Esta é a maneira de Yosemite resfriar sua máquina.
Activity Monitor may show that a process named kernel_task is using a large percentage of your CPU, and during this time you may notice a lot of fan activity. This process helps manage temperature by making the CPU less available to processes that are using the CPU intensely. In other words, kernel_task responds to conditions that cause your CPU to become too hot. When the temperature decreases, kernel_task automatically reduces its activity.
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O problema subjacente pode ser outro processo, ou até mesmo que o fluxo de ar esteja bloqueado, causando superaquecimento.