SLI - Random easy question...

SideWinder · 27 Jul 2007 at 23:49

Can someone explain why having 2 cards in SLI doesn't equal double the power? I don't actually know and
i'd like to.

Jokester · 27 Jul 2007 at 23:50

Because the it takes some GPU computational power to combine the work done by both cards.

Jokester

Baka^Ni · 28 Jul 2007 at 00:50

Whilst that is true, it doesn't explain why some games achieve 90%+ performance increases and some vary wildy, depending on what they're rendering on screen.

sid · 28 Jul 2007 at 00:52

Baka^Ni said:
Whilst that is true, it doesn't explain why some games achieve 90%+ performance increases and some vary wildy, depending on what they're rendering on screen.

Some games are cpu limited other gpu

Rroff · 28 Jul 2007 at 02:00

On a very basic note... SLI uses 2 main modes for rendering games using 2 cards - one is AFR where each card builds and renders an alternate frame and in many cases this can give double the performance of a single card minus a very small amount to overhead... the other main method is SFR where each frame is split between the 2 cards (usually top half handled by one card and bottom by the other) - this gives very variable speed increases depending on how busy each part of the scene is... and generally only gives a 1.6x performance boost at best... fortunatly most games can use AFR...

The other thing that does prevent you from getting double the performance of a single card is as previously mentioned above CPU limitation...

truebluecfc · 28 Jul 2007 at 03:59

Another question, is SLI/Crossfire only to increase performance or can it actually improve picture quality?

Jokester · 28 Jul 2007 at 08:44

truebluecfc said:
Another question, is SLI/Crossfire only to increase performance or can it actually improve picture quality?

It can actually be used to improve picture quality, as you can configure it to to use the second card to do the very high AA.

Jokester

SideWinder · 28 Jul 2007 at 11:15

Makes a bit more sense...i've heard a bit about 'Master' and 'Slave'...so it's basically the 2 cards doing different jobs?

Jokester · 28 Jul 2007 at 11:18

Depends on how you configure it, you can either set it up so that each card renders a half of the screen, using 2 or 3 different methods or use the second card to do AA (upto x32 if I remember correctly).

Jokester

gareth170 · 28 Jul 2007 at 12:16

Jokester said:
Depends on how you configure it, you can either set it up so that each card renders a half of the screen, using 2 or 3 different methods or use the second card to do AA (upto x32 if I remember correctly).

Jokester

i take thats only on SLI? if its on crossfire too wheres the options to choose the configurations?

Jokester · 28 Jul 2007 at 12:19

gareth170 said:
i take thats only on SLI? if its on crossfire too wheres the options to choose the configurations?

Aye sorry that's just SLI, not sure if Crossfire has any configurable options.

Jokester

Rroff · 28 Jul 2007 at 13:41

While 32x FSAA looks very nice... its not worth the money of a second card if thats the only thing you use it for :S but normal SLI mode would allow you to max out the settings that the single card alone might not be able to handle with playable framerates and get better visual quality that way too

SideWinder · 29 Jul 2007 at 17:27

Jokester said:
Depends on how you configure it, you can either set it up so that each card renders a half of the screen, using 2 or 3 different methods or use the second card to do AA (upto x32 if I remember correctly).

Jokester

Makes sense...so if i had the 2 cards powering each half of the screen..why wouldn't it be 'double' in terms of performance compared with the single card? THIS is where i'm getting confused. :confused:

You mention something about computational power needed to combine the work...can you elaborate?

Jokester · 29 Jul 2007 at 17:32

SideWinder said:
Makes sense...so if i had the 2 cards powering each half of the screen..why wouldn't it be 'double' in terms of performance compared with the single card? THIS is where i'm getting confused.

You mention something about computational power needed to combine the work...can you elaborate?

Two things that stop you getting a 100% increase in power:-

1. Main one is that one of the cards has to deal with combining the work done by both cards into a useable image for outputting to the monitor, in the case of SLI this pretty much limits you to at most an 80% increase.

2. Having two GPUs won't increase performance where the bottleneck is the CPU (which is nearly always the case at lowish resolutions like 1024x768 or even 1280x1024 on a 8800GTX).

Jokester

Rroff · 29 Jul 2007 at 17:58

SideWinder said:
so if i had the 2 cards powering each half of the screen..why wouldn't it be 'double' in terms of performance compared with the single card? THIS is where i'm getting confused.

Because the scene the game is rendering is never uniformly busy

SideWinder · 29 Jul 2007 at 18:00

Makes more sense.

Thankyou.

setter · 30 Jul 2007 at 02:31

i had an sli setup once (2x6800gt) and i always found when the next generation of graphics card came out a single card rendered sli obsolete. i know some people like the idea of dual cards but i found it to be expensive, power hungry and very few games supported it

Rroff · 30 Jul 2007 at 03:33

There are few games that don't support it actually... prolly 1 game in every 100 that doesn't work with SLI at all, and about 40 in every 100 that see double performance (minus a small amount to overhead - typically less than 1% on high end cards) the other 59 see some boost from SLI tho typically somewhere between 1.2x and 1.6x - fortunatly the games that see the biggest performance gains are for the most part the more popular ones.

6800 weren't the greatest cards for SLI as it was still a developing technology at the time, plus the overheads had more of an impact on the final performance... with high end 7 and 8 series cards theres much less lost to overheads, etc. tho it is more likely to run into CPU bottlenecks.

setter · 30 Jul 2007 at 03:39

for all the time i had it going, my cards were leadtek 6800gt's both went boom inside 2 weeks so my sli experience didnt last to long lol ive actually still got both, never bothered to rma them like a clown

Perfect_Chaos · 30 Jul 2007 at 10:03

Even quad sli isnt 100% better than one card.

2 cards = usually 30% increase in minimum fps (where it counts) and thats at higher resolutions anyway

not really that great, its a shame they cant make it better if they could make 2 cards 60+% better in all games then it would be well worth the money.