Memory bandwidth not allowing for 3D games running at native on iPad 3?

[Steelbom]

Hi,

I'm wondering if anyone can elaborate further on what Anand wrote in his Analysis of the new Apple iPad, about memory bandwidth not allowing 3D games to run at native resolution.

Also keep in mind that memory bandwidth limitations will keep many titles from running at the new iPad's native resolution. Remember that we need huge GPUs with 100s of GB/s of memory bandwidth to deliver a high frame rate on 3 - 4MP PC displays. I'd expect many games to render at lower resolutions and possibly scale up to fit the panel.

Does anyone know the bandwidth of the PowerVR SGX543? Wouldn't four of them be enough to handle 3.1 million pixels? According to Wikipedia, the PowerVR SGX543 can do 35 million triangles per second and 1 GPixel per second, at 200MHz. The iPad 3's clock speed is about 250MHz, and it has four of them. Do either of those two things have anything to do with the memory bandwidth of the GPU?

Would appreciate any information! Thanks!

(Link: http://www.anandtech.com/show/5663/analysis-of-the-new-apple-ipad/1)

EDIT:

Real Racing 2 (on iPad 2) is rendered at 1080p w/ 30 FPS when connected via HDMI to an HDTV. That's 2 million pixels, wouldn't the extra two SGX543's in the iPad 3 allow for enough bandwidth to push 3.1 million pixels? (The only difference I know of between Real Racing 2 on iPad 2 or 4S (768/640p) vs HDTV (1080p) is the latter doesn't have any AA.)

 

[runawayprisoner]

He's alluding to the fact that if the iPad 3 were to have 3D games with graphics as good as the XBox 360 or the PS3, it'll have to render at lower resolution because RAM bandwidth in the iPad 3 isn't fast enough even though the GPU might actually be fast enough. That's partially why the PS Vita has a resolution of 960 x 540 and not something higher like 1280 x 720.

Regarding Real Racing 2, the game is actually surprisingly simple. It doesn't employ a lot of shading techniques, textures are low resolution, and the geometric of car models and track models are pretty primitive. Try to up those and you'll see the iPad 2 run into severe RAM bandwidth, which is why it's limited to 1920 x 1080 @ 30fps.

While it's definitely possible for the iPad 3 to do something similar at its native resolution, it'll need a lot more RAM bandwidth just to maintain 30fps in a game that's as simple as Real Racing 2, and then you'll miss out on more complex shaders, higher resolution textures, and etc...

Actually, textures would be the main reason for the bandwidth bottleneck.

Unless Apple also did the impossible and put in significantly faster RAM, I don't think the iPad 3 would be able to render many 3D games at its native resolution.

Do note that many desktop-grade GPUs struggle and cough up blood at such resolution. Right around 1920 x 1080 is where 3D gaming tops out and beyond that, you need significantly faster cards and a lot more VRAM to maintain 60fps.

 

[Steelbom]

He's alluding to the fact that if the iPad 3 were to have 3D games with graphics as good as the XBox 360 or the PS3, it'll have to render at lower resolution because RAM bandwidth in the iPad 3 isn't fast enough even though the GPU might actually be fast enough. That's partially why the PS Vita has a resolution of 960 x 540 and not something higher like 1280 x 720.

Regarding Real Racing 2, the game is actually surprisingly simple. It doesn't employ a lot of shading techniques, textures are low resolution, and the geometric of car models and track models are pretty primitive. Try to up those and you'll see the iPad 2 run into severe RAM bandwidth, which is why it's limited to 1920 x 1080 @ 30fps.

While it's definitely possible for the iPad 3 to do something similar at its native resolution, it'll need a lot more RAM bandwidth just to maintain 30fps in a game that's as simple as Real Racing 2, and then you'll miss out on more complex shaders, higher resolution textures, and etc...

Actually, textures would be the main reason for the bandwidth bottleneck.

Unless Apple also did the impossible and put in significantly faster RAM, I don't think the iPad 3 would be able to render many 3D games at its native resolution.

Do note that many desktop-grade GPUs struggle and cough up blood at such resolution. Right around 1920 x 1080 is where 3D gaming tops out and beyond that, you need significantly faster cards and a lot more VRAM to maintain 60fps.

Mmmm, I see. So this "bandwidth" is the bandwidth of the GPU? Just like on the desktop? Except much less? Or is it just RAM? Hopefully Apple's managed to do something that'll allow for native res games, because they'll look the best.

 

[runawayprisoner]

The simple answer is that it's bandwidth of the RAM.

But I think it's worth noting that currently, the iPad 2 has bandwidth clocking in at 12.8GB/s. If Apple moves to LPDDR2-1066 then they can up that to 17GB/s, which would put the iPad 3 closer to the PS3 in terms of bandwidth, which is about 22.4GB/s for VRAM.

 

[Steelbom]

The simple answer is that it's bandwidth of the RAM.

But I think it's worth noting that currently, the iPad 2 has bandwidth clocking in at 12.8GB/s. If Apple moves to LPDDR2-1066 then they can up that to 17GB/s, which would put the iPad 3 closer to the PS3 in terms of bandwidth, which is about 22.4GB/s for VRAM.

Ah, I see. What's the Vita's VRAM bandwidth? It's certainly possible for them to move up to 1066 isn't it? I expect they may do that if 800 wasn't sufficient.

 

[runawayprisoner]

Yeah, it's definitely possible for them, since Tegra 3 also supports LPDDR2-1066.

I'd actually love to see if Apple didn't move to DDR3L-1500, which would give them a very nice bandwidth boost to 24GB/s on dual-channel configuration, but that's just wishful thinking.

 

[Steelbom]

Yeah, it's definitely possible for them, since Tegra 3 also supports LPDDR2-1066.

I'd actually love to see if Apple didn't move to DDR3L-1500, which would give them a very nice bandwidth boost to 24GB/s on dual-channel configuration, but that's just wishful thinking.

I'm assuming you mean "did", but yeah that would be pretty cool.