CXL™ is needed to overcome CPU-memory and memory-storage bottlenecks faced by computer architects. Future data centers need heterogeneous compute, new memory and storage hierarchy, and an agnostic interconnect to tie it all together. CXL maintains memory coherency between the processor memory space and memory on attached devices to enable pooling and sharing of resources to provide higher performance, reduce software stack complexity, and lower overall system cost.​

Traditional DRAM and persistent storage class memory (SCM) are supported, allowing for flexibility between performance and cost.

Compute Express Link™ (CXL™) is an open industry standard interconnect offering high-bandwidth, low-latency connectivity between the host processor and devices including accelerators, memory expansion, and smart I/O devices. CXL utilizes the PCIe® 5.0 physical layer infrastructure and the PCIe alternate protocol to address the demanding needs of high-performance computational workloads in Artificial Intelligence, Machine Learning, communication systems, and HPC through the enablement of coherency and memory semantics across heterogeneous processing and memory systems.​

The CXL™ protocol supports three different type of devices:

• Type 1 Caching Devices / Accelerators
• Type 2 Accelerators with Memory
• Type 3 Memory Buffer

• Memory tiering in which additional capacity is applied with a variable mix of lower-latency direct-attached memory and higher-latency large capacity memory
• Higher VM density per system by having more memory capacity attached
• Large databases can use a caching layer provided by SCM to improve the performance

• CXL.io is used for initialization, link-up, device discovery and enumeration, and register access. It provides a non-coherent load/store interface for I/O devices similar to PCIe® 5.0.
• CXL.cache defines interactions between a Host and Device, which allows CXL devices to cache host memory with low latency.
• CXL.mem provides a Host processor with direct access to Device-attached memory using load/store commands.

CXL™ runs on PCIe® 5.0 electrical signals. CXL runs on PCIe PHY and supports x16, x8, and x4 link widths natively.

CXL™ 2.0 adds support for switching, persistent memory, and security as well as memory pooling support to maximize memory utilization, reducing or eliminating the need to over-provision memory.

In traditional servers, memory is directly connected to a specific CPU or GPU (i.e., locked behind the host) and can result in over-provisioning of memory resources when applications are not using the available memory. When the memory is over-provisioned to a specific host, the memory is now stranded and cannot be accessed by other hosts, thereby increasing data center costs. In addition, when memory is locked behind a host, the data being processed by the application needs to be copied through high latency interconnects if a different CPU or GPU needs access to the data.

Memory pooling allows multiple hosts in a heterogenous topology to access a common memory address range with each host being assigned a non-overlapping address range from the “pool” of memory resources. Memory pooling allows system integrators to dynamically allocate memory from this pool, which reduces costs by reducing stranded memory and increasing memory utilization. Memory pooling is part of a growing trend for resource disaggregation or composability for heterogeneous solutions.

Memory sharing allows multiple hosts in a heterogeneous topology to access a common memory address range with each host being assigned the same address range as the other host. This improves memory utilization similar to memory pooling, but also provides an added benefit of data flow efficiency since multiple hosts can access the same data. With memory sharing, coherency needs to be managed between the hosts to ensure data is not overwritten by another host incorrectly.

CXL™ 2.0 supports Integrity and Data Encryption (IDE) and key exchange protocols for to provide end-to-end protection of data on the CXL link.

“RAS” is the ability of the system to provide resilience starting from the underlying hardware all the way to the application software through three components collectively referred to as “RAS” features: ​

1. Reliability: the ability of the system to detect and correct faults​
2. Availability: how the system guarantees uninterrupted operation with minimal degradation
3. Serviceability: the ability of the system to proactively diagnose, repair, upgrade or replace components at scale​