Re-applying CPU thermal paste fixed my CPU throttling issues
Back at the start of May, my office workstation started reporting thermal throttling problems when I had all four cores fully busy:
kernel: CPU1: Core temperature above threshold, cpu clock throttled (total events = 1) kernel: CPU3: Package temperature above threshold, cpu clock throttled (total events = 1) kernel: CPU2: Package temperature above threshold, cpu clock throttled (total events = 1) kernel: CPU0: Package temperature above threshold, cpu clock throttled (total events = 1) kernel: CPU1: Package temperature above threshold, cpu clock throttled (total events = 1)
As usual, I stuck these messages into some Internet searches, and the general advice I found was to re-apply thermal paste to the CPU, because apparently inexpensive thermal paste can dry out and get less effective over time (this assumes that you've exhausted obvious measures like blowing out all of the dust and making sure your case fans are working). I put this off for various reasons, including that I was going on vacation and the procedure seemed kind of scary, but eventually things pushed me into doing it.
The short version: it worked. I didn't destroy my office machine's CPU, it was not too annoying to get the standard Intel CPU fan off and then on again, and after my re-application my office machine's CPU doesn't thermally throttle any more and runs reasonably cool. As measured by the CPU itself, when I build Firefox using all four cores the temperature now maxes out around 71 C, and this was what previously ran headlong into those thermal throttling issues (which I believe happen when the CPU reaches 90 C).
(Note that this is with an i5-2500 CPU, which has a 95 W TDP, and the stock Intel cooler. I could probably have gotten the temperature lower by also getting a better aftermarket cooler, but I didn't feel like trying to talk work into spending the extra money for that. Especially when I want to replace the machine anyway.)
In fact my office machine's CPU is now clearly cooler than my identical home machine's CPU while doing the same Firefox build. The situation is not completely comparable (my home machine has probably been exposed to more dust than my work machine, although I try to keep it cleaned out), but this suggests that maybe my home machine would also benefit from me redoing its CPU thermal paste. Alternately I could get around to replacing it with a new home machine, which would hopefully render the issue mostly moot (although if I wind up assembling said new home machine myself, I'll get to apply CPU thermal paste to it).
(It wouldn't be entirely moot, because I'd like to have my current home machine be a functioning backup for any new machine since I don't have a laptop or other additional spare machine lying around.)
PS: I used ArctiClean to clean off the CPU's old thermal paste and Arctic Silver 5 as the new thermal paste. I didn't do any particular research on this, I just picked products that I'd heard of and people seem to talk about favorably.
(This sort of follows up my earlier mention of this.)