Computer Graphics Graphics Hardware CO2409 Computer Graphics Week 12

Lecture Contents Graphics Architecture Processing: Memory: GPU vs CPU Memory: Video Memory vs System Memory Hardware Comparisons Motherboard Interface Parallelism / Concurrency

Graphics Architecture The basic graphics architecture for all modern PCs and game consoles is similar: Two key parts: Main System Graphics Unit Local RAM for each processor Fast access Interface between components slow Compared to local access

Graphics Unit The graphics unit describes the graphics processing components of a system PC: usually a separate graphics card Console: built-in custom components The core of a graphics unit is the Graphics Processing Unit (GPU) Occasionally called a Graphics Adapter Equivalent of a CPU for graphics PS4 GPU

Graphics Unit The graphics unit also contains: Local RAM (a.k.a. video memory, VRAM, etc.) Output sockets for monitor, TV etc. Can be integrated into the motherboard Consoles, some PC’s and laptops Attached via an interface: PCs use PCI Express sockets Can connect several together (SLI or Crossfire) for more parallelism NVidia GeForce GTX 980 Graphics Cards [Four connected by SLI]

GPU vs CPU A GPU is a dedicated graphics processor Much more parallel than a typical CPU i.e. Many more cores (100s or 1000s compared to 4-8 on a CPU) Much faster than CPU for graphics algorithms Particularly vector/matrix operations But worse at general operations, esp. conditional instructions Driven a programmable pipeline (shaders) Can now perform some general programming using the flexibility of this – GPGPU programming (General Purpose GPU) Power of a GPU is measured by its: Clock speed Parallelism. E.g. number of simultaneous shader, texture and output operations

Graphics Memory Many graphics units have local memory To store vertices, primitives and textures for the GPU This memory is similar to standard CPU memory Usually more closely coupled to the GPU I.e. Bandwidth to the GPU will be greater Graphics memory can be measured by: Clock speed (how fast can it serve data) Bandwidth to the GPU (clock speed * width of the bus between graphics memory and the GPU) Some GPUs have no dedicated memory Must share memory with the CPU This can be a benefit or a penalty…

Graphics vs System Memory GPUs can usually access system memory i.e. CPU-local memory Bandwidth is often lower Usually better to have graphics data in GPU memory System memory can be used as a back-up But expect lag if relied on too heavily Some architectures share CPU & GPU memory Xbox 360 / One / PS4: entire console architecture designed around this sharing – performance is high On-board video cards: Lack of GPU memory is a cost-cutting feature – memory access is slow

Graphics Unit - Specifications The key specifications for a graphics unit are: GPU clock speed (MHz ) – speed of processor Amount of Memory (Mb) Memory clock speed (MHz / GHz) GPU<-> Memory Bandwidth (GB / s) Speed of transfers between GPU and local memory, determined by memory clock speed and bus width Other factors to take into account: Pixel / Texture Fill Rate (Giga-Pixels / s) Speed it can output pixels to screen or textures Amount of parallelism in the pipeline E.g. Number of shader threads/cores – how many vertices / pixels can be processed at the same time

Historical Comparison (Indicative) Platform PS2 XBox Xbox 360 PS3 Xbox One PS4 GeForce GTX 680 GTX 980 CPU / GPU Clock (MHz) 294/147 733/233 3 x 3200 / 500 1+7x3200 / 500 8x1750 / 853 8x1600 / 800 - / 1058 - / 1216 CPU / GPU Memory (Mb) 32 / 4 64 (Shared) 512 (Shared) 256 / 256 8000 (Shared) - / 2048 - / 4096 Memory Clock (MHz) ? 200 700 650 2133 (GDDR3) 5500 (GDDR5) 6000 7000 Memory Bus (GB/s) 3.2 / 48 6.4 22.4 20.8 / 22.4 68 / 102x2 176 192 224 Pixel / Texture Fill Rate (GPx/s) ? / 0.932 ? / 4 (No AA) 12.8 / 40.9 25.6 / 57.6 128.8 144 Parallelism - 4 / 2 Shader 48 Shader 24 / 8 Shader 768 Cores 1152 Cores 1536 Cores 2048 Cores

GPU / System Interface It is faster to keep graphics data local to the GPU But the CPU and system memory still need to interface with the graphics unit: To get data into the graphics memory To issue instructions to the GPU (DrawPrimitive) For dynamic geometry / textures So the interface between the graphics unit and the rest of the system is critical Games consoles have the graphics unit built into the motherboard and so are closely coupled E.g. Xbox 360/One, & PS3/4 have fairly symmetrical performance between GPU, CPU & memory

PC Graphics Interfaces Interface between system and GPU on PC: PCI Express Uses serial ‘lanes’ to transfer streams of data in parallel Version 3 supports 1GB/s transfer rates (theoretical). Version 4 specs are being finalised Has superseded the earlier interfaces AGP & PCI But still can be a bottleneck (Console memory bus much faster) Slow compared to local GPU memory PCs must rely on more graphics memory

Parallelism / Concurrency GPUs are parallel architectures Processing many pixels / vertices at the same time A graphics application is typically a concurrent system Graphics rendered while main application does other processing Concurrent = different tasks performed simultaneously Parallel = the same task split up and performed simultaneously Need to program carefully to get best performance: Ensure both CPU and GPU working all the time Neither should be waiting for the other to complete its current task But watch out for problems with shared data Implications about programming graphics applications Games students will see this in the 3rd year

General Purpose GPU A GPU is a massively parallel processor Especially for vector maths operations So it can be used for certain non-graphics tasks Physics simulation, video processing, weather forecasting, etc. Anything with massive amounts of mathematical calculation Called General Purpose GPU processing (GPGPU) Several APIs for this: CUDA (from NVidia) Extension to C language Up to 40 times faster than same code on CPU OpenCL for ATI and NVidia platforms Compute Shaders as part of DirectX11