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(Get the code!)
It is normally always preferable to use the intel-pstate driver compared to pcc-cpufreq or acpi-cpufreq on modern Intel hardware.
Some HP ProLiant platforms implement the PCC interface [1] which can be disabled by a BIOS setting in which case the PCC driver will not load and the acpi-cpufreq driver can be used instead.
The intel-pstate driver is presumed to be better for Sandybridge CPUs and later. Unlike the the cpufreq drivers, it uses P-states rather than cpu frequency [2]. It also has access to CPU performance metrics so in theory it has finer control than the traditional BIOS table driven frequency scaling.
So for HP Proliants that are pre-Sandybridge, pcc-cpufreq may be the best bet, providing the firmware is doing the right thing. If not, acpi-cpufreq maybe better, as long as the BIOS has the correct control data in the ACPI tables.
[1] Processor Clocking Control, https:/
[2] https:/