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I have upgraded my laptop CPU and gave it a stress test. The weird thing is that during the test the CPU temperature readings fluctuate wildly in a periodic manner. At idle, the CPU temperature is ~45 degrees C. When I start the stress test, CPU starts running at %100 load (133MHz x 22) while the temperature climbs to ~100 degrees at a normal rate. I observed these two steps:

  1. At ~100 degrees, core multiplier drops to 9x and at the exact same time, temperature reading drops to ~69 degrees. Core multiplier values stay at 9x while temperature drops to ~65 degrees in about 4 seconds.

  2. Core multiplier jumps back to 22x and again, at the exact same time, temperature readings jump to ~96 degrees. Core multiplier stays at 22x for about 6 seconds while temperature slowly climbs to ~100 degrees.

This 2 step, 10-second cycle repeats itself no matter how long I run the test. When I idle, the computer CPU slowly cools down to ~45 degrees. I am experienced in installing CPUs and whatnot so I am sure there is nothing wrong with the cooling system. Below is a temperature vs time graph of the stress test but because it collects data at 1s intervals, the temperature drop is not represented accurately. The temperature literally 'jumps' up and down whenever the clock changes.

I am suspecting a defective temperature sensor. Could it be that the power fluctuation during the clock alternation is affecting the sensor's accuracy? Any ideas as to what might be giving me the headache? A solution, hopefully?

T vs. time

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Your cooling system is unable to adequately cool your CPU

  • The processor should not reach 95 °C even under load and ideally should be operating below 90 °C. The processor will throttle automatically when its operating temperature is too high, then return to normal when temperatures drop to safe levels. This behavior explains the fluctuations in clock multiplier and temperature.

  • Make sure you've properly installed the heatsink and fan and that you've properly applied the thermal paste. If this does not help, it is likely that your laptop's cooling system just doesn't have enough capacity to keep up with your upgraded processor's heat output.

  • actually, the new CPU has the same TDP with the manufacturer-installed CPU but has a higher Tj at 105 degrees. The thermal paste should start working better in a few days, I have just applied it. Anyway, that does not explain the wild fluctuations in temperature readings. It is not physically possible that the CPU cools down 35 degrees in under a second so obviously there is something wrong with the sensor or who knows what. I have checked with several software, all read the same. – uygar Dec 20 '14 at 20:33
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    My first suggestion would have been that the paste is old or cracked - if it's new, then it's not working as it should, or was mis-applied etc etc. I agree with DragonLord that the CPU is cycling to try keep cool – Tetsujin Dec 20 '14 at 20:39
  • Whu not. If multiplier throttled from 22x to 9x temp go down asymptotically tends to ~ 64. When throttle switched off it asymptotically tends to ~ 105 – Mikhail Moskalev Dec 20 '14 at 21:18
  • @uygar It's a bit of a myth that thermal paste performance improves appreciably after some kind of break-in period. At best you might get 1~5 degrees improvement which won't really help at all in your case. – misha256 Dec 23 '14 at 3:16
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It's clear to me that the temperature sensor is fine. Look again at how the temperature increases smoothly over time (quickly at first as it should) until it hits a critical threshold. That's a temperature sensor in good working order.

What happens after critical temperature is reached is interesting but has nothing to do with the temperature sensor. That's a hard-coded BIOS reaction to a critical temperature situation. The BIOS clocks the CPU right down to achieve maximum possible cooling. Once the critical condition is resolved, the BIOS allows the CPU to clock back up.

It is not physically possible that the CPU cools down 35 degrees in under a second

Actually it is. Massive temperature fluctuations, over time-frames of fractions of a second, are exactly what you can expect when you pack billions of transistors into such a tiny space.

I'm afraid there are two likely explanations for your problem. Either:

  • DragonLord is right and your laptop's cooling cannot cope with the greater thermal output of your new CPU, or

  • Your laptop is not compatible with the CPU you installed. The cooling system might well be capable of cooling your CPU, but the BIOS does not have a suitable cooling profile for the CPU.

EDIT: BIOS updates can resolve cooling issues in laptops. Wouldn't be the first time. Definitely worth trying.

  • I am aware that clocking down is as a normal response to high temperature but I am surprised to hear that such drastic cooling is possible in a tiny time frame -never seen that before. I know for a fact that the CPU is compatible because it is listed in manufacturer's list of compatible hardware so I will assume I have missed something while installing the CPU -possibly applied too much paste. Too bad, accessing the CPU of a Lenovo B560 is a real PITA. Thanks everyone for your input. – uygar Dec 26 '14 at 5:18
  • @uygar Yeah, it really is true, within the core of a CPU you can get significant temperature swings over really short time frames. Not sure if you remember AMD Athlon CPUs, but if you pulled the heatsink off those while they were under load, they'd totally fry (smoke and all) within a couple of seconds. In the same vain, if you massively reduce CPU load (clock down, shut down most cores, etc.) while the heatsink fan is going full blast, the internal die temperature will drop dramatically even in just a second. – misha256 Dec 26 '14 at 7:02
  • @uygar Incidentally, if there is a newer BIOS available for your laptop you should absolutely try updating that before re-seating the heatsink/cooler. – misha256 Dec 26 '14 at 7:04

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