Strix Halo's Memory Subsystem: Tackling iGPU Challenges

Summary

Editor’s Note (11/2/2025): Due to an error in moving the article over from Google Docs to Substack, the “Balancing CPU and GPU Bandwidth Demands” section was missing some Cyberpunk 2077 data. Apologizes for the mistake!AMD’s Strix Halo aspires to deliver high CPU and GPU performance within a mobile device. Doing so presents the memory subsystem with a complicated set of demands. CPU applications are often latency sensitive with low bandwidth demands. GPU workloads are often latency tolerant and bandwidth hungry. Then, multitasking requires high memory capacity. Mobile devices need low power draw. Finally, the whole package has to fit within a price tag acceptable to consumers. Investigating how AMD dealt with those challenges should make for a good time.ASUS has kindly sampled the ROG Flow Z13, which implements Strix Halo in a tablet form factor with 32 GB of LPDDR5X. They’ve made deep dives like this possible, and we greatly appreciate their support.RX 7600 results were provided by Azralee from the Chips and Cheese Discord.Strix Halo’s GPU uses a similar cache setup to AMD’s older and smaller mobile chips. As on Strix Point and Hawk Point (Zen 4 mobile), Strix Halo’s GPU is split into two Shader Arrays. Each Shader Array has 256 KB of L1 mid-level cache, and a 2 MB L2 services the entire GPU. Latencies to those GPU-private caches are in line with other RDNA3 and RDNA3.5 implementations. AMD likely kept L2 capacity at 2 MB because a 32 MB memory side cache (Infinity Cache, or MALL) takes over as the GPU’s last level cache.The L2 only has to catch enough traffic to prevent the Infinity Cache from getting overwhelmed. The resulting cache setup is similar to the one in the RX 7600, a lower midrange RDNA3 discrete card.The Infinity Cache on Strix Halo has slightly higher latency compared to implementations in AMD’s discrete cards. DRAM latency from the GPU is higher as well. Compared to AMD’s other mobile CPUs with iGPUs though, the 32 MB Infinity Cache offers a large c...