Ubuntu 14.04 vs 12.04.4 + prime95 or LLR
 

Did anyone encountered any stability problems running prime95 / LLR under latest Ubuntu 14.04?

Couple of months ago I have put together HTPC based on i7-4770s + 4GB RAM. I've installed Ubuntu 12.04.4 and with only one 140mm exhaust fan in the case I've had a perfectly stable machine running 4 LLR instances 24/7 at stock speeds.

When Ubuntu 14.04 showed up I have installed it on the same HTPC, and at the same time I put an additional 120mm fan on the Thermalright True Spirit 140 CPU cooler lowering temperatures from 84-85C to 73-74C.

Everything was fine until recent increase in ambient temperature (summer is coming :smile:), which caused the increase of CPU temperature to ~78C, and generated first round-off error in LLR (I haven't seen those in years). I have later confirmed stability issue under prime95 (usually I was getting an error after 6-8 hours of stress-testing using both small-FFT and large-FFT, blend test was not affected).

This made me very suspicious and after I have performed a lot of double/triple checks I've found out that some results under 14.04 were in fact bad, whereas all results under 12.04.4 are OK (even those when CPU was cooled passively and operated above 80C).

Does 12.04 support the latest cpu instructions? I doubt it. 14.04 will. This will push your cpu harder and produce more heat etc.

Both under 12.04.4 and 14.04 I was using FMA3 FFTs. Iteration times are the same, temperature under the same conditions is the same, the only difference is instability.

Did you reinstall 12.04 and check whether the stability issues are effected by the higher ambient temperatures? Maybe the higher ambient temperatures cause higher stress on the PSU or VRMs on the motherboard, but that is just a wild guess....

Yes, it is the 4th day after reinstallation of 12.04.4 and no errors during stress testing have occurred so far. Today evening (CET) I will start doublecheck of all the work performed on this machine from beginning of April.

All in all I don't think it is related to ambient temperature, as earlier I was using 12.04.4 on passively cooled CPU and core temperatures were even higher than lately - time will tell, as my doublecheck will be done during hottest time of the year :devil: