Navigating Cache Coherence: The Back-Invalidate Feature in CXL 3.0

By _Cadence Blog

November 30, 2023

In the rapidly evolving landscape of data centers, ensuring cache coherence in multi-host environments is imperative. The Compute Express Link (CXL) 3.0 specification introduces a robust mechanism known as the Back-Invalidate feature to uphold cache coherence across multiple hosts and devices. This blog delves into the technical complexity of the Back-Invalidate feature, explaining how it contributes to the efficient functioning of modern data center architectures.

Introduction to CXL 3.0

CXL 3.0 is an open-standard interconnect technology that builds upon PCIe 6.0 to facilitate high-speed communication between CPUs and peripheral devices. With a doubled bandwidth of 64 GT/s and enhanced fabric capabilities, CXL 3.0 aims to optimize system-level flows, resource utilization, and enable new device types for composable disaggregated infrastructure.

Unveiling the Back-Invalidate Feature:
At the heart of CXL 3.0's cache coherence strategy lies the Back-Invalidate Snoop (BISnp) feature. This mechanism allows a CXL 3.0 memory device to issue a Back-Invalidate snoop to the host(s) to change the cache state, ensuring cache coherence in shared memory scenarios.

BISnp feature provides a more direct and efficient mechanism for cache invalidation, offering improvements in system performance, scalability, and flexibility compared to the device/host biasing approach in CXL 2.0, which requires more back-and-forth communication between the device and the host to maintain coherence. As systems continue to grow in complexity and size, features like BISnp will become increasingly important for maintaining high performance and efficient resource utilization.

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