Intel has revealed that Windows 7 features new and improved multi-threading, which will help to improve power consumption and battery life.
Previous versions of Windows often swapped threads around cores, which prevented them from entering lower power states and caused cache thrashing as separate cores raced to grab data processed by others.
The Windows 7 kernel changes this by improving thread affinity, locking threads to particular cores in order to allow unused CPU cores to enter low power C-states when they’re not in use – called thread parking - providing the CPU and motherboard supports this of course.
Both Intel’s Nehalem and AMD’s Phenom II processors do support per-core low power states, so power consumption should improve (decrease) dramatically when using single or lightly threaded applications. Moreover, in the case of Intel’s Nehalem based chips, it should also mean good things for Turbo Mode.
Turbo Mode enables the processor to optimise its performance based on the workload – if only one core is being used, the CPU can run at a higher frequency without stepping outside of its power envelope - this is all hardware controlled by the Power Control Unit (PCU) on each processor. Intel admitted that turning off HyperThreading can give a fraction more Turbo Mode in certain, limited situations, because less of the core is in use at anyone time meaning less heat, but the PCU is "global" to each CPU and doesn't monitor sub-sections within the pipeline, making the performance with HyperThreading on generally better.
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