Vertex Performance of Xenos and RSX+EDGE Comparison

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mech
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Vertex Performance of Xenos and RSX+EDGE Comparison

Post by mech »

Pretty interesting for technically minded folks...

http://forums.e-mpire.com/showthread.php?t=73148" onclick="window.open(this.href);return false;
cpiasminc wrote:
mokmok wrote:1. The Xenos Vertex Performance is up to 6x greater than the RSX's.
So I've talked about this one a few times before, but it is a flat-out absurdity in the sense that it ignores all other limitations of the respective hardware. So the assumption is that the smallest possible vertex shader is 4 dot products (basically, transform the vertex). And since you've got 8 vertex shader pipes in RSX, and 8/4 = 2 * 500 MHz = 1 billion verts.
On Xenos, you've got 48 ALUs which if you assume are all dedicated to vertex processing (this is actually impossible, but for the sake of theory we'll ignore that), you get 48/4 = 12 * 500 MHz = 6 billion verts.

Sounds that way, but unfortunately, it's completely untrue. The thing is vertices do not get moved in at unlimited speed. You can only move vertex attributes at a fixed number of attributes per clock cycle, and that means in 99% of all *major* render passes, that a single vertex takes more than one clock cycle to get in. So no matter what, it doesn't matter how much you can theoretically process because the data doesn't move through the system fast enough. The real theoretical advantage is still there for Xenos, but it is by no means 6:1. In reality, they both suck pretty bad. RSX simply sucks a little worse.

In the end, Xenos can only set up one triangle per cycle, while RSX can set up 1 every two cycles. It should be noted, though, that because of things like a post-transform cache, if you're smart, you can actually exceed the theoretical limits. And since RSX's post-transform cache is about 8x larger than Xenos', it has more potential for gain. To be fair, though, RSX needs it far more badly than Xenos does. The vertex attribute read rate on RSX is incredibly god-awful, but it's not an insurmountable wall. Xenos simply hits fewer internal limits.

BTW, about the 6 billion verts figure... that kinda ignores a little detail. This may come as a shock to a lot of people, but vertices consist of this thing called DATA. If you take a pretty average-sized vertex, 6 billion vertices per second requires more than double the bandwidth than the entire Xbox360 has... and that's including the totally internal busses which don't actually connect any two separate devices (you know how people like to pretend the the 256 GB/sec on the eDRAM die can be treated like a point-to-point link). You want to move that much data over a main memory bus (which is the real bus of concern for this purpose), that's not going to happen within the next 3 or 4 console generations. Memory architectures simply don't grow that quickly. Currently you can't move 6 billion of even the smallest possible vert (per second) over the main memory bus, and I don't see that happening on Xbox720 or PS4 either.
2. The use of the Edge Tools and SPEs brings the Vertex Performance of the PS3 on par with the 360 but prevents the SPEs from being used for Physics, AI etc.
They're kind of assuming a lot of things because the demos, which were meant for a technical audience, used all the available SPEs in order to demonstrate the concept and showcase techniques that can keep all the SPEs busy. If you actually did it like that in a real game, yeah, you'd certainly tie up all the SPEs for that period of time within the frame. Something that I think nobody outside the industry actually realizes is that the CPU side of rendering does NOT take up a huge portion of the time between frames. Physics, AI, etc. take up much more time than rendering. It's a little hard to see that with the PC as a reference point, of course, because Windows and the API layer robs you of so much.

That aside, the point of Edge is not to fill up all the SPEs. You certainly don't NEED to use more than 1 or 2 SPEs in order to get a huge gain out of it. More importantly, while Edge was specific to graphics, a lot of the same principles can be applied to physics (Havok's tech talks demonstrated that quite handily and nobody talks of Havok precluding the use of Edge) and AI and so on.
Just looking at the raw specs of the Xenos GPU it does seem to have a Vertex processing advantage.
For all you might say about the dynamic allocation of vertex pipes, you end up limited by a lot more external things than anything internal to the GPU. Also, no matter what, on major passes, you're going to end up spending more effort on pixels anyway, and RSX has a moderate advantage over Xenos in that area. All the same, getting a billion triangles per second to the GPU in the first place is basically impossible. It doesn't matter how much power the GPU has to work with them because it can't get to that point. In general, the challenge in getting 100-150 million tris per second moved through the pipe is hard enough whether you're on PS3 or 360, and it's not the GPU itself that's the problem.

