System Management Unit
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The System Management Unit (SMU) is an advanced internal subsystem introduced in late 2004 with the iMac G5 and Power Mac G5 series computers. Its manages the functions previously governed by the PMU (Power Management Unit) as well as additional cooling functions.
The SMU manages thermal and power conditions to optimize the power and airflow while keeping audible noise to a minimum. Power consumption and temperature are monitored by the operating system, which communicates the necessary adjustments back to the SMU. The SMU makes the changes, slowing down or speeding up fans as necessary.
Resetting the SMU may fix certain problems during troubleshooting.
To shed more light on the SMU, see the article on the Power Management Unit ( PMU ), the SMU's predeccesor.
References
"Desktop and Portable Systems: Second Edition" Edited by Owen W. Linzmayer ISBN 0-321-33546-5