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HBM Write (Aggressive)

Overview

The hbm_write_agg test pushes VRAM and Infinity Fabric bandwidth to the absolute limit while intentionally maximizing electrical noise on the bus.

Execution Mechanics

This kernel modifies a standard sequential write by introducing three aggressive changes:

  1. Crosstalk Pattern: It rapidly alternates writing 0xAAAAAAAA (10101010...) and 0x55555555 (01010101...). Switching between these forces every single bit on the physical bus to flip relative to its neighbor, maximizing Inter-Symbol Interference (ISI) and crosstalk.
  2. Massive Unrolling: It utilizes a 64x unroll factor to ensure the instruction pipeline consists of nothing but pure STORE commands.
  3. Cache Bypass: It leverages Non-Temporal (NT) stores to write directly to the HBM modules, bypassing the L2 cache entirely.

Target Subsystems

  • Primary Target: VRAM modules and interconnect fabric.
  • Secondary Target: Unified Memory Controllers (UMCs).

Failure Symptoms

Critical Failures

  • System Freeze / Driver Timeout: The electrical noise causes signal integrity to collapse, hanging the memory controller.
  • Visual Artifacts: Bit flips occur on the bus before reaching the physical memory cells.