AI & GPU
CPU vs GPU: What's the Difference for AI?

CPU vs GPU: What's the Difference for AI?

Introduction: Understanding the GPU Architecture

In the rapidly evolving landscape of computing, the Graphics Processing Unit (GPU) has emerged as a crucial component, particularly in the fields of artificial intelligence (AI) and machine learning (ML). But what exactly is a GPU, and why has it become so vital in these domains?

At its core, a GPU is a specialized type of processor designed to handle the complex mathematical calculations required for rendering images, videos, and 3D graphics. However, the parallel processing capabilities of GPUs have made them invaluable for a wide range of applications beyond just graphics, including scientific computing, cryptocurrency mining, and most notably, AI and ML.

The rise of deep learning and neural networks has fueled the demand for GPUs, as their highly parallel architecture is ideally suited for the massive computational requirements of training and running these models. In this article, we'll explore the architecture of GPUs, compare them to CPUs, and examine their pivotal role in the AI revolution.

GPU Architecture Overview: Designed for Parallel Processing

The unique architecture of a GPU sets it apart from a CPU and enables its parallel processing capabilities. While CPUs are designed for general-purpose computing and excel at serial processing, GPUs are built for parallel processing and are optimized for throughput.

Streaming Multiprocessors: The Heart of GPU Parallelism

The foundation of a GPU's parallel processing power lies in its Streaming Multiprocessors (SMs). Each SM contains hundreds of simple cores, allowing the GPU to execute thousands of threads simultaneously. This contrasts with a CPU, which typically has fewer, more complex cores optimized for serial processing.

              GPU Architecture Diagram
              ========================

               +-----------------------+
               |    Streaming          |
               |   Multiprocessors     |
               |         (SMs)         |
               +-----------+-----------+
                           |
                           |
               +-----------v-----------+
               |                       |
               |   Shared Memory       |
               |                       |
               +-----+------------+----+
                     |            |
                     |            |
       +-------------v+           +v-------------+
       |                                        |
       |    L1 Cache            L1 Cache        |
       |                                        |
       +-------------+           +-------------+
                     |           |
                     |           |
                     v           v
               +-----------+-----------+
               |                       |
               |      L2 Cache         |
               |                       |
               +-----------+-----------+
                           |
                           |
                           v
               +-----------------------+
               |                       |
               |    High Bandwidth     |
               |    Memory (HBM)       |
               |                       |
               +-----------------------+

The simple cores within an SM are designed to perform a single operation on multiple data points simultaneously, a concept known as Single Instruction, Multiple Data (SIMD). This allows GPUs to efficiently process large amounts of data in parallel, making them ideal for tasks like rendering graphics, where the same operation needs to be performed on millions of pixels.

Memory Hierarchy: Optimized for High Bandwidth

To keep its thousands of cores supplied with data, a GPU requires an immense amount of memory bandwidth. This is achieved through a memory hierarchy that includes:

  • High Bandwidth Memory (HBM): A type of stacked memory that provides a wide interface for transferring data to and from the GPU.
  • L2 Cache: A larger, shared cache that is accessible by all SMs.
  • L1 Cache: Each SM has its own L1 cache for fast access to frequently used data.
  • Shared Memory: A fast, on-chip memory that allows threads within an SM to communicate and share data.

This memory hierarchy is designed to provide the GPU with the high bandwidth it needs to keep its cores busy and optimize throughput.

Comparison to CPU Architecture

While GPUs are designed for parallel processing, CPUs are optimized for serial processing and general-purpose computing. Some key differences include:

  • Number and Complexity of Cores: CPUs have fewer, more complex cores, while GPUs have thousands of simple cores.
  • Cache Size: CPUs have larger caches to reduce latency, while GPUs have smaller caches and rely more on high bandwidth memory.
  • Control Logic: CPUs have complex branch prediction and out-of-order execution capabilities, while GPUs have simpler control logic.

These architectural differences reflect the different priorities of CPUs and GPUs. CPUs prioritize low latency and single-threaded performance, while GPUs prioritize high throughput and parallel processing.

GPU Parallelism: SIMT and Warps

GPUs achieve their massive parallelism through a unique execution model called Single Instruction, Multiple Thread (SIMT). In this model, threads are grouped into "warps" or "wavefronts," typically containing 32 or 64 threads. All threads in a warp execute the same instruction simultaneously, but on different data.

This execution model is well-suited for data parallel problems, where the same operation needs to be performed on many data points. Some common examples include:

  • Graphics Rendering: Each pixel on the screen can be processed independently, making it an ideal candidate for parallel processing.
  • Deep Learning: Training neural networks involves performing the same operations on large datasets, which can be parallelized across the GPU's cores.

By leveraging the SIMT execution model and warp-based processing, GPUs can achieve massive parallelism and high throughput on data parallel workloads.

GPU Computing and GPGPU

While GPUs were originally designed for graphics processing, their parallel processing capabilities have made them attractive for general-purpose computing as well. This has led to the rise of General-Purpose computing on Graphics Processing Units (GPGPU).

GPGPU has been enabled by the development of programming models and APIs that allow developers to harness the power of GPUs for non-graphics tasks. Some popular GPGPU platforms include:

  • NVIDIA CUDA: A proprietary platform developed by NVIDIA for programming their GPUs.
  • OpenCL: An open standard for parallel programming across heterogeneous platforms, including GPUs, CPUs, and FPGAs.

