why linus said in youtube "whe you running two gpu's only can use half memory"

[ahmaden2]

hey guys ,
i got shocked when heard this in from linus .... ( in 7970 unboxing vid )

he exaclty said :
whe you running two gpu's in sli/cross you only able of to take advantage of half of that frame buffer ,
if you take two of this 3 gig card and run them in crossfire you would have 3 of effective memory , whereas if you took a gtx 590 and ran it in your system (it's running in sli by default ) you actualy only have 1.5 gig which is fine for single display....
--------------------------------------------

i thought that if we have two 3gig cards , and when run'em in sli/cross , we have 6 gigabyte of memory for rendering graphic content ....

i put the video ( look around 4:20 )
Powercolor Radeon HD 7970 3GB Video Card Unboxing & First Look Linus Tech Tips - YouTube


someone pls describe this law for me in detail....
and what's exacly " effective memory " ?
does this mean if we have 2 3gb card we can only use half of total volume ?
and if graphic content pass 1.5 gig ( for example 2 gig ) , do we have bad rendering quality in result ?

[Bond007]

every gpu needs its own memory. if you have two gpu in sli/ crossfire or two gpu on a singe card (like a gtx 590 or hd 6990) each physical gpu needs memory. So a 4gb hd 6990 actually only has 2gb memory per gpu.

[great_big_abyss]

Take a moment to imagine how crossfire works. Alternate frames will be rendered by each card.

Imagine a sequence where each frame is named: A, B, C, D, E, F, G, and H.

Card 1 will render frames A, C, E, and G.
Card 2 will render frames B, D, F, and H.

So, each card renders an entire frame. Now, each frame will require a certain amount of video memory before, during or after the rendering process. Let's say that a rendered frame requires 1GB of video memory before making it up to your 3 monitors.

Since each card is rendering its own frame, the individual card will use 1GB of memory per frame. Having 2 cards simply means that each card is using 1GB of it's own memory to render separate frames. Imagine now that you want to play on 6 monitors. That's double the resolution of 3 monitors, so essentially each frame will require double the memory. An individual frame would now require 2GB of memory. If your video card has 2GB of memory, then all is good. if your video card only has 1GB of memory, you'll run into problems.

Just because you have two video cards, your memory doesn't double. Remember, each video card renders its own frame, and each individual card has to have enough memory to meet that one entire frame's requirement.

Hope I explained it to you well enough.

[ahmaden2]

yes helpfull for me , tank you great_big_abyss , and Bond007
but not complete....

in the end you said :
Just because you have two video cards, your memory doesn't double ....

but i can't understand that !
for example if required memory for playing batman arkham city on specific res. on high setting is 1gig ,
and i have two 512mb video card.... funny question is can i play the game smoothly and without frame drop ?

[Squeetard]

Your system is basically only using 512mb. Duplicating the stuff on each card. Your effective memory is only 512mb.

Once you fill the on-board memory of a video card you may or may not experience noticable stuttering/ frame drop. As always, try it on high, if you don't like it then back it down.

[ahmaden2]

Originally Posted by Squeetard

Your system is basically only using 512mb. Duplicating the stuff on each card. Your effective memory is only 512mb.

Once you fill the on-board memory of a video card you may or may not experience noticable stuttering/ frame drop. As always, try it on high, if you don't like it then back it down.

someone describe this "effective memory" guys......

does this mean if located graphic content pass 512mb , GPU's cant work to their full potential ? is it true ? what's exactly that "effective memory " ?
should i expect best rendering for half of totall vram , and once pass this limitation rendering quality drop down or something else ?!!!!!

respond separately by quote if possible .

[SKYMTL]

The issue is the fact that AFR mode (alternate frame rendering) results in each card rendering one frame at a time, effectively using the framebuffer allocated to each card instead of doubling them up. This means that in memory limiting situations, you may get significant drops in minimum framerates while the average performance results will be much, much higher than with a single card. HOWEVER, in non memory bottlenecked situations, you will get amazing performance.

[sswilson]

OP was banned for inappropriate posts a month ago, second account was banned shortly afterwards, but apparently didn't stick. That situation has been rectified.