Multi-GPU SLI/CF Scaling: Lynnfield's Blemish

When running in single-GPU mode, the on-die PCIe controller maintains a full x16 connection to your graphics card:


Hooray.

In multi-GPU mode, the 16 lanes have to be split in two:

To support this the motherboard maker needs to put down ~$3 worth of PCIe switches:

Now SLI and Crossfire can work, although the motherboard maker also needs to pay NVIDIA a few dollars to legally make SLI work.

The question is do you give up any performance when going with Lynnfield's 2 x8 implementation vs. Bloomfield/X58's 2 x16 PCIe configuration? In short, at the high end, yes.

I looked at scaling in two games that scaled the best with multiple GPUs: Crysis Warhead and FarCry 2. I ran all settings at their max, resolution at 2560 x 1600 but with no AA.

I included two multi-GPU configurations. A pair of GeForce GTX 275s from EVGA for NVIDIA:


A coupla GPUs and a few cores can go a long way

And to really stress things, I looked at two Radeon HD 4870 X2s from Sapphire. Note that each card has two GPUs so this is actually a 4-GPU configuration, enough to really stress a PCIe x8 interface.

First, the dual-GPU results from NVIDIA.

NVIDIA GeForce GTX 275 Crysis Warhead (ambush) Crysis Warhead (avalanche) Crysis Warhead (frost) FarCry 2 Playback Demo Action
Intel Core i7 975 (X58) - 1GPU 20.8 fps 23.0 fps 21.4 fps 41.0 fps
Intel Core i7 870 (P55) 1GPU 20.8 fps 22.9 fps 21.5 fps 40.5 fps
Intel Core i7 975 (X58) - 2GPUs 38.4 fps 42.3 fps 38.0 fps 73.2 fps
Intel Core i7 870 (P55) 2GPUs 38.0 fps 41.9 fps 37.4 fps 65.9 fps

 

The important data is in the next table. What you're looking at here is the % speedup from one to two GPUs on X58 vs. P55. In theory, X58 should have higher percentages because each GPU gets 16 PCIe lanes while Lynnfield only provides 8 per GPU.

GTX 275 -> GTX 275 SLI Scaling Crysis Warhead (ambush) Crysis Warhead (avalanche) Crysis Warhead (frost) FarCry 2 Playback Demo Action
Intel Core i7 975 (X58) 84.6% 83.9% 77.6% 78.5%
Intel Core i7 870 (P55) 82.7% 83.0% 74.0% 62.7%

 

For the most part, the X58 platform was only a couple of percent better in scaling. That changes with the Far Cry 2 results where X58 manages to get 78% scaling while P55 only delivers 62%. It's clearly not the most common case, but it can happen. If you're going to be building a high-end dual-GPU setup, X58 is probably worth it.

Next, the quad-GPU results from AMD:

AMD Radeon HD 4870 X2 Crysis Warhead (ambush) Crysis Warhead (avalanche) Crysis Warhead (frost) FarCry 2 Playback Demo Action
Intel Core i7 975 (X58) - 2GPUs 25.8 fps 31.3 fps 27.0 fps 70.9 fps
Intel Core i7 870 (P55) 2GPUs 24.4 fps 31.1 fps 26.6 fps 71.4 fps
Intel Core i7 975 (X58) - 4GPUs 27.0 fps 57.4 fps 47.9 fps 117.9 fps
Intel Core i7 870 (P55) 4GPUs 24.2 fps 50.0 fps 36.5 fps 116 fps

 

Again, what we really care about is the scaling. Note how single GPU performance is identical between Bloomfield/Lynnfield, but multi-GPU performance is noticeably lower on Lynnfield. This isn't going to be good:

4870 X2 -> 4870 X2 CF Scaling Crysis Warhead (ambush) Crysis Warhead (avalanche) Crysis Warhead (frost) FarCry 2 Playback Demo Action
Intel Core i7 975 (X58) 4.7% 83.4% 77.4% 66.3%
Intel Core i7 870 (P55) -1.0% 60.8% 37.2% 62.5%

 

Ouch. Maybe Lynnfield is human after all. Almost across the board the quad-GPU results significantly favor X58. It makes sense given how data hungry these GPUs are. Again, the conclusion here is that for a high end multi-GPU setup you'll want to go with X58/Bloomfield.

A Quick Look at GPU Limited Gaming

With all of our CPU reviews we try to strike a balance between CPU and GPU limited game tests in order to show which CPU is truly faster at running game code. In fact all of our CPU tests are designed to figure out which CPUs are best at a number of tasks.

However, the vast majority of games today will be limited by whatever graphics card you have in your system. The performance differences we talked about a earlier will all but disappear in these scenarios. Allow me to present data from Crysis Warhead running at 2560 x 1600 with maximum quality settings:

NVIDIA GeForce GTX 275 Crysis Warhead (ambush) Crysis Warhead (avalanche) Crysis Warhead (frost)
Intel Core i7 975 20.8 fps 23.0 fps 21.4 fps
Intel Core i7 870 20.8 fps 22.9 fps 21.5 fps
AMD Phenom II X4 965 BE 20.9 fps 23.0 fps 21.5 fps

 

They're all the same. This shouldn't come as a surprise to anyone, it's always been the case. Any CPU near the high end, when faced with the same GPU bottleneck, will perform the same in game.

