8/28/2023 0 Comments Cxl cache coherence![]() ![]() The multiple hosts and device agent caches and the accompanying memories must remain coherent in a CXL.cache/mem design.The host bias and the device bias are the two states for device-attached memory defined by the bias-based coherency model for a Type 1 CXL device. Applicable transaction types: CXL.mem MemRd and MemWr transactions.The typical application is a memory expander for the host.Only has CXL host-managed device memory.Applicable transaction types: All CXL.cache/mem transactions.Typical applications are devices that have high-bandwidth memories attached.Implements an optional coherent cache and host-managed device memory.Applicable transaction types: D2H coherent and H2D snoop transactions. ![]() We May need to implement a custom ordering model.Extends PCIe protocol capability (for example, Atomic operation).Implements a fully coherent cache but no host-managed device memory.Additionally, they share a cache coherence domain, which significantly benefits heterogeneous workloads.The final Type 3 devices use case made possible by the Compute Express Link.io and CXL.memory protocols is memory expansion.When performing high-performance applications, a buffer connected to the CXL bus could be utilized to increase memory sharing bandwidth, add persistent memory, or expand DRAM capacity without taking up valuable DRAM slots.Through CXL specification, high-speed, low-latency storage devices that would have previously replaced DRAM can now complement it, making add-in card, U.2, and EDSFF form factors available for non-volatile technologies.Let’s examine the several CXL device types and the particular CXL interconnect verification issues, such as preserving cache coherency between a host CPU and an accelerator. io and CXLcache protocol and has its DDR or High Bandwidth CXL Memory devices.When all three protocols are active, the memory pooling of the host processor and the accelerator are both made locally accessible to the CPU. Each requires the Rambus Compute Express mory protocol and Compute Express Link. This type of device uses the CXL.io protocol, which is required, and CXL.cache to communicate with the host processor’s DDR memory capacity as if it were it’s own.One possible example is a smart network interface card that can benefit from caching.GPUs, ASICs, and FPGAs are Type 2 devices. The Compute Express Link Consortium refers to the first as a Type 1 device, consisting of accelerators without host CPU memory. ![]() This is helpful for Type-1 and Type-2 CXL protocol devices. If the device caches the location, caches offer high-bandwidth and low-latency accesses.Accessing a local memory location through a non-cached connection is substantially more effective than the 500-ns access latency and 50-GB/s bandwidth. Only the data that the gadget needs can be cached. Hence all CXL devices need to support it since the link cannot function without it.The Compute Express Link OR CXL standard defines and may support three distinct Compute Express Link device types that come from various combinations of the other two protocols.The three distinct Compute Express Link memory device types are together with the corresponding protocols, typical applications, and supported memory access types. The CXL.io protocol is used for initialization and link-up. This article focuses on the Compute Express Link device types.Compute Express Link uses three protocols: CXL.io, CXL.cache, and CXL.mem. Introduction to Compute Express Link Device Types: Compute Express Link is anticipated to be used in heterogeneous computing systems with hardware accelerators that deal with artificial intelligence, machine learning, and other specialized activities. Compute Express Link (CXL) is an open interconnect standard that enables fast, coherent memory access between a host device, such as a CPU, and a hardware accelerator, which manages a heavy workload.With the recent release of Compute Express Link 1.0, a consortium to enable this new standard was established, and its initial interface definition was made available.
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