The ARM64 Linux kernel today supports up to 256 cores. A server with two AmpereOne CPUs has 384 cores, which the operating system cannot handle.

Can there ever be too much of something when we talk about data center hardware? Apparently yes, if you build a system with many CPU cores. Ampere’s new AmpereOne ARM CPU has 192 CPU cores. For a two-chip system server, that’s a total of 384 ARM cores. The ARM64 Linux kernel only supports up to 256 cores or less.

To solve this problem, Ampere proposed a patch that increases the core limit of the Linux kernel to 512 cores using the “CPUMASK_OFFSTACK” method. This method allows Linux to overlook this limit without changing the memory footprint of the image. According to Tom’s Hardware, each core increases the kernel image size by 8KB.

Ampere is the first manufacturer to reach this limit. Other manufacturers are also likely to experience problems in the near future. The new AmpereOne series consists of ARM chips with all high core counts: 136, 144, 160, 176 and therefore also 192, their flagship.

Waiting for new kernel

Ampere’s latest chip now has the most CPU cores ever on board. Even AMD’s latest Zen 4c EPYC chips have 128 cores on board. In a dual-socket system, you end up with 256 cores available, which is exactly the limit that Linux can handle.

We may have to wait a while for an increase to more CPU cores. Back in 2021, a patch was submitted to increase the ARM64 CPU core limit to 512, but this was refuted by the Linux team due to the lack of CPU hardware at the time. Support for 512 CPU cores could arrive no sooner than 2024 with the introduction of the Linux kernel 6.8.

However, this timeline only considers adding 512 core support in the normal way, without using the CPU mask off-stack method. Technically, the outgoing Linux kernel already supports the off-stack method of CPU mask to increase CPU core count limits. It is now up to Linux administrators to enable this feature by default.