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I was looking at the task manager in Windows 10 and for the first time noticed a secondary dashed line below the solid? What does this line indicate?

Windows 10 Task Manager CPU

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That looks like the kernel time graph. Right-click the graph and see if this option is ticked.

Kernel time has been available for a while but it was more hidden.

Seems it was available from at least Windows XP : https://blog.codinghorror.com/everything-you-always-wanted-to-know-about-task-manager-but-were-afraid-to-ask/

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    Ahh, it seems it might have been a bug. It wasn't check, but when I checked it, nothing happened, and then when I unchecked it again (back to its original state) they went away. Basically, it was showing them when it wasn't enabled. – Brian Leishman Aug 25 '16 at 14:58
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The light blue line represents the total amount of time your CPU is spending on tasks. The dark blue line represents what percentage of that time involves the kernel.

What's the difference?

If an application maxes out the %CPU load but the kernel time is still low, then the system still feels responsive and snappy. This is because the kernel's primary job is to schedule CPU time for processes and it can preempt one process to run another when necessary.

Only when the kernel pegs the CPU at 100% does your computer feel slow and sluggish. That's why the option is there to select because it gives a more realistic view from a performance perspective than the regular graph does.

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    @anatolyg; Wow. I can’t believe this answer has been up for over 4 years and nobody ever caught that I had them backwards this whole time. Fixed. – Wes Sayeed Nov 17 '20 at 16:34

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