Power Management and SMT Behind “Kaby Lake” and “Zen” Windows 10 Restrictions

Last week we reported that Microsoft will only support upcoming CPU architectures from Intel and AMD on Windows 10. Keep in mind this is support and not compatibility, which means Microsoft will offer customer support and serve updates to Intel “Kaby Lake” and AMD “Zen” machines that are running Windows 10, and not older versions of the Windows operating system. These processors are compatible with any x86 operating system, Windows on Linux, 32-bit or 64-bit. So do not think they are incompatible with older operating systems.

The decision to not offer support from Microsoft is likely because of the new power management and SMT features of the new processors. With Intel’s “Kaby Lake” microarchitecture they are introducing a new feature called Speed Shift Technology. The allows the processor to adjust its clock speed to match processing loads at a response time of 15 ms. This operation likely requires OS-level hooks so that the on-die power management components can poll for processing loads accordingly and raise or lower clock speeds 66.66 times each second, at no CPU cost.


On the AMD side they had spoke of fine-tuned, multi-domain clock-gating on its “Zen” based processors.


AMD “Zen” processors will also introduce simultaneous multi-threading, a feature that exposes each physical core as two logical CPUs to the OS. Intel has been using SMT with their HyperThreading Technology for will over a decade now. AMD’s implementation of SMT is not identical to Intel’s. This process cannot work without the OS Kernal and scheduler being aware of the method. Microsoft actually had to update the kernel and scheduler of Windows 7 in a similar way for “Bulldozer” and they will need to do the same for “Zen”.


For these reasons Microsoft is limiting the support for new CPU microarchitectures to Windows 10.

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