GPU Pixel and Texel write speed - graphics

Question

Many of the embedded/mobile GPUs are providing access to performance registers called Pixel Write Speed and Texel Write speed. Could you explain how those terms can be interpreted and defined from the actual GPU hardware point of view?

Answer 1

I would assume the difference between pixel and texel is pretty clear for you. Anyway, just to make this answer a little bit more "universal":
A pixel is the fundamental unit of screen space.
A texel, or texture element (also texture pixel) is the fundamental unit of texture space.
Textures are represented by arrays of texels, just as pictures are
represented by arrays of pixels. When texturing a 3D surface (a
process known as texture mapping) the renderer maps texels to
appropriate pixels in the output picture.
BTW, it is more common to use fill rate instead of write speed and you can easily find all required information, since this terminology is quite old and widely-used.
Answering your question
All fill-rate numbers (whatever definition is used) are expressed in
Mpixels/sec or Mtexels/sec.
Well the original idea behind fill-rate was the number of finished
pixels written to the frame buffer. This fits with the definition of
Theoretical Peak fill-rate. So in the good old days it made sense to
express that number in Mpixels.
However with the second generation of 3D accelerators a new feature
was added. This feature allows you to render to an off screen surface
and to use that as a texture in the next frame. So the values written
to the buffer are not necessarily on screen pixels anymore, they might
be texels of a texture. This process allows several cool special
effects, imagine rendering a room, now you store this picture of a
room as a texture. Now you don't show this picture of the room but you
use the picture as a texture for a mirror or even a reflection map.
Another reason to use MTexels is that games are starting to use
several layers of multi-texture effects, this means that a on-screen
pixel is constructed from various sub-pixels that end up being blended
together to form the final pixel. So it makes more sense to express
the fill-rate in terms of these sub-results, and you could refer to
them as texels.
Read the whole article - Fill Rate Explained
Additional details can be found here - Texture Fill Rate
Update
Texture Fill Rate = (# of TMU - texture mapping unit) x (Core Clock)
The number of textured pixels the card can render to the
screen every second.
It is obvious that the card with more TMUs will be faster at processing texture information.

Answer 2

The performance registers/counters Pixel Write Speed and Texel Write speed maintain stats / count operations about pixel and texel processed/written. I will explain the peak (maximum possible) fill rates.
Pixel Rate
A picture element is a physical point in a raster image, smallest
element of display device screen.
Pixel rate is the maximum amount of pixels the GPU could possibly write to the local memory in one second, measured in millions of pixels per second. The actual pixel output rate also depends on quite a few other factors, most notably the memory bandwidth - the lower the memory bandwidth is, the lower the ability to get to the maximum fill rate.
The pixel rate is calculated by multiplying the number of ROPs (Raster Operations Pipelines - aka Render Output Units) by the the core clock speed.
Render Output Units : The pixel pipelines take pixel and texel information and process it, via specific matrix and vector operations, into a final pixel or depth value. The ROPs perform the transactions between the relevant buffers in the local memory.
Importance : Higher the pixel rate, higher is the screen resolution of the GPU.
Texel Rate
A texture element is the fundamental unit of texture space (a tile of
3D object surface).
Texel rate is the maximum number of texture map elements (texels) that can be processed per second. It is measured in millions of texels in one second
This is calculated by multiplying the total number of texture units by the core speed of the chip.
Texture Mapping Units : Textures need to be addressed and filtered. This job is done by TMUs that work in conjunction with pixel and vertex shader units. It is the TMU's job to apply texture operations to pixels.
Importance : Higher the texel rate, faster the game renders displays demanding games fluently.
Example : Not a nVidia fan but here are specs for GTX 680, (could not find much for embedded GPU)
Model Geforce GTX 680
Memory 2048 MB
Core Speed 1006 MHz
Shader Speed 1006 MHz
Memory Speed 1502 MHz (6008 MHz effective)
Unified Shaders 1536
Texture Mapping Units 128
Render Output Units 32
Bandwidth 192256 MB/sec
Texel Rate 128768 Mtexels/sec
Pixel Rate 32192 Mpixels/sec