1. GRAPHICS PROCESSING UNIT
Shashwat Shriparv
InfinitySoft
9/19/2018

2. Presentation Overview
Definition
Comparison with CPU
Architecture
GPU-CPU Interaction
GPU Memory
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3. Why GPU?
To provide a separate dedicated graphics resources including a graphics processor and memory.
To relieve some of the burden of the main system resources, namely the Central Processing Unit, Main Memory, and the System Bus, which would otherwise get saturated with graphical operations and I/O requests.
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4. There comes
GPU
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5. What is a GPU?
A Graphics Processing Unit or GPU (also occasionally called Visual Processing Unit or VPU) is a dedicated processor efficient at manipulating and displaying computer graphics .
Like the CPU (Central Processing Unit), it is a single-chip processor.
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6. HOWEVER,
The abstract goal of a GPU, is to enable a representation of a 3D world as realistically as possible. So these GPUs are designed to provide additional computational power that is customized specifically to perform these 3D tasks.
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7. GPU vs CPU
A GPU is tailored for highly parallel operation while a CPU executes programs serially.
For this reason, GPUs have many parallel execution units , while CPUs have few execution units .
GPUs have singificantly faster and more advanced memory interfaces as they need to shift around a lot more data than CPUs.
GPUs have much deeper pipelines (several thousand stages vs for CPUs).
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8. BRIEF HISTORY Fifth-Generation GPUs - GeForce 8X:DirectX10.
First-Generation GPUs
Up to 1998; Nvidia’s TNT2, ATi’s Rage, and 3dfx’s Voodoo3;DX6 feature set.
Second-Generation GPUs
; Nvidia’s GeForce256 and GeForce2, ATi’s Radeon7500, and S3’s Savage3D; T&L; OpenGL and DX7;Configurable.
Third-Generation GPUs
2001; GeForce3/4Ti, Radeon8500, MS’s Xbox; OpenGL ARB, DX7/8; Vertex Programmability + ASM
Fourth-Generation GPUs
2002 onwards; GeForce FX family, Radeon 9700; OpenGL+extensions, DX9; Vertex/Pixel Programability + HLSL; 0.13μ Process, 125M T/C, 200M T/S.
Fifth-Generation GPUs
- GeForce 8X:DirectX10.
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9. GPU Architecture How many processing units? How many ALUs?
Do you need a cache?
What kind of memory?
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10. GPU Architecture How many processing units? How many ALUs?
Lots.
How many ALUs?
Do you need a cache?
What kind of memory?
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11. GPU Architecture How many processing units? How many ALUs?
Lots.
How many ALUs?
Hundreds.
Do you need a cache?
What kind of memory?
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12. GPU Architecture How many processing units? How many ALUs?
Lots.
How many ALUs?
Hundreds.
Do you need a cache?
Sort of.
What kind of memory?
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13. GPU Architecture How many processing units? How many ALUs?
Lots.
How many ALUs?
Hundreds.
Do you need a cache?
Sort of.
What kind of memory?
very fast.
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14. The difference…….
Without GPU
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With GPU

15. The GPU pipeline
The GPU receives geometry information from the CPU as an input and provides a picture as an output
Let’s see how that happens…
host
interface
vertex
processing
triangle
setup
pixel
memory
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16. Details………..
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17. Host Interface
The host interface is the communication bridge between the CPU and the GPU.
It receives commands from the CPU and also pulls geometry information from system memory.
It outputs a stream of vertices in object space with all their associated information (texture coordinates, per vertex color etc) .
host
interface
vertex
processing
triangle
setup
pixel
memory
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18. Vertex Processing
The vertex processing stage receives vertices from the host interface in object space and outputs them in screen space
This may be a simple linear transformation, or a complex operation involving morphing effects
No new vertices are created in this stage, and no vertices are discarded (input/output has 1:1 mapping)
host
interface
vertex
processing
triangle
setup
pixel
memory
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19. Triangle setup
In this stage geometry information becomes raster information (screen space geometry is the input, pixels are the output)
Prior to rasterization, triangles that are backfacing or are located outside the viewing frustrum are rejected
host
interface
vertex
processing
triangle
setup
pixel
memory
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20. Triangle Setup (cont…..)
