it in our organization. thanks for the reply. Reply
  • snowmyr - Monday, September 25, 2017 - link

    Check that link again and you'll see that it's not really an anandtech email address and might not get forwarded to the right people. Reply
  • mmrezaie - Monday, September 25, 2017 - link

    I sent it to advertisement link I found in the "contact us" page. Sorry by saying support. That's what we call it in our organization. thanks for the reply. Reply
  • AdditionalPylons - Monday, September 25, 2017 - link

    Just sent you a tweet with a screenshot, Ryan. I've been very annoyed with these clickbait ads for a
    very long time as well.
    Reply
  • hughc - Monday, September 25, 2017 - link

    Wasn't sure what you were referring to. I have AdBlock whitelisting the domain, so I see all the display advertising.

    I'm also using ClickbaitKiller. Disabled it, and now I can see the unit in question - very happy to hide this trash.
    Reply
  • thesavvymage - Monday, September 25, 2017 - link

    I get inappropriate ads as well. I'm not sure where I can send a screenshot, but the one I have on this page under the article is "This Is Better Than Adderall, According to US College Students. Try It!"

    Like what? This is a professional tech site and ads like that have no business being on here. Banner ads for tech companies? Good. Side ads for relevant products? Good. This BS thats always underneath every article? Absolutely unacceptable.
    Reply
  • Gothmoth - Monday, September 25, 2017 - link

    do you really think anandtech cares how they make money.. maybe when anad was still here.

    i see these ads too, a website who cares about it´s reputation would distance itself from such crap.. but not anandtech.
    Reply
  • ddriver - Monday, September 25, 2017 - link

    "maybe when anad was still here" LOL, if he didn't care about money, he'd not sell the site to money makers for money. That's the N1 business model, and the sole motivation for doing anything - get it to get popular, then sell it out, and all its users with it. Reply

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