Cerebras CS3 Simulator

A high-fidelity architectural simulator for the Cerebras CS3 Wafer-Scale Engine (WSE). This project models the hardware architecture, interconnects, and execution environment to enable performance analysis and software development for massively parallel 2D mesh architectures.

🚀 Project Overview

The Cerebras-Sim is designed to model the CS3 WSE, featuring a massive array of 720,000 processing elements (PEs). It provides a full-stack simulation environment, from a high-level Python DSL down to a custom 32-bit ISA binary.

Key Goals

• Performance Analysis: Estimate total runtime and identify bottlenecks using a hybrid performance/functional model.
• Software Development: Verify kernel correctness via a CUDA-like programming model before deploying to hardware.
• Architectural Exploration: Model the impact of mesh bisection bandwidth, latency, and SRAM constraints.

---

🏗️ Architecture

Hardware Model

• Processing Element (PE): Each core implements an 8-wide SIMD unit, vector registers, and a private 48KB local SRAM.
• Interconnect: A 2D Mesh (800x900) where communication occurs via SEND/RECV primitives and global address space abstractions.
• Memory Hierarchy:
  • Local SRAM: Private high-speed memory per PE (analogous to CUDA Shared Memory).
  • Weight Server: External DRAM accessed via a global address space for large-scale model weights and data.
• Host-Device Interface: A driver model implementing a command queue (CS3Queue) and memory movement (cs3_memcpy).

Execution Model

The simulator employs a Bulk Synchronous Parallel (BSP) model, dividing execution into discrete "supersteps":

1. Compute: PEs perform local SIMD operations.
2. Communicate: PEs exchange data across the mesh or with the Weight Server.
3. Synchronize: A global barrier (SYNC) aligns the execution state.

To balance accuracy and speed, the simulator uses a hybrid execution track:

• Performance Track (Global): All PEs are tracked for cycle counts and timing.
• Functional Track (Sampled): A stochastic sampling strategy is used where only a subset of blocks is fully simulated functionally to verify correctness.

---

💻 Software Stack

The project implements a complete toolchain: Python DSL $\rightarrow$ Tungsten-IR $\rightarrow$ ISA Binary $\rightarrow$ Simulator

Programming Example

Kernels are written in a CUDA-like Python DSL. For example, a simple SAXPY ($\mathbf{z} = \alpha \mathbf{x} + \mathbf{y}$) kernel:

@cs3_kernel(block_w=16, block_h=16)
def saxpy_kernel(ctx):
    # Load inputs from global memory (Weight Server)
    x = ctx.load_global(None, 0)
    y = ctx.load_global(None, 4)
    
    # Compute: z = 2.0 * x + y
    z = 2.0 * x + y
    
    # Store result back to global memory
    ctx.store_global(None, 8, z)

1. Frontend: A CUDA-like DSL embedded in Python using @cs3_kernel decorators.
2. Intermediate Representation (Tungsten-IR): A dataflow-centric IR mapping compute nodes and synchronization points.
3. Compiler Backend:
   • Mapping & Scheduling: Assigns IR nodes to the physical 2D mesh and manages the SRAM budget.
   • Assembler: Emits the final 32-bit binary stream.
4. Simulator Engine: A Python-based engine that decodes the ISA and drives the hardware model.

---

⏱️ Performance Modeling

Instead of exhaustive packet-level simulation, the system uses a latency-and-bandwidth-aware abstract model:

• Latency: Calculated based on physical Manhattan distance: $$\text{Latency}_{\text{op}} = \text{Base Latency} + (\text{Manhattan Distance} \times \text{Hop Latency})$$
• Bandwidth & Congestion: The simulator enforces a Bisection Bandwidth Constraint. If total bytes transferred per superstep exceed network capacity, a congestion multiplier is applied to "stretch" the superstep duration.

---

🛠️ Current Status

Component	Status	Details
ISA Decoder	✅ Complete	Full implementation of Compute, Mesh, Control, System, DSD, and Global memory opcodes.
Hardware Model	✅ Functional	Core logic, SRAM, 2D Mesh, and Host-Device IO are implemented.
Compiler	✅ Functional	AST parsing, IR generation, register allocation, and assembly are operational.
Simulation Engine	✅ Functional	Hybrid Performance/Functional tracks and BSP scheduling are implemented.
Advanced Mapping	🚧 In Progress	Optimizing spatial mapping for complex kernels.
Weight Server	🚧 In Progress	Integration with external weight servers for real-world model weights.

🧪 Testing & Validation

The project uses a Dual-Execution strategy to verify the compiler:

1. Python Path: Executes the kernel as a Python function via KernelContext (Golden Reference).
2. Binary Path: Compiles the kernel $\rightarrow$ executes the resulting binary on the BSPScheduler.
3. Comparison: Bit-exact comparison of final memory states.

To run integration tests:

python3 -m unittest discover tests/integration