AMD’s First Hybrid “big.LITTLE” Processors are Out: Zen 4c and Phoenix 2

AMD has launched its first hybrid core “big.LITTLE” processors in the notebook segment. Codenamed Zen 4c, the low-power “E” cores on these chips are identical to Zen 4 in ISA. Instead, they opt for a reduced L3 cache to save the die area and squeeze in more cores. AMD has released two heterogeneous CPUs within the Ryzen 7040 “Phoenix” lineup. These chips replace/complement existing Zen 4 designs with more Zen 4c cores.

The Ryzen 5 7545U features two Zen 4 and four Zen 4c cores. It can be thought of as a modification of the 7540U, a homogeneous design with six Zen 4 cores. The Ryzen 3 7440U replaces the existing 7440U with a single Zen 4 and three Zen 4c cores. The latter was likely faster with four full-fat Zen 4 cores.

In addition to the large L3 (LLC), Zen 4c is built atop highly dense libraries optimized for low-clock compute workloads. This change limits the peak operating clocks of these cores to sub-4GHz regions. The IPC and performance per clock are identical to Zen 4, with the same set of instructions and features. Both AVX512 and SMT2 are supported.

Unlike Intel’s hybrid designs, AMD has adopted a relatively straightforward scheduling system for Phoenix 2. So straightforward that the Windows scheduler won’t distinguish between the Zen 4 and Zen 4c cores. They will be viewed as lower-clocked cores of the same cluster, ignored in the case of high-performance workloads, and used for less sensitive tasks.

This may be a simple fix to a complex problem, but it has its limitations. For example, unlike Intel’s E-cores, they won’t be (by default) used for background “efficiency” tasks. Scheduling on the heterogeneous/hybrid Phoenix 2 CPU will be based on clocks. High-frequency cores for high-performance threads and low-frequency cores for lightly threaded tasks.

According to AMD’s internal testing, the Zen 4c cores are best suited for low-power, multi-threaded workloads. At 15W and lower, these compact cores offer superior performance than their larger Zen 4 brethren. Conversely, the latter are better suited for 25W and above, where they outmaneuver the denser cores.

AMD is already shipping its Phoenix 2 notebooks, and it won’t be long before they show up in the retail market. Unfortunately, there isn’t any way of telling them apart from the outside. You’ll need to power on the machine and check the SoC name to confirm the same.

Areej Syed

Processors, PC gaming, and the past. I have written about computer hardware for over seven years with over 5000 published articles. I started during engineering college and haven't stopped since. On the side, I play RPGs like Baldur's Gate, Dragon Age, Mass Effect, Divinity, and Fallout. Contact: areejs12@hardwaretimes.com.
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