Evaluating the Infinity Cache in AMD Strix Halo

Summary

Strix Halo is the codename for AMD’s highest end mobile chip, which is used in the Ryzen AI MAX series. It combines a powerful CPU with 16 Zen 5 cores and a large GPU with 20 RDNA 3.5 Workgroup Processors (WGPs). The sizeable iGPU makes Strix Halo particularly interesting because GPUs have high bandwidth requirements. Strix Halo tackles that with a 256-bit LPDDR5X-8000 setup combined with 32 MB of memory side cache. The latter is often referred to as Infinity Cache, or MALL (Memory Attached Last Level). I’ll refer to it as Infinity Cache for brevity.Infinity Cache has been around since RDNA2 in AMD’s discrete consumer GPU lineup, where it helped AMD hit high performance targets with lower DRAM bandwidth requirements. However, Infinity Cache’s efficacy has so far been difficult for me to evaluate. AMD’s discrete GPUs have performance monitoring facilities accessible through AMD’s developer tools. But those tools stop providing information past L2. Strix Halo stands out because it has an Infinity Cache implementation, and all the accessible performance monitoring features typical of a recent AMD GPU. That includes programmable performance counters at Infinity Fabric and memory controllers. It’s an opportunity to finally get insight into how well AMD’s Infinity Cache does its job in various graphics workloads.Special thanks goes out to ASUS for sampling their ROG Flow Z13. This device implements AMD’s Ryzen AI MAX+ 395 with 32 GB of LPDDR5X in a thin and light form factor. It superficially represents a convertible tablet from Microsoft’s Surface line, and is remarkably portable for a device with gaming credentials. Without ASUS’s help, this article wouldn’t have been possible.AMD’s Infinity Fabric aims to abstract away details of how data travels across the chip. It does so by providing endpoints with well defined interfaces to let blocks make or handle memory requests. Infinity Fabric also provides a set of programmable performance counters. AMD documents a single DAT...