CPU% limit is very disproportional when host frequency scaling is in effect

[LekKit]

The issue

• Setting RVVM_OPT_MAX_CPU_CENT to a value below 70-80% leads to guest CPU slowing down much more than actual CPU percentage requested on host machines with frequency scaling

Steps to reproduce

• Set non-performance governor
• rvvm_set_opt(machine, RVVM_OPT_MAX_CPU_CENT, 50);
• The guest is actually much slower than half of the peak CPU speed

Investigation

• RVVM has the assumption that during any given timeslice host CPU does the same amount of work
• Host governor has the assumption that when a process consumes <100% of CPU, frequency might be lowered since that process is likely doing fixed amount of work
• Lowering frequency reduces the amount of work performed during RVVM guest CPU timeslice, which is now disproportionate to the actual peak host CPU speed

Workarounds

• Use performance governor on Linux host
• Set performance mode on Windows host