DISABLE turbo boost

[Wei_K_]

Hi, All,

first of all, I always write my own driver when I need to read something from the model-specify register and I am successfully read some PMU counters before, say for instruction retired, cache misses and so on.

Recently I am conducting experiment using SPECCPU2006 benchmark and I notice the running time varies (larger than 10%) for different round. the program usually run faster iteration by iteration. I suspect that turbo boost may did the trick. In order to reproduce the same execution time, I need it to run without having freq changes. I did some search and noticed that by modify the register 0x1A0 from 0x850089 to 0x4000850089 can set the turbo boost off, in other word, set the bit 38 to 1 can disable turbo boost. So I first read the content from 0x1A0 and then set it to 0x4000850089 and write back. I am aware that I may need to do this for each core.

after that, I re-read the value, I found:

core0: 0x4000850089

core1: 0x4000850089

core2: 0x0

core3: 0x4000850089

and I test my benchmarks, it still run faster after iterations. (I pin the benchmark running on core0 using taskset -c 0 ./benchmark_script )

So my question is: is it possible that the os can re-write the msr so I cannot turn off turbo boost forever? Am i wrong on modify the turbo boost.

other thing is: any suggestion for the program execution time other than turbo boost? say smart cache technology? prefetcher? I am not expert on this side so any suggestion and idea are welcome

By the way, my machine is intel i7-920 (Nehelam) and os is linux centos5.10. I go into my bios and can only disable speedstep and it didn't have any turbo boost option for me to enable/disable in bios. And this cpu do have turbo boost feature in it.

Thanks in advance.

[McCalpinJohn]

It is best to use the OS options to control frequency if they are available.   On Linux systems this is usually via the "acpi-cpufreq" loadable kernel module and its associated files.   I don't know if this existed as far back as Centos 5.10, but it is certainly supported in Centos 6 & 7.

If the "acpi-cpufreq" module is loaded, then for each core (${P}) you want to execute these two commands:

echo "userspace" >/sys/devices/system/cpu/cpu${P}/cpufreq/scaling_governor

echo [frequency_in_kHz] > /sys/devices/system/cpu/cpu${P}/cpufreq/scaling_setspeed

On some systems the BIOS retains control of the frequencies, in which case the "acpi-cpufreq" module will probably not be loaded and you will need to find the magic BIOS option to tell it to give frequency control back to the OS.