Nvidia GeForce GTX 260/280 Review

The Architecture in Detail

A SIMT Architecture?

You’re familiar with the terms SIMD and MIMD, but with the GT200 Nvidia describes its Shader Multiprocessors as "SIMT units." So what are they? The acronym stands for Single Instruction Multiple Threads, and the main difference between it and SIMD mode is that the size of the vectors being processed has no predefined width. Concretely, with a sufficient number of threads, the processor behaves like a scalar processor. To grasp the difference, remember how pixel shader units operated in previous architectures.

The rasterizer generated quads – squares of 2x2 pixels, where each pixel is made up of a vector with four single-precision floating-point values (R,G, B, A) or (X, Y, Z, W), which are the formats most often used in 3D calculations. These quads then moved to an ALU, which was operating in 16-way SIMD mode – applying the same instruction to all 16 floating-point numbers. This is a simplification for the purpose of illustrating the principle; in practice GeForce 6 and 7 had a mode called co-issue for executing two instructions per vector.

Since the G80, this mode of operation has been reworked – the rasterizer still generates quads, which are placed in a buffer. When 8 quads (32 pixels, a "warp" in CUDA terminology) are present in the buffer, they can be executed by a multiprocessor in SIMD mode. So what’s the difference? It’s in how the data are organized: Instead of working on four vectors of four floating-point numbers organized like this: (R, G, B, A, R, G, B, A, R, G, B, A, R, G, B, A), the multiprocessors work on vectors of 32 floating-point numbers, each made up of a single component of each of the 32 threads:

(R, R, R, R, R, R, R, R, R, R, R, R, R, R, R, R, R, R, R, R, R, R, R, R, R, R, R, R, R, R, R, R) then(G, G, G, G, G, G, G, G, G, G, G, G, G, G, G, G, G, G, G, G, G, G, G, G, G, G, G, G, G, G, G, G) etc.

In SIMD programming, the first data alignment is called AoS (Array Of Structures), and the second SoA (Structure of Arrays). This second organization results in better performance. Provided there’s sufficient data to fill a vector, the processor behaves, from the programmer’s point of view, like a scalar processor since the SIMD units are always used at 100% regardless of the width of the data being processed. Conversely, AoS achieves peak performance only when the same instruction is applied to all four components of each vector.

  • BadMannerKorea
  • Lunarion
    what a POS, the 9800gx2 is $150+ cheaper and performs just about the same. Let's hope the new ATI cards coming actually make a difference
  • foxhound009
    woow,.... that's the new "high end" gpu????
    lolz.. 3870 x2 wil get cheaper... and nvidia gtx200 lies on the shelves providing space for dust........
    (I really expectede mmore from this one... :/ )
  • thatguy2001
    Pretty disappointing. And here I was thinking that the gtx 280 was supposed to put the 9800gx2 to shame. Not too good.
  • cappster
    Both cards are priced out of my price range. Mainstream decently priced cards sell better than the extreme high priced cards. I think Nvidia is going to lose this round of "next gen" cards and price to performance ratio to ATI. I am a fan of whichever company will provide a nice performing card at a decent price (sub 300 dollars).
  • njalterio
    Very disappointing, and I had to laugh when they compared the prices for the GTX 260 and the GTX 280, $450 and $600, calling the GTX 260 "nearly half the price" of the GTX 280. Way to fail at math. lol.
  • NarwhaleAu
    It is going to get owned by the 4870x2. In some cases the 3870x2 was quicker - not many, but we are talking 640 shaders total vs. 1600 total for the 4870x2.
  • MooseMuffin
    Loud, power hungry, expensive and not a huge performance improvement. Nice job nvidia.
  • compy386
    This should be great news for AMD. The 4870 is rumored to come in at 40% above the 9800GTX so that would put it at about the 260GTX range. At $300 it would be a much better value. Plus AMD was expecting to price it in the $200s so even if it hits low, AMD can lower the price and make some money.
  • vochtige
    i think i'll get a 8800ultra. i'll be safe for the next 5 generations of nvidia! try harder nv crew