I look back at how things looked when the 360 was still a little while shy of release, and back then, the notion of even drawing a scene of up to 750,000 polygons per frame at 30 fps was looking pretty much impossible to almost every developer out there. Nowadays we talk of nearly double that pretty freely. It's certainly not because the GPU suddenly got more powerful or we learned how to dedicate more ALUs to vertex processing. It's because we're doing better on the *CPU* side that we're able to keep that push buffer pushing more often.
My understanding is that pixel shaders are mainly used for bump mapping etc. whereas Vertex shader are used to render complex lighting effects etc.
Vertex shaders are simply for things you would do that operate at the level of a single vertex (transformation, positioning, and more often than not, setting up data for the pixel shaders to use). Pixel shaders are for things you would do at the level of a single pixel (all texturing, all lighting, etc). On hardware prior to programmable shaders, of course, you'd probably do just about everything at the vertex level because that's what you have available to manipulate both at the hardware and software level.
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Post by Talez »

BTW, about the 6 billion verts figure... that kinda ignores a little detail. This may come as a shock to a lot of people, but vertices consist of this thing called DATA. If you take a pretty average-sized vertex, 6 billion vertices per second requires more than double the bandwidth than the entire Xbox360 has... and that's including the totally internal busses which don't actually connect any two separate devices (you know how people like to pretend the the 256 GB/sec on the eDRAM die can be treated like a point-to-point link). You want to move that much data over a main memory bus (which is the real bus of concern for this purpose), that's not going to happen within the next 3 or 4 console generations. Memory architectures simply don't grow that quickly. Currently you can't move 6 billion of even the smallest possible vert (per second) over the main memory bus, and I don't see that happening on Xbox720 or PS4 either.
FYI, this is why the PS2 didn't look all that much better than the Dreamcast despite the PS2's specs looking overwhelming. Sure you can pump 50 million polys a second through the GS but good luck stuffing the vertex data over the bus.
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Post by mech »

PS2 didn't look all that much better than Dreamcast? Huh?
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Post by Talez »

It looked better but it wasn't the quantum leap that N64 to Dreamcast was.
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Post by mech »

Was it meant to be?

PS2 still looked better than Dreamcast once devs got a handle on it. I would say "much better" than Dreamcast too.
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Post by t0mby »

*insert pathetic troll comment here*
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Post by Pointy Cat »

That was quite some time after the DC was dead though. I wonder what would have happened had the DC got the same amount of attention and time investment as the PS2.
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Post by mech »

Mmm I was waiting for someone to say that - the thing about the Dreamcast was that its architecture was much more easily exploitable. Yeah they probably could have pushed it further, but I doubt they could have done GT4 or GOW2 on it. Nothing came close to these on Dreamcast.
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Post by lestat »

I remember melbourne house had an engine doing 8-10m polygons in development on DC, much higher than the modest 3m sega claimed the SH processors could handle. And I think as time went on ps2 games dropped in polygon count, trading off poly details for better textures to increase perceived detail. Because of the limited bus to the GPU and edram, you had to trade off one for the other.

Clearly you can see with the tekken games on ps2, later iterations used far less polys but looked much nicer.
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Post by Mavy »

So after reading that I just wanted to make sure of something. The Xbox360 is more powerful than the PS3?

Just need to be sure.

*hovers hand over PS3/Xbox360 Fanboy hat*
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Post by mech »

No.
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Post by Mavy »

So the PS3 hat then?

(Not until Uncharted and R&C)
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Post by mech »

They're just very different. From what I've read it's possible for PS3 to outdo the 360, but you really need to do some special code to do it.

I also read that PS3->360 porting is relatively easy, and the way you optimise things on the PS3 also works well on the 360, but the reverse isn't true.

I personally think the most impressive titles from both platforms will look amazing, and you'd be hard pressed to tell which came from which.
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Post by Mavy »

I think its pretty impressive that the 360 has been able to hold its own power wise against the PS3. Especially since it was released so much earlier.

There is going to be exclusive games on both systems that will be must haves for me. I think I'm getting a little impatient with the PS3 though. I want it but I don't see any reason to buy it.

Lestat, wasn't Leman's 24hour on DC the game that used that engine? I remember something about it.

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Post by Talez »

lestat wrote:I remember melbourne house had an engine doing 8-10m polygons in development on DC, much higher than the modest 3m sega claimed the SH processors could handle. And I think as time went on ps2 games dropped in polygon count, trading off poly details for better textures to increase perceived detail. Because of the limited bus to the GPU and edram, you had to trade off one for the other.
What are you talking about? The PS2 had massive bus bandwidth at all stages to compensate for the complete lack of caches and texture memory.
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Post by lestat »

Mavy wrote:I think its pretty impressive that the 360 has been able to hold its own power wise against the PS3. Especially since it was released so much earlier.

There is going to be exclusive games on both systems that will be must haves for me. I think I'm getting a little impatient with the PS3 though. I want it but I don't see any reason to buy it.

Lestat, wasn't Leman's 24hour on DC the game that used that engine? I remember something about it.

Why does your avatar look like he's giving the invisible man a blow job?
That was a post lemans engine that never saw light of day. :lol: fps doug is an interesting bloke.
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Post by Corey »

Transformers on PS2 by Melbourne House was pretty cool.
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Post by flipswitch »

MGS2> anything.And remember the cut scenes are pre rendered.
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