These platforms provide abstractions and libraries that allow developers to write parallel code that can be executed on GPUs, without needing to understand the low-level details of the GPU architecture.

GPGPU has found applications in a wide range of domains, including:

  • Scientific Computing: GPUs are used for simulations, data analysis, and other computationally intensive tasks in fields like physics, chemistry, and biology.
  • Cryptocurrency Mining: The parallel processing capabilities of GPUs make them well-suited for the cryptographic calculations required for mining cryptocurrencies like Bitcoin and Ethereum.
  • Machine Learning and AI: GPUs have become the platform of choice for training and running deep learning models, which require massive amounts of parallel computation.

The rise of GPGPU has driven the development of more powerful and flexible GPU architectures, as well as closer integration between GPUs and CPUs in modern computing systems.

GPUs in Machine Learning and AI

Perhaps the most significant impact of GPUs in recent years has been in the field of machine learning and AI. The parallel processing capabilities of GPUs have made them ideally suited for the computational demands of deep learning, which involves training neural networks on large datasets.

Deep Learning and Neural Networks

Deep learning is a subset of machine learning that involves training artificial neural networks with many layers. These networks can learn hierarchical representations of data, allowing them to perform complex tasks like image classification, natural language processing, and speech recognition.

Training deep neural networks is a computationally intensive task that involves performing matrix multiplications and other operations on large datasets. This is where GPUs shine, as they can parallelize these operations across their thousands of cores, allowing for much faster training times compared to CPUs.

Some key advantages of GPUs for deep learning include:

  • Faster Training Times: GPUs can train deep neural networks in a fraction of the time it would take on a CPU, enabling researchers to experiment with larger models and datasets.
  • Larger Models: The memory capacity and bandwidth of modern GPUs allow for training larger and more complex neural networks, which can lead to better performance on challenging tasks.
  • Scalability: Multiple GPUs can be used together to further parallelize training, allowing for even larger models and datasets.

The impact of GPUs on deep learning cannot be overstated. Many of the recent breakthroughs in AI, from AlexNet to GPT-3, have been enabled by the massive parallelism and computing power of GPUs.

GPU Architectures for AI

As the demand for GPU computing in AI has grown, GPU manufacturers have started designing architectures specifically optimized for machine learning workloads. NVIDIA, in particular, has been at the forefront of this trend with their Volta and Ampere architectures.

Some key features of these AI-optimized GPU architectures include:

  • Tensor Cores: Specialized cores designed for matrix multiplication and convolution operations, which are the backbone of deep learning workloads.
  • Mixed Precision: Support for lower precision data types like FP16 and BFLOAT16, which can speed up training and inference without sacrificing accuracy.
  • Larger Memory Capacities: Up to 80 GB of HBM2e memory in the NVIDIA A100, allowing for training of larger models.
  • Faster Interconnects: High-bandwidth interconnects like NVLink and NVSwitch, which enable faster communication between GPUs in multi-GPU systems.

These architectural innovations have further cemented the role of GPUs as the platform of choice for AI and deep learning workloads.

The Future of GPU Architecture

As the demand for GPU computing continues to grow, driven by advancements in AI, graphics, and high-performance computing, GPU architectures will continue to evolve to meet these challenges. Some key trends to watch include:

Increasing Parallelism and Specialization

GPU manufacturers will continue to push the boundaries of parallelism, with designs that incorporate even more cores and specialized units for AI and graphics workloads. NVIDIA's Hopper architecture, for example, introduces new features like the Hopper Transformer Engine and a new Thread Block Cluster for improved parallelism and efficiency.

Tighter Integration with CPUs

As GPUs become more central to computing workloads, there will be a push for tighter integration between GPUs and CPUs. This could take the form of heterogeneous architectures like AMD's APUs, which combine CPU and GPU cores on a single chip, or high-bandwidth interconnects like Intel's CXL, which enable faster communication between CPUs and accelerators.

Competition from Other Architectures

While GPUs have been the dominant platform for AI and parallel computing, they will face increasing competition from other architectures like Field Programmable Gate Arrays (FPGAs) and Application-Specific Integrated Circuits (ASICs). These architectures offer the potential for even greater efficiency and specialization for specific workloads.

Sustainability and Energy Efficiency

As the energy demands of GPU computing continue to grow, there will be an increasing focus on sustainability and energy efficiency. This could involve innovations in chip design, cooling systems, and power delivery, as well as a shift towards more efficient algorithms and software.

Conclusion

The GPU has come a long way from its origins as a specialized graphics processor. Today, it is a critical component of the modern computing landscape, powering everything from gaming and visualization to scientific computing and artificial intelligence.

The parallel architecture of GPUs, with their thousands of simple cores and high memory bandwidth, has made them ideally suited for the massive computational demands of these workloads. As the demand for GPU computing continues to grow, driven by advancements in AI and other fields, GPU architectures will continue to evolve and innovate.

From the rise of GPGPU and the impact of GPUs on deep learning, to the development of specialized AI architectures and the push for greater integration with CPUs, the future of GPU computing is bright. As we look ahead, it is clear that GPUs will continue to play a central role in shaping the future of computing and enabling the next generation of breakthroughs in AI and beyond.

7 Reasons for Low GPU Utilization in AI Model TraningGoogle TPU: A Beginner's Walkthrough