Friday, September 7, 2012

CPU Overheating Legacy Mode

CPU Overheating in Legacy Mode

When in Legacy (non-protected) mode it seems some CPUs are particularly susceptible to overheating. This is before they start frequency scaling, sometimes before dynamic FAN speed control, perhaps before use of the HLT instruction, and definitely before use of any core parking. Thus, these mitigation strategies may be all that keeps some CPUs from overheating. I've always noticed an increase in thermal emissions when my CPU is running full speed in legacy mode, but today is the first time I accidentally spent too much time in legacy mode, causing my system to shutdown pre-boot due to the CPU temperature (which had raised to 74C, though post-boot, with mitigation strategies in place, is back at 45C). Now, this is actually an indication of some trouble on this PC, despite the heavy load I place on it. Either I must allow for such extreme temperatures, or perhaps re-seat the Heatsink. Still, on a PC with a heavy lead that has no issues running continuously at 100% CPU frequency in High Performance mode, this is a surprising - and scary - discovery. I can't help but wonder, how long would your PC last in legacy mode?

UPDATE: In my case, since I was in a pre-boot RAID configuration tool, I believe the failure of the CPU fan speed to ramp up may have been the primary cause. The UEFI was likely not fully initialized at this stage, and thus left me in a vulnerable state, while the CPU ran at 100% of its clock speed, with 100% utilization of a single core at least (as it does in legacy mode).

UPDATE2: I have reproduced this many time by entering the pre-boot configuration on a HighPoint RAID controller. During its configuration interface, either the fan isn't running fast enough, or other cooling mitigation technologies aren't working. CPU temperature quickly escalates, and will eventually halt the PC. Otherwise, in the temperature in between, it will warn you at next bootup if it hasn't cooled already.

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