Intel TurboBoost in practice

Alexander Motin mav at
Sat Jul 24 23:06:56 UTC 2010

Rui Paulo wrote:
> On 24 Jul 2010, at 14:53, Alexander Motin wrote:
>> Here is my test case: FreeBSD 9-CURRENT on Core i5 650 CPU, 3.2GHz + 1/2
>> TurboBoost steps (+133/+266MHz) with boxed cooler at the open air. I was
>> measuring building time of the net/mpd5 from sources, using only one CPU
>> core (cpuset -l 0 time make).
>> Untuned system (hz=1000):     14.15 sec
>> Enabled ACPI C2 (hz=1000+C2): 13.85 sec
>> Enabled ACPI C3 (hz=1000+C3): 13.91 sec
>> Reduced HZ (hz=100):          14.16 sec
>> Enabled ACPI C2 (hz=100+C2):  13.85 sec
>> Enabled ACPI C3 (hz=100+C3):  13.86 sec
>> Timers tuned* (hz=100):       14.10 sec
>> Enabled ACPI C2 (hz=100+C2):  13.71 sec
>> Enabled ACPI C3 (hz=100+C3):  13.73 sec
>> All numbers tested few times and are repeatable up to +/-0.01sec.
>> PS: In this case benefit is small, but it is the least that can be
>> achieved, depending on CPU model. Some models allow frequency to be
>> risen by up to 6 steps (+798MHz).
> The numbers that you are showing doesn't show much difference. Have you tried buildworld?

If you mean relative difference -- as I have told, it's mostly because
of my CPU. It's maximal boost is 266MHz (8.3%), but 133MHz of them is
enabled most of time if CPU is not overheated. It probably doesn't, as
it works on clear table under air conditioner. So maximal effect I can
expect on is 4.2%. In such situation 2.8% probably not so bad to
illustrate that feature works and there is space for further
improvements. If I had Core i5-750S I would expect 33% boost.

If you mean absolute difference, here are results or four buildworld runs:
hw.acpi.cpu.cx_lowest=C1: 4654.23 sec
hw.acpi.cpu.cx_lowest=C2: 4556.37 sec
hw.acpi.cpu.cx_lowest=C2: 4570.85 sec
hw.acpi.cpu.cx_lowest=C1: 4679.83 sec
Benefit is about 2.1%. Each time results were erased and sources
pre-cached into RAM. Storage was SSD, so disk should not be an issue.

Alexander Motin

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