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I have seen that is a common problem:

but there is no solution or any relevant update. the parallel runs my win7 and when it startup it consume all the cpu power immediately and doesn't stop to do that for hours. does anyone know what is the problem?

I have the most up to date max os X and parallel.

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Have you tried uninstalling and reinstalling the Parallels Tools? Also what happens if you create another Virtual Machine (and install linux for example) Does that virtual machine consume all your CPU too? This may be a specific problem with that VM. – Martín Marconcini Jul 14 '12 at 12:25
How much memory did you allocate to the Windows 7 VM? If it's swapping a lot (causing excessive disk I/O), it will kill the host OS performance. – EmmEff Jul 14 '12 at 21:26

You could try using renice with the application/process, which allows you to adjust it's CPU priorty. There are several applications that can assist in doing this, or you can do it from the command line in Terminal.

In Terminal the syntax is:

# renice ## -p PID 

where ## is priority (value from -20 to 20, -20 being HIGHEST). PID is the process ID of the application. (For more information # man renice)

So if I open Activity monitor and notice Parallels is process ID 1222 and want to lower it's burden on the CPU I would use something such as:

# renice +5 -p 1222

Several Applications (that I know of) also have the ability to do this too:

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