Sign in with
Sign up | Sign in
Your question
Closed

Crossfire and Memory Usage

Last response: in Graphics & Displays
Share
January 16, 2013 3:15:02 AM

Well I asked this question on the ROG forums and so far I haven't got a response, so I'll give it a try here.

In 2010 I bought 2 HD6870's w/1 GB of GDDR5 each to run 3-24in monitors in eyefinity. I was under the common misconception that 2 cards with 1 GB of vram apiece would equal 2GB of usable vram. Although to my dismay this misconception was cleared up shortly after the purchase.

Recently while looking at dual GPU cards like the HD7990 I have found myself questioning my understanding of vram usage and Grossfire. Now I'm not sure I have a complete understanding of what happens to that vram on the 2nd card or in the case of a dual GPU card the vram that's dedicated to the 2nd GPU (if any)

Some of what makes me question my understanding is this Club 3D Radeon HD 7990 6GB review http://www.bit-tech.net/hardware/gra...0-6gb-review/1

The review states "The two GPUs are each served by six 64-bit memory controllers, which are in turn connected to two 256MB GDDR5 memory modules each, for a grand total of a whopping 6GB of available graphics memory (3GB per GPU). The memory frequency has also been left at the stock HD 7970 3GB setting of 1.375GHz (5.5GHz effective), giving both GPUs a total memory bandwidth of 264GB/s."

Since a dual GPU card is 2-GPU's in crossfire via a PLX chip why would the rules for memory usage be any different or does the total of 6GB on this card or a card like the ARES II really translate into only 3GB's usable GDDR5? And if that's the case does the vram associated with the 2nd GPU (or in the case of CF on two physical cards, the 2nd card) do any thing at all or is it just wasted?

And before anyone responds with something silly like: Did you read the sticky? Re-read (and try to understand) the question in it's entirety. The sticky explains what I wrote in the first paragraph but not the question (or inconsistency's) I'm asking about. :sol: 

More about : crossfire memory usage

February 5, 2013 2:25:04 AM

Bump
Score
3

Best solution

a c 171 U Graphics card
March 22, 2013 1:07:18 PM

any card that has 2 gpu's on one board like the ares or gtx690, you take the advertised amount of ram it has and half of that is useable as its essentially 2 cards in crossfire, just on the one circuit board. 2 cards in crossfire the ram in each card is mirrored, like hard drives in mirrored raid, you only get the capacity of the smallest drive, it does not add up together. So anytime you see 2 or more gpu's together in crossfire/sli, weather on one card or two or three cards, the amount of vram useable will be the ram of a single card.
Share
Related resources
March 22, 2013 2:19:25 PM

Yea, that's kind of what I thought. What threw me was when the reviewer said "for a grand total of a whopping 6GB of available graphics memory (3GB per GPU)". IMO the word "available" shouldn't have been used in that statement.
Anyway thanks for the response.

Score
0
a c 216 U Graphics card
March 22, 2013 5:45:13 PM

Idonno said:
Yea, that's kind of what I thought. What threw me was when the reviewer said "for a grand total of a whopping 6GB of available graphics memory (3GB per GPU)". IMO the word "available" shouldn't have been used in that statement.
Anyway thanks for the response.


That's marketing for you. The full truth is, it is possible to utilize them together to some extent in non-AFR configurations. However, AMD and Nvidia has pushed all developers to utilize AFR, which basically requires both cards to have their own copy of all the data. In crossfire modes which split the screen up for each card, each cards memory would go a bit further.

Score
0
March 23, 2013 9:22:46 PM

Alright then here's another question for you, since most of the time (when they are both being used by a app in CF) the cards are operating in AFR mode does the memory on the 2nd card help at all since each card is only rendering one frame at a time at alternating intervals, therefore sharing the workload?
Score
0
a c 216 U Graphics card
March 23, 2013 9:47:41 PM

Idonno said:
Alright then here's another question for you, since most of the time (when they are both being used by a app in CF) the cards are operating in AFR mode does the memory on the 2nd card help at all since each card is only rendering one frame at a time at alternating intervals, therefore sharing the workload?


In AFR, each card renders alternating frames. That means each card must do all the things the other card does to render the frame. That means, yes, each cards memory is equally important.
Score
0
!