Why doesnt Crossfire/SLI Give you 2x performance?

[Sc1mitar]

Just wondering,it just seems strange to me that...

if you have 2 HD4870;s or 2 GTX260's or w/e, why dont you get double the framerate? shouldnt you theoretically have double the processing power with which you can crunch raw visual data n such?

so whats the major bottleneck then? cause on paper 1+1=2, but with xfire/sli its more like 1+1=1.2-1.4

what gives?

 

[trickson]

In theory yes you would think that this would be the case but this is a great question and I have no answer for you but I would like to know as well .

 

[newtekie1]

The simple answer is that a lot of that processing power is lost keeping the two cards in sync.

Long Answer:

Think of it like this, two cars are on the road, and both are going as fast as they possibley can, cut can't pass eachother, they both have to stay exactly side by side. They have to go slower than if it was just the single car going as fast as it could. If one car has to slow down for some reason(Pot Holes, puddle of water, bumpy road, etc) the other car has to slow down also.

Now with SLi/Crossfire, there are technigues they use to keep the load on both cards equal, but it isn't perfect.

 

[Sc1mitar]

then that is the major bottleneck? the syncing of the gfx cards?

i dunno, personally i cant see shelling out another 2-3 bills for 15-20 more fps, then thers all the driver issues n stuff.

just wish they;d figure out a way to make it work properly, then i could have an excuse 2 get another card an increase my e-peen.

lol

 

[TRIPTEX_CAN]

Many times the CPU is the reason you don't see more than 1.4x performance from Crossfire and SLi. There are other things like resolution, filtering methods, and biased game engines. Driver optimization is very important. Sometimes you need to rename a game to trick the driver into enabling crossfire.

I can tell you that with my cards in crossfire (if the game works or can be tricked) I always get more than 1.6x performance and some games are actually close to 2x.

The new FEAR 2 demo that was just released is a good example of excellent scaling. I get an AVG of 68fps with a single 4850 but renaming the game gives me 130fps AVG showing that both cards are working and at much more than 1.4x performance.

 

[Zehnsucht]

SLI/Crossfire are in some games x2 the performance. Others ranges from 1.1-1.9.
COD4 scales good on both crossfire and SLI.
http://www.anandtech.com/GalleryImage.aspx?id=3186

 

[mikek75]

Best explanation I've heard.

 

[TRIPTEX_CAN]

SLI/Crossfire are in some games x2 the performance. Others ranges from 1.1-1.9.
COD4 scales good on both crossfire and SLI.
http://www.anandtech.com/GalleryImage.aspx?id=3186

COD4 is a good example, even COD:WaW scales well. Surprisingly enough Crysis Warhead scales almost a 2x. The game is still shit but it does scale.

 

[HolyCow02]

it usually about a cpu bottleneck. If your cpu can't handle all the data it is being sent, it won't be able to put out that same amount of data at a certain time

 

[insider]

Quads with lots of cache helps considerably in CF/SLI, which is why you shouldn't pair those crap Intel Q7xxx/Q8xxx (2MB/4MB cache) quads with a CF/SLI setup, the older Q6xxx quads (8MB cache) are better suited due to the larger cache size, the Q9xxx with 12MB of cache even better.

 

[imperialreign]

rendering method also plays a big part in how fast both cars can work together. AFR (IIRC) isn't exactly the fastest method, but reduces the possiblities of visual anomalies and glitches (such as tearing). Chessboard is one of the faster methods, but leads to being more prone to areas being out of synch with each other.

There is a TON that goes into the whole equation - newtekie explained it best, IMO.

 

[hat]

Well, you have things like lackluster driver support, and games which are particularly optomized for a specific company, like Crysis. Crysis starts up with an Nvidia logo, which means Nvidia sponsors it. Of course they are going to make sure any games they sponsor are designed to do better on Nvidia hardware than ATi to make them look good. A lot of times ONE powerful graphics card will bottleneck a system, such as the 8800gtx when it came out, the 4870x2, 7950GX2, 9800GX2, GTX280, GTX295... these cards are cards I suspect are major bottlenecking cards. So if you use TWO or MORE of them at the same time then they are bound to not scale to the 2x they should even with the best hardware, like the Core i7 extreme. The Core i7 extreme will be bottlenecked by probably even one GTX295, so using two at the same time will got net you a 2x performance boost.

 

[JC316]

then that is the major bottleneck? the syncing of the gfx cards?

i dunno, personally i cant see shelling out another 2-3 bills for 15-20 more fps, then thers all the driver issues n stuff.

just wish they;d figure out a way to make it work properly, then i could have an excuse 2 get another card an increase my e-peen.

lol

That is why if you crossfire, do it with older tech. I did it with 2600xt's and matched an HD3870's performance for a grand total of $35.

 

[hat]

Exactly... so for someone like me with a high-end card like my 9800gt, it probably won't really be worth getting another one for another year or two. If you're not exactly happy with your performance and you have highish-end hardware, just drop a safe overclock on it. The E8400 is still a grand processor. They're easy to crank to 4GHz and once you get past 4GHz there's really not too much more performance to be had. I myself would probably go for 4.4GHz just to be cool though.