How can I lock my Xeon CPU frequency in R730xd?

Hi, 

Recently I'm trying to set my CPU frequency to a fixed value for my research. It seems a simply issue but there is a problem.

My machine is Dell PowerEdge R730xd. The CPU is Intel Xeon E5 2630 v4 2.2GHz. The OS and kernel is Ubuntu 16.04 and 4.15.0-33-generic. The current settings are as follows:

1) intel_pstate driver is disabled (Add "intel_pstate=disable" to /etc/default/grub)

    cat /sys/devices/system/cpu/cpu*/cpufreq/scaling_driver will show acpi-cpufreq

2) userspace governor is used (sudo cpupower frequency-set -g userspace)

    cat /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor will show userspace

3) the target frequency is set (sudo cpupower frequency-set -f 2.2G)

    cat /sys/devices/system/cpu/cpu*/cpufreq/cpuinfo_cur_freq will show 2200000

4) the current BIOS power management settings are as follows:

• CPU Power Management -> OS DBPM (i.e., OS Control)
• Memory Frequency -> Maximum Performance
• Turbo Boost -> Disabled
• C1E -> Disabled (When set to Enabled, the processor is allowed to switch to minimum performance state when idle)
• C States -> Disabled (C States allow the processor to enter lower power states when idle)
• Collaborative CPU Performace Control -> Enabled
• Uncore Frequency -> Maximum
• Energy Efficient Policy -> Performance (And there are other options, e.g., Balanced Performance / Balanced Energy / Energy Efficient）
• Monitor/Mwait -> Enabled    

However, by using watch -n 1 "cat /proc/cpuinfo | grep MHz" to monitor the current CPU frequency, I find that the frequency is always changing. (Using i7z is the same). Only when the load becomes higher (e.g., use stress -c 40) the CPU frequency will remain stable in 2.2GHz.

So, how can I lock my CPU frequency (For my current research, the cpu frequency must be locked)? Any help is really appreciated!