Intel ARCHITECTURE IA-32 User Manual download pdf (Page 430)

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IA-32 Intel® Architecture Optimization

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thread enables the physical processor to operate at lower frequency

relative to a single-threaded version. This in turn enables the processor

to operate at a lower voltage, saving battery life.

Note that the OS views each logical processor or core in a physical

processor as a separate entity and computes CPU utilization

independently for each logical processor or core. On demand, the OS

will choose to run at the highest frequency available in a physical

package. As a result, a physical processor with two cores will often

work at a higher frequency than it needs to satisfy the target QOS.

For example if one thread requires 60% of single-threaded execution

cycles and the other thread requires 40% of the cycles, the OS power

management may direct the physical processor to run at 60% of its

maximum frequency.

However, it may be possible to divide work equally between threads so

that each of them require 50% of execution cycles. As a result, both

cores should be able to operate at 50% of the maximum frequency (as

opposed to 60%). This will allow the physical processor to work at a

lower voltage, saving power.

So, while planning and tuning your application, make threads as

symmetric as possible in order to operate at the lowest possible

frequency-voltage point.

Thread Migration Considerations

Interaction of OS scheduling and multi-core unaware power

management policy may create some situations of performance anomaly

for multi-threaded applications. The problem can arise for

multi-threading application that allow threads to migrate freely.

When one full-speed thread is migrated from one core to another core

that has idled for a period of time, an OS without a multi-core-aware

P-state coordination policy may mistakenly decide that each core

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