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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  • tajmahal - Tuesday, September 8, 2009 - link

    You fail to mention that Microcenter prices are for IN STORE PURCHASE ONLY. If you live about 6 driving hours away from a Microcenter like i do, then you're screwed.
  • Chlorus - Tuesday, September 8, 2009 - link

    Shhh! Don't spoil his self-righteous post with your troublesome facts!
  • strikeback03 - Tuesday, September 8, 2009 - link

    Does Microcenter have a limit on how many processors people can buy? If not, why isn't anyone buying these things and reselling them for less than the ~$280 that Newegg (and every other online retailer) do?
  • Solema - Tuesday, September 8, 2009 - link

    Anand,
    I know those prices you quoted are per-unit prices from Intel, but they are much more expensive than actual CPU costs. Given that I can get the following from Micro Center, and that I plan to overclock and run two 8800GTS 512's in SLI, what would you recommend?

    i5 750 - $179
    i7 920 - $199
    i7 860 - $229

    It still seems to me that the additional overclocking flexibility of the 920 (especially on stock voltage), coupled with the better multi-GPU performance would make that the best CPU to purchase, no? Given that P55 motherboards currently only retail for about $50 cheaper than many x58 boards, wouldn't the extra $70 cost for x58+i7 920 over a P55+i5 750 be worth it? You get better multi-GPU performance, better overclocking, better RAM performance, and future upgradeability to 6-core CPU's. What am I missing that would tip the scales in favor of the i5?
  • Pneumothorax - Tuesday, September 8, 2009 - link

    the 920 for sure as you get a HT CPU for even cheaper than the 860. Both should overclock the same.
  • dman - Tuesday, September 8, 2009 - link

    So, do they support Hardware Virtualization? And don't give slack about not targeted at that market, specifically, does this support Windows 7 virtualization mode?

    I searched and didn't see it covered. I've read that the i5 and lower do not support vt-d, but, I'm not sure how that translates to Windows 7 "XP mode" support... I do need to review a bit more, would be nice if this was covered in the review.

    I do know that this IS something that the Phenom family does support.

    http://www.virtualization.info/2009/07/intel-core-...">http://www.virtualization.info/2009/07/...core-i3-...

  • ash9 - Tuesday, September 8, 2009 - link

    No Virtalization!! That maybe huge for corporate setups,

    how did I miss that.

    It should have been reported
  • has407 - Wednesday, September 9, 2009 - link

    VT-x != VT-d. You want, and may need, VT-x. Most people don't need VT-d, much less know what to do with it or have a system that can make use of it--if you do, you're very unlikely to be using one of these CPUs.

    VT-x is Intel's name for processor virtualization features; it is part of the processor. All Core iX CPUs support it. VT-x is required for some hypervisors, (e.g., MSFT HYperV), but not all, although most (all?) require it for running 64-bit guests.

    VT-d is Intels name for for IO virtualization (specifically "directed IO"); it is, or has been, part of the northbridge. For VT-d to be useful, you need a chipset that supports it; a MB/BIOS that supports it; and a hypervisor that knows how to use it. VT-d is primarily of interest to VM's that want to dedicate direct access to hardware by guests, and avoid the overhead of the hypervisor for that IO.

    When you see "CPU X supports vT-d", it means the chipset for CPU X supports VT-d (the P55 supports VT-d). Whether MB/BIOS vendors choose to support it is another matter. Moreover, support for VT-d isn't simply yes or no; support varies by chipset (e.g., the P55, like the rest, support virtualizating a subset of interfaces).

    In short:

    1. Based on Core iX chipset capabilities (e.g., P55, X58), VT-d support is an MB vendor decision--not a function of the CPU model.

    2. Which vendors support VT-d, and to what extent, is more often than not clear as mud, and the topic of much discussion in some threads.

    3. If VT-d is important to you, you're probably running a heavy virtualized workload on an MP system with 10Gbe or very fast DAS--certainly not a Core iX. (Only exception of interest to others might be for access to GPU's by VMs)

    What the new processors throw into the mix is an integrated PCIe controller, which also means an integrated DMA controller (at least I hope it does). Whether that supports VT-d is unknown (I haven't been able to find a definitive answer).
  • Gary Key - Tuesday, September 8, 2009 - link

    VT-d is enabled on the i7/870-860. It is not enabled on the i5/750, just VT-x is available on it. I am working on Windows 7 XP mode as we speak.
  • Jakall78 - Tuesday, September 8, 2009 - link

    Reading this site for years, but there is something wrong going on here. Besides some slideshow pictures from Intel and 2-3 tests... there is nothing. That is not the way reviews are done. Look at the SSD reviews, THAT is a review(both of them actually). Now please look at this review
    http://translate.google.com/translate?prev=hp&...">http://translate.google.com/translate?p...mp;sl=ro...
    and say it`s not better...
    * I`m not making any false advertising here, I just found a better review.

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