SLI- Whys it not 2x as fast?

Ender · 23 Apr 2006 at 12:57

Slightly random thing, i know that SLI Rigs tend to give about 30% performance boost, but what stops them from literally being twice as fast? Is it like dual core cpus arent twice as fast, because games simply arent designed for dual gpus?

Durzel · 23 Apr 2006 at 13:08

Overhead in the implementation for starters, also I imagine most powerful SLI/Xfire setups are CPU-bound.

The Running Man · 23 Apr 2006 at 13:10

i think that in some circumstances SLi will give over 70% inc in performance...

it depends on the game and the sytse,m tbh.

if a cpu/or the ram isnt fast enough for the game it is CPU limited and thus having 2 gfx cards isnt going to improve outright performance as the system is being held back.

If the res was cranked up and 8x FSAA/16x AF etc was put on, then the system with only 2 gfx card would probably start to feel gfx strain and lag behind the SLI system

therefore u need to ensure that you are not system limited before the gfx card becomes the bottleneck.

also there is the sli software layer which makes the process not 100% efficient thus u arent going to get 100% extra boost in any games.

georges · 23 Apr 2006 at 13:14

Probably because some of their power has to go towards synching them together maybe?

chex · 23 Apr 2006 at 15:33

diminishing returns to scale

some power is now dedicated to splitting the signal and putting it back into one picture.

Goksly · 23 Apr 2006 at 16:03

As meantioned, putting the picture together again takes some work, as does working out which parts each card has to render. ie SLI doesnt just split up a single frame - it looks at the frame and works out the proper ratio of work load.
Also most resolutions are cpu bound so it only helps slightly.

Naffa · 23 Apr 2006 at 16:14

Goksly said:
As meantioned, putting the picture together again takes some work, as does working out which parts each card has to render. ie SLI doesnt just split up a single frame - it looks at the frame and works out the proper ratio of work load.
Also most resolutions are cpu bound so it only helps slightly.

SLI is a lot more simple than that, all it does is one GPU renders half of the screen and another GPU renders the other. The SLI bridge basically joins the cards and makes them work as one rather than Crossfire where theres a primary GPU and the secondary one assists it.

D.P. · 23 Apr 2006 at 16:20

naffa said:
SLI is a lot more simple than that, all it does is one GPU renders half of the screen and another GPU renders the other. The SLI bridge basically joins the cards and makes them work as one rather than Crossfire where theres a primary GPU and the secondary one assists it.

SLI is a lot more complex than that.

There are around 4 different SLI rendering modes, these can be split into the 2 main types: Split Frame Rendering & Alternate Frame Rendering.

SFR: not used much now, this mode has both casrds render the same frame with the load balancing logic which decide how much of each frame to render on each card (otherwise you will have 1 card rendering the sky and the other card rendering everything else!).

AFR: One card renders one frame, the ther card renders the next frame, lots of logic used to keep the timing right.

Fx-Overlord · 23 Apr 2006 at 16:22

naffa said:
SLI is a lot more simple than that, all it does is one GPU renders half of the screen and another GPU renders the other. The SLI bridge basically joins the cards and makes them work as one rather than Crossfire where theres a primary GPU and the secondary one assists it.

Are you sure?

I'm pretty sure in split frame rendering sli works as Golsky mentioned with the screen splitting up depending on workload. I can definately remember screenies showing splits of 70% in 1 card and 30% in another in farcry for instance.

For crossfire its slightly different because their split frame is baiscally splitting the frame in a checkerbox of 64, with each video card rendering different parts of that checkerboarding, to make sure each GPU gets a good split of rendering load.

Goksly · 23 Apr 2006 at 17:28

salutes the return of D.P -> prolly an imposter... agreeing with me an all

speeduk · 23 Apr 2006 at 18:18

Proper gpu limited games like FEAR and Oblivion give about 90% increase when using SLI or xfire. Other games limited somewhat by the CPU or lacking in driver support for SLI will not work as good.

Naffa · 23 Apr 2006 at 18:36

D.P. said:
SLI is a lot more complex than that.

There are around 4 different SLI rendering modes, these can be split into the 2 main types: Split Frame Rendering & Alternate Frame Rendering.

SFR: not used much now, this mode has both casrds render the same frame with the load balancing logic which decide how much of each frame to render on each card (otherwise you will have 1 card rendering the sky and the other card rendering everything else!).

AFR: One card renders one frame, the ther card renders the next frame, lots of logic used to keep the timing right.

No, SLI is not more complicated than that.

SLI or 'Scalable Link Interface' uses whats known as 'Scissors Rendering' to split the work load between the two cards. One card takes responsibility for the top half of the screen, more or less, and the other card renders the bottom half. If one of the halves is simpler to render, such as a cloudless sky, that card will take on some of the other card's burden to equalise their loads. This is as complicated as it gets.

Goksly · 23 Apr 2006 at 19:02

if it was that simple, why would nvidia include a function to show how much of the screen each card is rendering?

As you can see, when the screen is full of complex items (top pic) then its 50/50... but when there is more calculations to be done rendering the bottom half, the split is 70/30 (2nd pic). It can only work out how much each card should render by calculating it - hense the complexity :}

Naffa · 23 Apr 2006 at 19:10

Hence why I said that happened.

Goksly · 23 Apr 2006 at 19:11

that was what I said in my first post which you disagreed with. you said it was more simple and it just did half the frame.

Naffa · 23 Apr 2006 at 19:13

Ah yeah, I see. That's fair enough. Did seem like what I said was wrong. What I said in my longer post was what I meant to say in the first one.

Sorry about that.

Goksly · 23 Apr 2006 at 19:14

so i posted something, you disagreed with it.... we said it was right - you said it was wrong and then said the same theory ;p
ah - all in the day of the gc forum :x

Naffa · 23 Apr 2006 at 19:20

lol.. I didn't understand that but I liked the smileys

HangTime · 23 Apr 2006 at 20:04

-cpu (and to a lesser extent, chipset/memory) limitations
-implementation overheads (driver issues etc)

In cases were games are very gpu limited (i.e. modern titles at high res/AA/AF), you will see much larger gains than you do normally:

http://www.firingsquad.com/hardware/2560_1600_gaming_preview/page4.asp

Here you can see that SLI/Crossfire is extremely efficient, giving around 80%+ performance improvement. To be honest, I think SLI is fine, the problem is that most people are running them in cpu limited situations, e.g. on slow cpus and/or not in high res/AA/AF. To get the most out of SLI you really need a high end monitor (22"+) and a decent cpu (FX-57+).

In fact for me, the fact that we see such large gains from SLI is proof that current graphics cards are simply too slow and that we need MORE POWER!

D.P. · 23 Apr 2006 at 20:32

Goksly said:
salutes the return of D.P -> prolly an imposter... agreeing with me an all

Final year exams.....