AnandTech: AMD's CrossFireX: Tri & Quad GPU Preview

The Test

Test Setup
CPU	AMD Phenom @ 2.6GHz
Motherboard	MSI K9A2 Platinum
Video Cards	ATI Radeon HD 3870 X2 ATI Radeon HD 3870
Video Drivers	Catalyst 8.1 (Modified for CrossFireX Support)
Hard Drive	Seagate 7200.9 300GB 8MB 7200RPM
RAM	2x1GB Corsair XMS2 DDR2-800 4-4-4-12
Operating System	Windows Vista Ultimate 64-bit

We ran the following games in their corresponding quality settings:

Game	Resolution	AA	AF	Detail Settings
Half Life 2: Episode Two	2560 x 1600	8X	16X	Highest in-game
Unreal Tournament 3	2560 x 1600	0X	16X	Highest in-game
Bioshock	2560 x 1600	0X	1X	Highest in-game
Call of Duty 4	2560 x 1600	4X	16X	Highest in-game
Crysis	1920 x 1200	0X	1X	High Quality defaults

The graph below shows the performance levels you can get from a single GPU all the way up to four:

Obviously the biggest jump comes from one to two GPUs, but we still see some reasonable gains going from two to three. The value of a fourth GPU is simply nonexistent in most of the cases.

Number of GPUs	HL2	UT3	Bioshock	CoD4	Crysis
1 x Radeon HD 3870	39.3	46.7	36.9	25.3	14.0
2 x Radeon HD 3870 (1 X2)	71.9	84.1	63.2	50.0	26.2
3 x Radeon HD 3870 (2 X2 + 1)	93.3	112.7	86.7	72.2	26.4
4 x Radeon HD 3870 (2 X2)	102.2	114.6	92.5	93.2	27.7

Let's put some percentages with the graph above to put things into perspective:

Configuration	HL2	UT3	Bioshock	CoD4	Crysis
2-way CF Improvement over 1 card	83%	80%	71%	98%	87%
3-way CF improvement over 2 cards	30%	34%	37%	44%	0%
4-way CF improvement over 3 cards	10%	3%	7%	29%	4%
4-way CF improvement over 1 card	160%	150%	151%	268%	98%

The move from one to two cards generally yields a healthy performance improvement, but the gains taper off as we look at the performance added by a third GPU. Call of Duty 4 is the only game that shows solid gains with 4 GPUs (29% over a 3-GPU configuration), the rest of the titles show mostly single-digit percentage improvements. Once again we see that Crysis simply needs new, faster GPU architectures - four GPUs does absolutely nothing for this game.

Remember our 3-way SLI review from earlier in the year? Although the test systems were very different, the GPU-bound scaling should be comparable (if anything, NVIDIA should have the advantage of being run on an Intel system). Here's a quick look back at some of the comparable benchmarks:

Configuration	UT3	CoD4
2-way SLI Improvement over 1 card	84%	90%
3-way SLI improvement over 2 cards	20%	28%
3-way SLI improvement over 1 card	121%	143%

Single to dual GPU scaling is similar with SLI as CrossFire, but the 2-way to 3-way gain is better on CrossFireX. NVIDIA's quad-SLI is not yet available so we're not sure how the 4-way scaling will compare, but so far it seems like CrossFireX is doing quite well in the pure numbers game.