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[SLI] Vram Debate... - Page 3

post #21 of 26
Quote:
Originally Posted by brettjv View Post
The reason that a 512MB (oh, lets just say an) 8800GT will outperform a 1GB model in a majority of situations (typically by <1%) is because the 1GB card has looser VRAM timings than the 512MB version.
Correct, that is indeed a contributeing factor. In the majority of cases if you were to compare two similar priced graphics cards from the same manufacturer, such as the 8800GT 512MB and 8800GT 1GB, you'd find that the card with larger memory capacity has lower quality memory. By lower quality I mean lower speed capability and higher timeings in general.

Quote:
Originally Posted by brettjv View Post
And to the extent performance is 'hindered' in a 1GB 8800GT in any absolute sense, it has just as much if not more to do with the inherent limitations of the GPU as it does with the memory BANDWIDTH ... cuz remember, bus width is only one part of the bandwidth equation.
Indeed, bandwidth is measures how fast data can be sent to and from memory to the processor. However, motherboards have one interface for each individual RAM DIMM while graphics cards only have a single interface between the GPU and the respective set of memory. As I said earlier, if two cards are priced the same, but one has a larger amount of memory, you can be sure that the larger capacity card is going to have lower frequency capable RAM on it. This obviously effects overall bandwidth.

Memory's effective speed x number of interfaces x bus width x bits per clock cycle

Bandwith of typical 8800GT 1GB with GDDR3 1800Mhz memory;

900 x 1 x 256 x 2 = 460800megabits / 8 = 57600MB / 1024 = 56.25GB p/s

Bandwidth of typical 8800GT 512MB with GDDR3 2000Mhz memory;

1000 x 1 x 256 x 2 = 512000megabits / 8 = 64000MB / 1024 = 62.5GB p/s

As you can see, the card with lower memory is able to transmit 6.25GB more data every second between the GPU and memory. Considering that's every second, that's quie a sizeable amount of data.

Quote:
Originally Posted by brettjv View Post
Hopefully you see why I take issue when people say this about the memory bus width 'limiting' performance of a larger memory card.
Quote:
Originally Posted by brettjv View Post
It really has more to do with the core and it's capabilities overall, as well as the fact that the extra memory only gets USED in limited situations, thus, giving the appearance that it's 'not working' to improve performance. Extra VRAM NEVER HAS and NEVER WILL give you 'across the board' performance improvements. It's only when you NEED IT due to the game and the settings that it makes ANY difference having more of it.
I understand what you're saying, but the fact of the matter is that it does limit performance somewhat, as the majority of games out now don't seem to require more than 512MB of graphics memory. Yes you are correct with regards to more memory only haveing an effect in games/applications that require it. However, the fact that more memory costs more means that manufacturers resort to useing lower quality memory. This in turn means that the only area that could be improved to bring bandwidth up to par is the memory bus.

You can't increase the amount of interfaces, and there's no such thing as triple data rate memory yet. With this in mind the bus width is what needs to be focused on if larger capacity cards expect to perform as well as their lower capacity equivilents in the majoirity of applications.
post #22 of 26
Okay, so we basically don't disagree, which I suspected since you seem like a smart cat based on other posts of yours I've seen.

But would you not agree that its probably more accurate, when comparing performance between, say the two cards we've used as examples, to talk about the effect of lower quality memory being present on the larger vram capacity card, and the limited usefulness of having >512MB in todays games, than it is to simply say that the memory bus width is hindering the performance of the larger capacity card?

And I mean, a wider bus on ANY card is going to be preferable, regardless of the amount of VRAM, right?

Not baggin on you man, it's just that I see SO many people on these boards like *authoritatively* stating that the (relatively small) 256bit bus is *why* the 512MB card usually beats the 1MB card, and, frankly, that is just incorrect. And it bugs me ... because if you used the exact same vram at the exact same timings and clocks (which you probably *could*, assuming the mem controller is up to it), the cards would perform IDENTICAL ... until you started using over 512MB.

Anyways, good discussion, man
    
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post #23 of 26
brettjv has been pretty much spot on about why memory capacity does not stack in SLI and with regards to performance on a 256-bit bus.

The whole "more than 512MiB of memory is useless on a 256-bit bus" is a rough rule of thumb that has been taken to far.

For example, if you gave an 8800GT a 512-bit bus, 1GiB of video memory would still be mostly worthless on it. Very few games will use that kind of memory without so much AA that the GPU would not be able to render fast enough, no matter how much memory bandwidth it had access to.

On the other hand, If you just ramped up texture sizes, 1GiB of memory could theoretically be of great benefit to a card with only a 128-bit bus.

In this respect, it's a lot like system memory. A decent S754 AMD chip (single 64-bit memory channel) will see just as much relative benefit from 4GiB of ram when playing demanding games in Vista as a dual channel setup will.

Most recent video cards with 512MiB or 1GiB of memory will run into major fill rate limitations well before a lack of memory bandwidth cipples them.
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post #24 of 26
Quote:
Originally Posted by Blameless View Post
brettjv has been pretty much spot on about why memory capacity does not stack in SLI and with regards to performance on a 256-bit bus.

The whole "more than 512MiB of memory is useless on a 256-bit bus" is a rough rule of thumb that has been taken to far.

For example, if you gave an 8800GT a 512-bit bus, 1GiB of video memory would still be mostly worthless on it. Very few games will use that kind of memory without so much AA that the GPU would not be able to render fast enough, no matter how much memory bandwidth it had access to.

On the other hand, If you just ramped up texture sizes, 1GiB of memory could theoretically be of great benefit to a card with only a 128-bit bus.

In this respect, it's a lot like system memory. A decent S754 AMD chip (single 64-bit memory channel) will see just as much relative benefit from 4GiB of ram when playing demanding games in Vista as a dual channel setup will.

Most recent video cards with 512MiB or 1GiB of memory will run into major fill rate limitations well before a lack of memory bandwidth cipples them.
Couldn't have said it better myself. Thanks for having my back

Also, I want to say for that record that if you were looking for a setup that would really last awhile, wanted to play at high resolutions/AA levels, and the bucks to do it, getting 2x8800GT 1GB in SLI (or even better 2x8800GTS 1GB, if those can be found) makes a fair amount of sense. But I personally wouldn't spend much extra for a single 1GB 8800GT, because that's really too much VRAM for one g92 core. 2 cores in SLI, though ... that's a different story, I believe.
    
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post #25 of 26
Quote:
Originally Posted by brettjv View Post
It doesn't work that way I'm afraid.

As it stands now, no matter what frame-rendering configuration for doing SLI anyone has come up with, both cards have to have all the textures that COULD come into their 'field of view' if you will, otherwise, you'd have to move texture data across the PCI-express bus to move it from one card to the other, and your performance would immediately tank.
the memory is used for more then just texture though isn't it?
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post #26 of 26
Yes, but the point still remains the same. Both cards need to have all the same data in their memory in order to be able to perform their functions as quickly as they need to, because if they didn't, they'd be having to pass data back and forth across the pci-express bus to complete their functions ... this going to be too slow. I used textures as an example to keep the explanation simple and because texture loading at level-startup is a simple concept to grasp, and textures compose the largest chunk of memory usage.
    
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