ResolutionMany end-users get used to measure CPU power in Megahertz which is why it is more convenient for service providers to set CPU limits in Megahertz (MHz) rather than in per cent.
Generally the limit shows the amount of processor time can be used by a virtual environment, e.g. if host server has 4 core processor with 2000 MHz clock, the total CPU power will be considered as 8000 MHz.
A virtual environment with CPU limit equal to 500 MHz on such host will consume not more than 6.25% of CPU time. On that host the limit 500 MHz will mean the same as the limit 6.25%.
The major difference between limits Megahertz and in per cent is that the limit in MHz will be preserved after migration of virtual environment, that is if we migrate the virtual environment with 500MHz limit to the host with 2 cores with the clock of 2000 MHz the total CPU power will now be 4000 MHz so the virtual environment will not consume more than 12.5% of CPU time. If we set CPU limit equal to 6%, the virtual environment will consume not more than 6% of CPU time on any host it can reside. See the picture below which illustrates this behavior:
For more information refer to Parallels Server Bare Metal User Guide 5, page 105, chapter Managing CPU Shares and Limits