Kernel_task on CPU, MacPro

Question

Plenty of threads on the kernel_task issue, even here. But most of them are on MacBooks, and seem to think it is heat related. I see this issue twice a month or so, on a MacPro.
10.8.2
# top -u
PID
COMMAND 
%CPU  TIME
#TH   #WQ  #PORTS #MREGS RPRVT  RSHRD  RSIZE
0 
kernel_task  400.7 109 hrs  91/12 0
2 
737
25M
0B
1018M
54170  top     
21.5  00:00.63 1/1   0
24+
36+
3056K+ 220K-  3812K+
22871  seamonkey
12.1  05:40:20 31/1  3
363-   18285- 754M-  156M-  1176M-
50207- iTerm   
9.9   18:26:24 8/2   4
171-   383-   36M-   28M-   65M-
Brendans hotkernel say:
mach_kernel`pmap_remove_range                          
8   0.1%
mach_kernel`0xffffff801a921080                        
10   0.1%
mach_kernel`lck_mtx_lock                              
22   0.2%
mach_kernel`pmap_flush_tlbs                           
37   0.4%
mach_kernel`_rtc_nanotime_read                        
55   0.6%
mach_kernel`machine_idle                            
4182  42.3%
mach_kernel`ml_set_interrupts_enabled               
5408  54.7%

Answer 1

The kernel is using excessive processor cycles. Below is a partial list of causes for this condition.
Thermal overload
This can be due to accumulation of dust, high ambient temperature, or to the malfunction of a cooling fan or temperature sensor. The kernel tries to compensate by throttling back the CPU's. You might see messages like the following in the Console window:
SMC::smcHandleInterruptEvent WARNING status=0x0 (0x40 not set) notif=0x0
The timestamps of those messages (if any) indicate the times, since the log was last cleared, when a processor was being throttled because of high temperature.
Note that if the problem is caused by a faulty sensor that reads too high, there may be no actual overheating.
The Apple Hardware Test, though not very reliable, is sometimes able to detect a bad fan or temperature sensor.
Using Apple Hardware Test
Encryption
Transferring large amounts of data to or from an encrypted disk image or FileVault volume puts an extra load on the kernel. If both the source and the destination are encrypted, the load may be doubled. If you transfer data from an encrypted disk image on an encrypted partition to another such image on another encrypted partition, the load may be quadrupled.
This issue probably doesn't affect late-model Macs with an Intel i-series, recent Xeon, or later processor. Those processors support hardware-accelerated encryption. You can determine what kind of processor you have by selectingAbout This Mac from the Apple menu in the menu bar.
Installed software
User-installed software that includes a device driver or other kernel code may thrash the kernel. Some system-monitoring applications, such as "iStat," can also contribute to the problem. You can test for this possibility bycompletely disabling or removing the software according to the developer's instructions, or booting in safe mode (with the shift key held down at the startup chime.) Note, however, that disabling a system modification without removing it or booting in safe mode may not be as easy as you think it is.
Corrupt NVRAM or SMC data
In some cases the issue has reportedly been resolved by resetting the NVRAM or the SMC.