White Paper - Power Management in Intel Architecture Servers

Turbo Boost

Introduction to Intel® Turbo Boost Technology

A processor with turbo mode capabilities can run at frequencies

higher than the advertised frequency of the processor if the physical

processor package is operating below its rated maximum temperature,

current, and power limits.

Turbo mode uses available power headroom to run the active

processor cores at higher frequencies. Its availability is independent of

the number of cores, but the turbo mode frequency depends on the

number of active cores. The amount of time a system spends in turbo

mode depends on the system workload and operating environment.

How Turbo Mode Works

CPUs typically operate at a fixed maximum frequency regardless

of the workload. However, most applications allow operation below

the maximum power rating. Headroom may also be available if some

cores are in idle mode as shown in Figure 3, or as long as the package

is operating within its thermal limit. Turbo mode speeds up the CPU

to utilize available power headroom, as needed, to get an extra

performance boost [3].

Turbo mode functionality is enabled and disabled using the BIOS

setup, which provides this option only when the processor supports

this functionality. When it is enabled, BIOS enables this feature in the

processor and publishes a _PST ACPI table with one extra P-state: p0.

Num of P-states in turbo mode = Num of P-states in non-turbo mode + 1

Turbo mode operates under OS control and is engaged only when the

OS requests a transition to a P0 state. No OS changes are required

to use turbo mode. The Intel® Xeon® processor 5500 series does

not support each core running at different ratios when turbo mode

engages, thus all active cores will run at the resolved p0 turbo mode

frequency. Active cores are in a C0 state (working state, not a sleep

state). Figure 4 explains how turbo mode works.

Core

Rated Speed Actual Power

Max Speed Max Power

Turbo Mode

CPU Speed Power

Core

Figure 3. If two cores are off, the remaining active cores can run at a higher frequency while the processor package stays within the overall power

and thermal limits�

White Paper: Power Management in Intel® Architecture Servers