Video Card Memory Analysis: 256MB vs. 512MB

[Jameson]

How many forums have you visited in the last month with threads discussing whether 512MB video cards are really necessary? Well hopefully this article will shed some light on this topic.

A few weeks ago I reviewed the Gainward BLISS 7800 GT Goes like Hell, which is a supercharged 7800 GT with 512MB memory. It's clocked at 450/650 ( standard GT is 400/500 ) and that is undoubtedly the main reason for its superior performance, but an interesting question arose after the review went live - Did the 512MB framebuffer make any difference? I don't believe anyone reading this would disagree that 512MB cards will become a firm requirement of high end gaming very soon, but the key question here is do 512MB framebuffers offer anything to us now, other than future proofing? To answer this we first need to understand exactly what happens to a 256MB card when you max out its memory (VRAM).

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[blackberry]

Thanks for this article James.

Another possible advantage of the 512mo could be that Windows Vista might need it to enable the 3D display.....

[blackberry]

Another question:

I wonder what would be the impact of 256mo vs 512mo if one is using 2 monitors instead of 1 ?

[ralgha]

Hello, and thank you for the article. I wanted to make a suggestion. Please reconsider your use of the word 'framebuffer'. A framebuffer is a buffer used to store a frame. In graphics cards, this is the frame that's displayed by the monitor. The entire memory on the graphics card is not the framebuffer. For example, my framebuffer at the moment is approximately 1600*1200*4=7.6MB, out of a total of 128MB of memory on my graphics card.

When doing 3D rendering, there are other buffers that are tightly associated with the framebuffer, such as Z-buffers and stencil buffers. You may or may not decide to lump these in with the framebuffer's memory, since they scale proportionally to the size of the framebuffer. But memory used for texture maps and geometry is clearly separate.

[Jameson]

Knew there would be a mistake Though I agree with you on this, almost all the websites i know and read are calling the entire memory a framebuffer, so i went along with this. Even Beyond 3D's artcle here is stating the card has a "512MB framebuffer". I'm not sure if to make things simpler the term has been generalized.

So in answer I'm not sure if swapping out framebuffer for another word is important. What word would you suggest?

Thanks!

[Jameson]

Thanks for this article James.

Another possible advantage of the 512mo could be that Windows Vista might need it to enable the 3D display.....

Ahh yes, Vista. From what I understand, Vista might need a ton of VRAM, and it's possible that if it starts virtualising VRAM to system memory it could slow the whole OS down.

I don't know much about what Vista will cause though, not following it to be honest.

[ralgha]

True, you're definitely not the first person to use framebuffer this way. But since it's so clearly wrong, and has very important implications for understanding how the memory is used and the performance characteristics, it seems like a poor generalization to me. That is, I don't think I'm really nit picking here.

There doesn't seem to be a single word or phrase that could be used in place of framebuffer in every case. I think the sentences using it would have to be modified. For example:

"Did the 512MB framebuffer make any difference?" ->
"Did the 512MB of memory make any difference?"

"do 512MB framebuffers offer anything to us now" ->
"do 512MB cards offer anything to us now"

"its just the way this particular game manages the framebuffer" ->
"it's just the way this particular game manages the video memory"

"while the 512MB card has no problems fitting the framebuffer within VRAM"
This is a particularly important example of the consequences of misusing the word framebuffer. Any normal video card with a significant amount of VRAM (i.e. not entirely shared, not "turbo cache" whatever) will never use system memory for the framebuffer. (I doubt it's even possible.) VRAM is always used for the framebuffer first, and textures+geometry get whatever's left over. To do otherwise would be a huge and unnecessary performance hit. So your statement should probably read:
"while the 512MB card has no problems fitting the textures within VRAM"
or maybe
"while the 512MB card has no problems fitting everything within VRAM"

Please remember that the amount of memory consumed by a framebuffer and its associated buffers is very small compared to texture data. Even a 2048x1536 framebuffer at 32bpp with a 32bpp z-buffer and 8-bit stencil buffer would still only come to 28MB. (Although, anti-aliasing can add significant overhead depending on its implementation, I think...)

By the way, I think it's very cool that you used Video Memory Watcher and shared the data and your interpretations. This is what made your article unique and interesting to me.

[Jameson]

Indeed, i think it's very clear that i should not follow the status quo on this one, it's inaccurate to use the term framebuffers in the ways i did. I will update the article to correct this.

Seriously though, thanks for your pointers

[ten]

Great article, many thanks!

Just a quick question.....How would 2x 256mb cards in SLI compare in this test? Would the 2 vmems be independent resources or combined to perform as a normal 512mb card?

Thanks,

ten

[Jameson]

Welcome

As far as I'm aware both SLI and CrossFire mirror the texture and geometry buffers on both cards, so effectivly you still have 256MB.

One card can't access the other cards memory for data because of limitations with bandwidth over the SLI/Crossfire connection methods used, so the same data needs to be available natively in both cards VRAM.

[MadBoris]

I'm glad to see you put this article together, hopefully more will come down the road.
I recently got tired of people talking about AGP aperture sizes and finally decided to do some testing on my own with some home grown tools.
I did some video memory testing like this with the game X3 which can be seen here if anyone is interested:
http://forum.egosoft.com/viewtopic.php?t=132164

So it was funny to come across this article today
I have been wondering why I don't see more articles like this on the web, I'm sure there will be more in the future as tools grow, keep up the good work.

[Jameson]

I like what you did there Madboris, good stuff

[razzmonid]

Great article! For once the question was answered.

They kept reviewing other things like comparisons of other card based on the FPS and other things like overclocking capacity, but the simple issues like 256MB vs 512MB was not being mentioned at all. This is good and I'm hoping that the other cards maybe reviewed as well for the justification of the added memory that does not seem to be having a visible effect on the performance like: GeForce 6600 and the 6600GT variant. Which may just be used on low res/low IQ kind of settings and also the Radeons maybe reviewed as well, so as to the sweet spot for the price. It looks like the added memory does not benefit that well and may well be just purchasing the next level of card rather than spend it on extra that may not be utilized that much at all...

Anyway hope that more cards and variations maybe explored... like other games performance, and like also the justification of buying 256MB cards with just 128bit wide memory pipeline.

[Jameson]

I might consider a part 2 to this subject including more games and perhaps looking at the effects on an ATI card, as I've heard they react differently to Nvidia cards. Games that might be added are Splinter Cell, Serious Sam 2 and COD2.

I'll see if i can arrange something for next month maybe.

[blackberry]

Hi James,

As you know on my side, I've decided to keep on with the 7900GT 256mo (Inno3D) instead of the 7800GT GLH 512mo.

I've tried Quake4 on 1280x1024 with AA and AF at max and it's perfect. As I don't intend to go higher in resolution, I think this was the best choice for me.

Especially for the price : £226 @ Dabs !!!!

[aerospyke]

Hi guys, after reading this article i was motivated to see what my system was doing whilst playing eq2. And this is what i found.

Whilst playing, my system memory is ALWAYS at max of 247MB, and my VRAM is the one that goes up and down. playing eq2 on 1024x768 8x AF and 2xAA. AMD64x2 4400+, 1GB RAM and a 7800GTX 256 are my gear. ANyone have an idea why this may be happening? Or have a solution?