The GPU and its Performance

We’ve gone through, in great depth, the CPU in the new iPhone 3GS. But the GPU is arguably the more interesting upgrade. I’ve already covered the GPU pretty well so I present you with a blatant copy/paste of what I’d already written:

Now that we’re familiar with the 3GS’ CPU, it’s time to talk about the GPU: the PowerVR SGX.

Those familiar with graphics evolution in the PC space may remember Imagination Technologies and its PowerVR brand by their most popular desktop graphics card: STMicro’s Kyro and Kyro II. The Kyro series used the PowerVR3 chips and while STMicro ultimately failed to cement itself as a NVIDIA competitor in the desktop, the PowerVR technology lived on in ultra-mobile devices.

The SGX is on Imagination Technologies’ fifth generation of its PowerVR architecture, and just like the Kyro cards we loved, the SGX uses a tile based renderer. The idea behind a tile or deferred renderer is to render only what the camera sees, not wasting clocks and memory bandwidth on determining the color of pixels hidden by another object in the scene. Tile based renderers get their name from dividing the screen up into smaller blocks, or tiles, and working on each one independently. The smaller the tile, the easier it is to work on the tile on-chip without going to main memory. This approach is particularly important in the mobile space because there simply isn’t much available bandwidth or power. These chips consume milliwatts, efficiency is key.

The MBX-Lite used in the original iPhone was also a tile based architecture, the SGX is just better.

Also built on a 65nm process the PowerVR SGX is a fully programmable core, much like our desktop DX8/DX9 GPUs. While the MBX only supported OpenGL ES 1.0, you get 2.0 support from the SGX. The architecture also looks much more like a modern GPU:

Pixel, vertex and geometry instructions are executed by a programmable shader engine, which Imagination calls its Universal Scalable Shader Engine (USSE). The “coprocessor” hardware at the end of the pipeline is most likely fixed-function or scalar hardware that’s aids the engine.

The SGX ranges from the PowerVR SGX 520 which only has one USSE pipe to the high end SGX 543MP16 which has 64 USSE2 pipes (4 USSE2 pipes per core x 16 cores). The iPhone 3GS, I believe, uses the 520 - the lowest end of the new product offering. Update: Thanks to BlazingDragon and psonice for pointing out that the 3GS may in fact use a PowerVR SGX 535 based on drivers on the iPhone 3GS. It could still use a lower end SGX core and use the 535's driver, but there's at least the possibility that the 535 is under the hood of the 3GS.

A single USSE pipe can execute, in a single clock, a two-component vector operation or a 2 or 4-way SIMD operation for scalars. The USSE2 pipes are upgraded that handle single clock 3 or 4 component vector operations, have wider SIMD and can co-issue vector and scalar ops. The USSE2 pipes are definitely heavier and have some added benefits for OpenCL. For the 3GS, all we have to worry about is the single USSE configuration.

  iPhone 3G (PowerVR MBX-Lite) PowerVR SGX @ 100MHz PowerVR SGX @ 200MHz
Manufacturing Process 90nm 65nm 65nm
Clock Speed ~60MHz 100MHz 200MHz
Triangles/sec 1M 3.5M 7M
Pixels/sec 100M 125M 250M

 

In its lowest end configuration with only one USSE pipe running at 200MHz, the SGX can push through 7M triangles per second and render 250M pixels per second. That’s 7x the geometry throughput of the iPhone 3G and 2.5x the fill rate. Even if the SGX ran at half that speed, we’d still be at 3.5x the geometry performance of the iPhone 3G and a 25% increase in fill rate. Given the 65nm manufacturing process, I’d expect higher clock speeds than what was possible on the MBX-Lite. Also note that these fill rates take into account the efficiency of the SGX’s tile based rendering engine.

Obviously all games on the app store were designed for the PowerVR MBX-Lite and the SoC in the iPhone/iPhone 3G/iPod Touch. Some do run faster/smoother on the 3GS thanks to the faster CPU and GPU, but the new hardware should enable an entirely new class of game in the future.

Developers will have to deal with a segmented user base, since there are around 20 million PowerVR MBX-Lite based phones and a bit over 1 million SGX based 3GSes in the market.

The GPU Comparison

There are obvious limitations to the iPhone 3GS being used as a gaming platform. But there are also obvious limitations to the 3GS being used as a camera or a video player. The point isn’t that the 3GS could replace the Nintendo DS or the Sony PSP, it’s that the iPhone could get you maybe 80 - 90% of the way there.

There are serious control and battery life limitations, but the platform has the right combination of hardware, a software delivery system and ubiquity to at least be considered a viable gaming system. It has a much more powerful CPU/GPU combination than a Nintendo DS, now what it needs is the iPhone equivalent of Mario. Perhaps Apple should buy a game developer.

The obvious smartphone GPU comparison here is the Palm Pre. The TI OMAP 3430 used in the Pre (good job Palm on full disclosure there, just add clock speeds and you’ll get an A+) is a very similar SoC to the Samsung chip in the iPhone 3GS. As such, it also has a PowerVR SGX core.

