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I recently built a new PC and after some cursory temperature monitoring in the BIOS I installed Windows 10 and everything looked fine.

I installed HWiNFO to get a better look at my CPU temperatures and noticed that if I'm performing low load tasks like downloading drivers or opening an application such as Steam or Chrome the CPU appears to rapidly fluctuate between 0.8Ghz and 4.7Ghz (also causing the CPU temperatures to rapidly jump between 30c and 45-50c). It stays like this until whatever low load work is being done stops.

Is this normal behavior for the 8700k?

If the system is completely idle this doesn't happen and after running Prime95 for 2 hours it sits at 4.7Ghz for the entire duration as you'd expect and the temperature remains stable, maxing out at 71c.

I read these spikes can be caused by an improperly mounted/over-tightened CPU heatsink/incorrectly applied thermal paste, but the stable temperatures under load make me think this isn't the case.

The CPU isn't overclocked and my cooler is the Corsair H100i V2. My motherboard is a Z370 AORUS Ultra Gaming.

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  • Don't quote me on that, but it sounds normal to me. You're ordering the CPU to do something, it starts doing the thing as efficiently as possible, then when it's done it goes idle again. Neither launching Chrome nor Steam are low load tasks, both of these programs are pretty CPU-demanding when starting. 8700k isn't a laptop CPU, it's built for performance, not power saving and low heat emission.
    – gronostaj
    Commented Jun 13, 2018 at 8:01
  • True - I suppose I expected to see the CPU load jump to 4.7Ghz and remain there until the task is complete. During a 5min driver download through GeForce Experience all 6 cores would alternate between 0.8 and 4.7 very rapidly until the download completed. I never noticed this behavior on my old CPU (4770k) and I tested it similarly at the time.
    – ResoundingBoom
    Commented Jun 13, 2018 at 8:11

1 Answer 1

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When you're running low-load tasks (basically every day tasks), the CPU will increase its clock speed to complete the task as soon as possible, and then go back to idle again. i5s and i7s go a step further, boosting the clock speed over the base clock when doing single-threaded tasks (see Intel Turbo Boost for more information). That's what CPU does, it's normal.

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  • Thanks for the quick answer. I'm probably paranoid but the CPU base clock speed of 3.7Ghz is never "settled" on, the 0.8Ghz or 4.7Ghz figures are the only clock speeds I see. I know there's Intel SpeedStep which is why (I assume) it idles at 0.8Ghz. I swear my 4770k idled at ~3.5Ghz.
    – ResoundingBoom
    Commented Jun 13, 2018 at 8:19
  • You're right, all CPU reduce their clock speed on idle (unless you disable that, or something is taking up CPU cycles in the background). And about the 3.7GHz base clock, it's the all core clock, which means that if you run the CPU at full load on all cores (or disable Turbo Boost), it'll run at 3.7GHz.
    – Kurumi Gaming
    Commented Jun 13, 2018 at 9:11
  • That makes sense. Apparently if the SpeedStep is enabled in BIOS, on Windows power management you can toggle SpeedStep on/off by changing between the min and max power settings. I'm beginning to think on my old system, the power management settings were set at max performance. So my CPU's clock speed "jump" was smaller during "lower load" tasks (3.5 - 3.9 compared with 0.8 - 4.7). I will check this when I get home. Link to the SpeedStep page I read link
    – ResoundingBoom
    Commented Jun 13, 2018 at 9:47
  • Yeah, when you set your power plan to maximum performance, Windows will not let the CPU reduce the clock speed, which is bad.
    – Kurumi Gaming
    Commented Jun 13, 2018 at 15:20

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