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Here are my results... Take it with a grain of salt, this is really amateur work - I tested only 7 games and ran ONE pass of each benchmark at max settings. I just wanted a rough but documented idea of GPU scaling in some of my currently installed and easy-to-bench games, so here goes.
My system:
i975 @ 4.08 GHz
6 GB DDR3 1700 MHz CAS9
EVGA X58 SLI
GTX 580 driver v263.09
Multi-mon display: 3x 1680x1050. Remember, that's 40% less pixels than 3x 1920, so your mileage - GPU load and SLI scaling - may vary.
PCI-E link speed: I found no difference between SLI 16x/16x and 16x/8x, so let's say it has no impact and leave it at that. 3-way SLI on my system works at 16x/8x/8x. PCI-E bandwidth couldn't explain the varied results below, anyway.
R.U.S.E.
Built-in benchmark.
Insane preset, 'Disable SLI-unfriendly settings' On (removes motion blur, apparently).
1) GPU Rating @ 2560x1600
Single GPU: 47.4
2-way SLI: 87.3
3-way SLI: 88.3
Typical peak VRAM usage: 1130 MB.
2) GPU rating @ 5220x1050
2-way SLI: 67.7
3-way SLI: 63.9
Typical peak VRAM usage: 1450 MB.
3) Quick conclusion
The additional GPU power of 3-way SLI is unused. Higher multi-mon resolutions may yield better scaling, but wouldn't VRAM usage become a problem then ?
DiRT 2
Built-in benchmark
DX11, Ultra High preset, 4x AA.
1) Avg framerate @ 2560x1600
Single GPU: 71.6
2-way SLI: 127.5
3-way SLI: 146.6
2) Avg framerate @ 5220x1050
2-way SLI: 95.7
3-way SLI: 123.2
Typical peak VRAM usage: 1400 MB.
3) Quick conclusion
Satisfactory scaling. Each additional GPU boosts both min & max framerates.
Mafia 2
Built-in benchmark
Max settings, APEX PhysX Medium.
1) Avg framerate @ 2560x1600
Single GPU + PPU: 37.7
2-way SLI: 45.4
2-way SLI + PPU: 64.0
3-way SLI: 50.4
Typical peak VRAM usage: 650 MB (500 when PPU is present).
2) Avg framerate @ 5220x1050
2-way SLI: 39.5
3-way SLI: 51.5
Typical peak VRAM usage: 820 MB.
3) Quick conclusion
Satisfactory scaling in multi-mon, but a dedicated PhysX card is even more useful performance-wise. This could be because of my "low" resolution and/or the level of PhysX content in the benchmark.
Apache Air Assault
Beginning of the same Free Flight mission
High preset
1) Avg framerate @ 2560x1600
Single GPU: 68
2-way SLI: 65
3-way SLI: 44
Typical peak VRAM usage: 450 MB.
2) Avg framerate @ 5220x1050
2-way SLI: 35
3-way SLI: 23
Typical peak VRAM usage: 600 MB.
3) Quick conclusion
Not only does this game have no SLI support yet, but multi-GPU actually degrades performance dramatically. Hopefully a driver update will fix this.
GTA 4
Built-in benchmark
Max settings
1) Avg framerate @ 2560x1600
Single GPU: 62
2-way SLI: 65
3-way SLI: 64
Typical peak VRAM usage: 1000 MB.
2) Avg framerate @ 5220x1050
2-way SLI: 64
3-way SLI: 61
Typical peak VRAM usage: 1200 MB.
3) Quick conclusion
Not a SLI-friendly game. GTA4 loves beefy CPUs and large amounts of RAM/VRAM above all, so the additional CPU power required to handle 3 cards has an obvious (albeit negligible) impact here.
Lost Planet 2
Benchmark Test B
DX11, max settings
1) Avg framerate @ 2560x1600
Single GPU: 35.9
2-way SLI: 59.8
3-way SLI: 63.3
Typical peak VRAM usage: 1050 MB.
2) Avg framerate @ 5040x1050 (bezel comp. not supported)
2-way SLI: 52.6
3-way SLI: 55.7
Typical peak VRAM usage: 1200 MB.
3) Quick conclusion
A third GPU brings remarkably little additional performance. SLI scaling is good though, so a more aggressive CPU overclock or higher resolutions should help.
Metro 2033
Benchmark tool (metro2033benchmark.exe)
DX11 High, AAA + Tesselation + PhysX.
1) Avg framerate @ 2560x1600
Single GPU + PPU: 39.72
2-way SLI: 65.29
2-way SLI + PPU: 66.24
3-way SLI: 68.67
Typical peak VRAM usage: 830 MB (750 when PPU is present)
2) Avg framerate @ 5220x1050
2-way SLI: 55.04
3-way SLI: 59.88
Typical peak VRAM usage: 990 MB
3) Quick conclusion
Again, SLI scaling is good up to two GPUs. A dedicated PhysX card has little impact at these settings.
Just Cause 2
Concrete Jungle benchmark
DX10, max settings incl. CUDA water
1) Avg framerate @ 2560x1600
Single GPU: 27.33
2-way SLI: 41.47
3-way SLI: 43.75
Typical peak VRAM usage: 750 MB
2) Avg framerate @ 5220x1050
2-way SLI: 32.87
3-way SLI: 32.97
Typical peak VRAM usage: 950 MB
3) Quick conclusion
Negligible gain of 3-way SLI. Again, is the CPU holding me back, would higher resolutions help ? Or are some of the advanced settings "3-way SLI-unfriendly" (shadows) ?
Summary of benchmark results vs. 3-way SLI
:!: Remember that this is for MY resolutions, with MY settings, on MY system, after only one run of each benchmark, in only seven games. Actual gameplay has often little to do with benchmark scenes, too so this is NOT a general SLI scaling evaluation of games, but a summary of GTX 580 3-way SLI results.
Good scaling:
Dirt 2
Mafia 2
Zero gain: (CPU-limited results @5M pixels)
R.U.S.E.
GTA 4
Lost Planet 2
Metro 2033
Negative gain: (SLI issues)
Apache Air Assault
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