AMD Enabling "Fast CPPC" For Even Greater Linux Performance & Power Efficiency On Some CPUs

While AMD Zen 4 processors whether it be the Ryzen 7000/8000 desktop/mobile series or EPYC 8004/9004 series server processors are already performing very well on Linux and with great power efficiency against the competition as shown in dozens of Phoronix articles at this point, it turns out there's been a minor power/performance optimization left untapped yet under Linux for select Zen 4 processors. A new patch series posted this Sunday allows for this "fast CPPC" feature to be utilized on supported processors.

The AMD P-State CPU frequency scaling driver makes use of the ACPI Collaborative Processor Performance Control (CPPC) data for describing performance scales and attributes on a per-CPU basis and in turn the kernel requesting desired performance levels. With some AMD processors beginning with current Zen 4 models, there is the notion of "fast CPPC". The Fast CPPC feature when indicated by a CPU bit allows for a faster CPPC loop thanks to architectural enhancements. In turn leveraging AMD Fast CPPC can make for higher performance at the same power level.

Detection of AMD Fast CPPC requires checking a particular bit to see if it's supported. I haven't seen any public list of what current Zen 4 processors support Fast CPPC, but given the kernel patch comments it's at least found with select Phoenix APUs like the Ryzen 7 7840HS.

The posted kernel patches allow the CPU frequency transition delay to be lowered from 1000 us to 600 us on systems with fast CPPC support. In turn depending upon the workload this can commonly lead to 0~2% better performance. On a performance-per-Watt basis some workloads can see as much as a 6% improvement.

Overall this AMD Fast CPPC feature appears to be a relatively modest optimization but again it's for further enhancing the already great Zen 4 performance on Linux. Any performance and power efficiency gains are always appreciated though a bit surprising it's taken until now to see this "fast CPPC" support emerge for the Linux kernel. At least it will presumably be ready for upcoming AMD Zen 5 processors.

This patch series is where the AMD Fast CPPC support is currently out for review before being mainlined in the upstream Linux kernel hopefully in the near-term for further enhancing Zen 4's performance and efficiency prowess.