A pixel is generated if and only if its center is inside the triangle
Every pixel generated has its attributes computed to be the perspective correct interpolation of the three vertices that make up the triangle
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21. Pixel Processing
Each pixel provided by triangle setup is fed into pixel processing as a set of attributes which are used to compute the final color for this pixel
The computations taking place here include texture mapping and math operations
host
interface
vertex
processing
triangle
setup
pixel
memory
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22. Memory Interface
Pixel colors provided by the previous stage are written to the framebuffer
Used to be the biggest bottleneck before pixel processing took over
Before the final write occurs, some pixels are rejected by the zbuffer .On modern GPUs z is compressed to reduce framebuffer bandwidth (but not size).
host
interface
vertex
processing
triangle
setup
pixel
memory
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23. Programmability in GPU pipeline
In current state of the art GPUs, vertex and pixel processing are now programmable
The programmer can write programs that are executed for every vertex as well as for every pixel
This allows fully customizable geometry and shading effects that go well beyond the generic look and feel of older 3D applications
host
interface
vertex
processing
triangle
setup
pixel
memory
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24. GPU Pipelined Architecture (simplified view)
… …
CPU
Vertex Setup
Vertex Shader
Rasterizer
Pixel Shader
Framebuffer
Texture Storage + Filtering
Pixels
Vertices
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25. GPU Pipelined Architecture (simplified view)
CPU
Vertex Setup
Vertex Shader
Rasterizer
Pixel Shader
Framebuffer
Texture Storage + Filtering
One unit can limit the speed of the pipeline…
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26. CPU/GPU interaction
The CPU and GPU inside the PC work in parallel with each other
There are two “threads” going on, one for the CPU and one for the GPU, which communicate through a command buffer:
CPU writes commands here
GPU reads commands from here
Pending GPU commands
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27. CPU/GPU interaction (cont)
If this command buffer is drained empty, we are CPU limited and the GPU will spin around waiting for new input. All the GPU power in the universe isn’t going to make your application faster!
If the command buffer fills up, the CPU will spin around waiting for the GPU to consume it, and we are effectively GPU limited
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28. Synchronization issues
In the figure below, the CPU must not overwrite the data in the “yellow” block until the GPU is done with the “black” command, which references that data:
CPU writes commands here
GPU reads commands from here
data
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29. Inlining data
One way to avoid these problems is to inline all data to the command buffer and avoid references to separate data:
CPU writes commands here
GPU reads commands from here
However, this is also bad for performance, since we may need to copy several Mbytes of data instead of merely passing around a pointer
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30. GPU readbacks
The output of a GPU is a rendered image on the screen, what will happen if the CPU tries to read it?
CPU writes commands here
GPU reads commands from here
Pending GPU commands
GPU must be synchronized with the CPU, ie it must drain its entire command buffer, and the CPU must wait while this happens
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31. GPU readbacks (cont)
We lose all parallelism, since first the CPU waits for the GPU, then the GPU waits for the CPU (because the command buffer has been drained)
Both CPU and GPU performance take a nosedive
Bottom line: the image the GPU produces is for your eyes, not for the CPU (treat the CPU -> GPU highway as a one way street)
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32. About GPU memory…..
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33. Memory Hierarchy CPU and GPU Memory Hierarchy Disk CPU Main Memory
GPU Video
Memory
CPU Caches
GPU Caches
CPU Registers
GPU Constant
Registers
GPU Temporary
Registers
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34. Where is GPU Data Stored?
Vertex buffer
Frame buffer
Texture
Texture
Vertex
Processor
Fragment
Processor
Frame
Buffer(s)
Vertex Buffer
Rasterizer
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35. CPU memory vs GPU memory
Registers
Read/write
Local Mem
Read/write stack
None
Global Mem
Read/write heap
Read-only during computation.
Write-only at end (to pre-computed address)
Disk
Read/write disk
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36. It looks like…..
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37. Some applications…..
Computer generated holography using a graphics processing unit
Improve the performance of CAD tools.
Computer graphics in games
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38. New…..
NVIDIA's new graphics processing unit, the GeForce 8X ULTRA, said to represent the very latest in visual effects technologies.
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39. THANK YOU Shashwat Shriparv dwivedishashwat@gmail.com InfinitySoft
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