The CPU and its Performance Camera and Video Capture
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  • MrJim - Wednesday, July 8, 2009 - link

    Why no mention of the heat issues? Reply
  • ViRGE - Wednesday, July 8, 2009 - link

    Anand, if you haven't already, jailbreak the 3GS and grab SysInfoPlus from Cydia. It may be able to tell you the clock speed of the 3GS's ARM, although to what extent I'm not sure since it hasn't been specifically programmed for the A8. Reply
  • ltcommanderdata - Wednesday, July 8, 2009 - link

    I don't suppose that program can also tell the GPU clock speed too?

    I always thought that the MBX work at bus speed, ie. 103MHz for the iPhone/3G and 133MHz for the 2nd Gen iPod Touch instead of the 60Mhz that Anand has speculated. Assuming the iPhone 3G S has a 150MHz bus speed, the SGX could run at 150MHz which is a reasonable compromise between Anand's 100MHz and 200MHz estimates.
    Reply
  • fyleow - Wednesday, July 8, 2009 - link

    How useful is the new GPU? The iPhone's performance has come a long way from the first generation but I don't see developers taking full advantage of the jump. If you bump up the graphics of your game it might run smoothly on the 3GS but end up lagging on the 1st gen iPhone.

    The increase in load times and battery life is much welcomed, but when do we get to see some apps that take advantage of the upgraded hardware in other more interesting ways? I can see a resolution increase as being one way to do that. The game would look better on a higher resolution screen but performance wouldn't suffer on the older models because the lower resolution would place less demand on the hardware.

    2010 will be an interesting year. There should be a bigger upgrade to the iPhone, most likely a resolution bump and a significantly modified OS that supports background tasks. Apple has been keeping all the devices on the iPhone platform on feature parity so far with the OS upgrades (minus obvious limitations due to hardware differences). It would be interesting to see how they handle the switch and the resulting two classes of phones that come from it (i.e. old "legacy" iPhones/Touch vs new iPhones/Touch).
    Reply
  • ltcommanderdata - Wednesday, July 8, 2009 - link

    You're right that it's difficult to take full advantage of the SGX without writing a separate dedicated code path for it and one for the MBX. However, there are simpler ways to take advantage of the iPhone 3G S power without writing 2 separate code paths. For example, you can scale draw distance based on hardware. Firemint demonstrated the iPhone 3G S accelerating 40 cars in Real Racing compared to 6 in the iPhone 3G, so the potential for better scalable AI is there. For a RPG, perhaps having more NPCs walking around to make the environment more lifelike. This can all be done using existing OpenGL ES 1.1 code playable on all iPhones/Touches, optimizing for each device, without making older iPhone users feel like they are playing some Lite version of the game as implementing shaders and HDR using OpenGL ES 2.0 in the iPhone 3G S might do.

    I believe the reluctance of Apple to change the resolution is that it could break the interface layout for existing apps and/or make things ugly if apps haven't used vector graphics. It would have been nice if they had enforced resolution independence early on, but I don't believe they did. Resolution independence is also what is needed for Apple to introduce an iPhone nano with a smaller screen and presumably smaller resolution.
    Reply
  • smallpot - Wednesday, July 8, 2009 - link

    Thanks for the article Anand. Your long-form articles are the reason Anandtech is my number one tech website. I'm thinking of articles such as this, your articles on SSD performance, and the long-form story behind the RV770. After reading such articles, I really feel like I've learned something, rather than just had performance metrics thrown at me without context. Reply
  • Baron Fel - Tuesday, July 7, 2009 - link

    Interesting article.

    As far as portable gaming goes, the Ipod Touch/iPhone/Zune HD dont have a chance against the DS or even the PSP. The software support just isnt there.

    PSP hardware runs circles around the DS, so why is the DS killing it in sales? Good games.

    and are we getting more SSD articles anytime soon? I think thats what we want to see :D
    Reply
  • ltcommanderdata - Tuesday, July 7, 2009 - link

    Given all the media attention about discoloration and possible heat issues with the iPhone 3G S, I was wondering if you could comment on your experience in this area. Do you think it's a real concern or just stories popularized to generate page hits as Apple related stories tend to do? The latest reports on discoloration indicate that it might actually be from a reaction with some third-party cases that may be reversed by cleaning with alcohol.

    Similarly, there have been lower-key reports of build quality issues with the Palm Pre having a wobbly screen from it's slide-out keyboard. Has this been a major issue for you and do you think it'll be an issue over time?
    Reply
  • Anand Lal Shimpi - Tuesday, July 7, 2009 - link

    I haven't seen anything to indicate heat as being a bigger concern with the 3GS. It's a new processor so there's bound to be some bad chips out there, but I wouldn't be too concerned.

    The build quality on the Pre did bother me. It's something that I think bothered me more because of my experience with the iPhone. The screen was a bit wobbly and overall the device just didn't feel as well put together. Part of it is because of the slide out keyboard, but part of it has to be cost/experience related. I think you'd get used to it over time, but if you then held an iPhone you'd quickly grow tired of the build quality issues once again :)

    Take care,
    Anand
    Reply
  • tomoyo - Tuesday, July 7, 2009 - link

    Btw Anand, the chart for number of stages in the cpus shows the Iphone 3GS as 8 stage instead of 13. Reply

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