Now that doesn't mean you should ignore performance data and buy a slower CPU. You always want to purchase the best performing CPU you can at any given pricepoint. It'll ensure that regardless of the CPU/GPU balance in applications and games that you're always left with the best performance possible.

The Test

Motherboard: Intel DP55KG (Intel P55)
Intel DX58SO (Intel X58)
Intel DX48BT2 (Intel X48)
Gigabyte GA-MA790FXT-UD5P (790FX)
Chipset: Intel X48
Intel X58
Intel P55
AMD 790FX
Chipset Drivers: Intel 9.1.1.1015 (Intel)
AMD Catalyst 9.8
Hard Disk: Intel X25-M SSD (80GB)
Memory: Qimonda DDR3-1066 4 x 1GB (7-7-7-20)
Corsair DDR3-1333 4 x 1GB (7-7-7-20)
Patriot Viper DDR3-1333 2 x 2GB (7-7-7-20)
Video Card: eVGA GeForce GTX 280
Video Drivers: NVIDIA ForceWare 190.62 (Win764)
NVIDIA ForceWare 180.43 (Vista64)
NVIDIA ForceWare 178.24 (Vista32)
Desktop Resolution: 1920 x 1200
OS: Windows Vista Ultimate 32-bit (for SYSMark)
Windows Vista Ultimate 64-bit
Windows 7 64-bit

Turbo mode is enabled for the P55 and X58 platforms.

The Best Gaming CPU? SYSMark 2007 Performance
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  • goinginstyle - Tuesday, September 8, 2009 - link

    Really? SuperPi, WPrime, Everest, 3Dmarks and LN2 overclocking defines a better review? How does any of that correlate into real world applications and what 99% of people use their computers for on a daily basis. I counted a lot more than three tests in the AT review, go spam elsewhere.
  • C'DaleRider - Tuesday, September 8, 2009 - link

    Really. All I play is SuperPi, Everest and 3DMarks....oh, and Furmark and OCCT.

    Who would ever use their computer to encode video, run Excel spreadsheets, play games like Far Cry or Crysis or Left For Dead, or actually use any other real world application?

    Don't you know? Real elite computer users just bench synthetic crap, over and over, for hours and hours, and scoff at anyone who dares do anything productive with their computer.

    /sarcasm
  • geok1ng - Tuesday, September 8, 2009 - link

    I am not pleased by the news; i5 needs more juice for overcloking. Thats terrible: my E8600 is still on top: it runs at 4.0Ghz with 1.16v on a watercooled setup with 4x120mm fans at 1600rpm. we are talking less than 30dB of noise and less than 55w of power consumption. At 1.25v my E8600 reaches 4.25Ghz and would go a little further if wasnt for the 4 sticks of ram burning the NB. no reason to exchange systems before the 32nm parts arrive. The ability to achieve high clocks with low voltages is crucial for a good system: not only will it consume less power, but it will also be quieter, and that is a point for choosing sub-65w dual-cores in gaming rigs.
  • papapapapapapapababy - Tuesday, September 8, 2009 - link


    honestly... i lol at all the core i7 core i5 "GAMERS"( and their punny 23 lcd with shitty TN panels) also LGA1366? LGA1156? and 285$ for a shitty motherboard? XD I have a better proposition for you INTEL. how about u SUCK MY BALLS XD. MKAY? the story: my old man needed a gaming setup ( mostly simracing) So i bought the cheapest E7200 i could find (oc3.2GHZ), 4gb of ram, the cheapest intel mobo i could find -g31- ( not even pcie2) and gave him my 4770 ... the price? ridiculously cheap... almost nothing. and with all that extra money i saved i got this > a nice 42" HDTV with a perfect s-ips panel and low 1366 X 768 resolution, and a g25 wheel.

    btw, the framerates? i never, ever drop below 30fps. ALWAYS 60FPS NO MATTER WHAT, ( with nice 8xAA) XD so the best gaming cpu? the cheapest !


  • erple2 - Tuesday, September 8, 2009 - link

    Interesting. Which 42" LCD TV did you get that has a S-IPS panel? Also, a 23" panel at about 3 feet looks bigger than a 42" screen at 6 feet.
  • papapapapapapapababy - Tuesday, September 8, 2009 - link

    LG. btw a 23" TN lcd looks like shit no matter how you look at it.
  • C'DaleRider - Tuesday, September 8, 2009 - link

    My, aren't you special?

  • papapapapapapapababy - Tuesday, September 8, 2009 - link

    yes, i know! btw enjoy your small screen gaming, mr sheep XD
  • chrnochime - Tuesday, September 8, 2009 - link

    I'd say the same about that barely big enough 42" LCD, but then again why bother...
  • papapapapapapapababy - Tuesday, September 8, 2009 - link

    my bad i meant this > E7300 3.2ghz (+ a cheap 24$ modded heatpipe cooler... 14 dba XD)

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