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Intel

Hyper-Threading Technology

Intel Nehalem-based processors re-introduce the support for HT Technology (simultaneous multi-

threading). HT Technology lets each core execute two computational threads at the same time, which

allows each four-core processor to simultaneously execute up to eight threads. In addition, the high-

bandwidth memory subsystem supplies data faster to the two computational processes, and the low-

latency cache hierarchy allows simultaneous processing of more instructions. HT Technology improves

performance-per-watt over previous generation Intel processor-based servers.

HT Technology achieves performance gains by reducing latency. The two threads are not executed in

parallel; rather, they share the resources of a single execution core. If one thread needs to use an

execution unit being used by the other thread, it must wait. As a result, processor throughput may only

increase by up to 30-percent and may vary based on the application and hardware platform. In

applications where software programmers minimize or effectively eliminate memory latencies through

cache optimizations, HT Technology may not yield measureable performance gains.

In comparison, a two-core processor running a single thread on each core provides true parallel

execution, delivering close to 100% performance improvement. In addition, a well-designed two-core

processor is more energy efficient than a single core processor running multiple threads. For these

reasons, processors based on the Intel Core microarchitecture did not support HT Technology.

Intel

Turbo Boost Technology

Turbo Boost Technology complements HT Technology by increasing the performance of both multi-

threaded and single-threaded workloads. The processor increases the clock frequency of all active

cores when it is operating below power and thermal design points set by the user. These design

points include the number of active cores desired, the estimated current consumption, the estimated

power consumption, or the processor temperature.

The three Turbo Boost control states are Off, Automatic, and Manual. When Turbo Boost is turned

Off, the processor operates only at the rated frequency. When Turbo Boost is set to Automatic, the OS

requests a higher performance state and the processor determines the optimum frequency. When

Turbo Boost is set to Manual, the user can manually disable cores using the BIOS (reboot required)

and increase the likelihood that Turbo Boost will be initiated (Figure 12).

Figure 12. Turbo Boost Technology: some cores turned off; remaining cores running at a higher frequency

There are at least three situations in which disabling processor cores can prove beneficial.

• Reducing power use. Disabling processor cores reduces processor power use. If a server is being

used in an application environment that does not depend heavily on multi-threading, disabling

cores can lower power consumption without materially affecting performance.

• Increasing overall performance. Some applications benefit from higher core frequency rather than

from additional cores. When Turbo Mode is enabled for Intel Nehalem processors, the power and