What is kernel_task, and why is it using so much CPU?

What kernel_task is

kernel_task is how Activity Monitor shows the macOS kernel: the part of the system that schedules every process, manages memory, and talks to the hardware and drivers. It's always running, it belongs to the system, and it has process ID 0 (you'll see it as PID 0 in Activity Monitor and in Terminal's top).

Because the kernel does work on behalf of everything else, kernel_task always uses some CPU. On the M2 Max MacBook Pro this guide was written on, it used between about 13% and 60% of one core at different moments while the Mac was cool and busy with ordinary work (macOS 27.0.1, sampled with top). Remember that % CPU is per core, so that's a small slice of a twelve-core Mac. Figures like that aren't a problem.

Why it shows high CPU: managing heat

The alarming case is kernel_task suddenly near the top of the list with a very large figure, the Mac warm, the fans loud and everything sluggish. Apple explains what's going on (Apple, If kernel_task is using a large percentage of your Mac CPU):

• One of kernel_task's jobs is to manage CPU temperature. It does this by making the CPU less available to the processes that are using it intensely.
• It reacts to conditions that make the CPU too hot, "even if your Mac doesn't feel hot to you".
• It doesn't cause those conditions itself.
• When the temperature comes down, it automatically reduces its activity.

In other words, the high figure is the brake, not the engine. The processor time it "uses" is time it's keeping away from the apps that were heating things up. That's also why the Mac feels slow: the busy apps are getting less of the processor on purpose.

Apple's fix is to find the processes using the CPU intensely in Activity Monitor and quit them. Sort the CPU tab by % CPU and look just below kernel_task: the app that was busy before the slowdown is your culprit. Our Activity Monitor guide explains the columns.

What's making the Mac hot

The usual causes are sustained heavy work and poor cooling:

• A heavy job: video export, a game, a large build, a virtual machine, or a stuck app at full CPU.
• Blocked airflow: a MacBook on a bed, cushion or lap, or a desktop Mac in a tight cabinet.
• A hot room or direct sun. Apple says MacBooks work best at ambient temperatures of 10 °C to 35 °C (50 °F to 95 °F) (Apple, Batteries – Maximizing Performance).
• Charging while working hard, which adds heat of its own.

Quit or pause the heavy job, move the Mac to a hard surface somewhere cooler, and kernel_task backs off as it cools. Why is my MacBook so hot? covers the causes and fixes in more depth.

Checking whether heat is the reason

Activity Monitor doesn't show temperatures. macOS tracks a thermal state that Apple's developer documentation describes in four levels: nominal ("within normal limits"), fair ("slightly elevated"), serious ("high") and critical, where it is "significantly impacting the performance of the system" (Apple Developer, ProcessInfo.ThermalState). Terminal's powermetrics tool has a thermal sampler that prints the current level; it needs an administrator password and only reads:

sudo powermetrics --samplers thermal -i 1000 -n 1

Its manual page (man powermetrics) says the value is taken at the end of the sample, so run it while kernel_task is high. For temperatures in degrees, see how to check your Mac's temperature.

When the Mac isn't hot

Apple's article only covers heat. If kernel_task stays very high while the Mac is cool and nothing heavy is running, you're outside what Apple documents, and the job is to narrow it down:

1. Restart. It's the simplest test, and on Apple silicon it's also what Apple means by resetting the SMC (Apple).
2. Disconnect accessories one at a time: docks, hubs, external drives and displays, audio interfaces. Work the kernel does for hardware and its drivers is counted under kernel_task, so a misbehaving device can show up here.
3. Try safe mode. Apple says it helps identify whether software that loads as your Mac starts up is causing a problem (Apple, Start up your Mac in safe mode). If kernel_task is calm there, look at drivers and system extensions you've installed, and update or remove the software they came with.
4. Update macOS in System Settings › General › Software Update.

What not to do

• Don't try to quit it. kernel_task is the kernel itself, not an app you can stop.
• Don't follow old advice to delete or edit system files to stop kernel_task throttling. On current macOS that means turning off System Integrity Protection, and the throttling is there to protect the hardware.
• Don't install "CPU optimizer" apps. kernel_task's behavior is macOS working as designed; a third-party app can't make the heat go away.

When to contact Apple

Run Apple Diagnostics if kernel_task stays high in safe mode with nothing connected, or if the fans run hard while the Mac is idle and cool. On a Mac with Apple silicon, press and hold the power button until "Options" appears, release it, then press and hold Command-D (Apple, Use Apple Diagnostics). If it reports a problem, or the behavior continues, contact Apple Support.

How Tend helps

kernel_task tells you there's heat, not where it comes from. Tend's free Energy & Apps page lists every app by CPU and estimated watts, so the app behind the heat is at the top, with 24 hours of history. With Pro, the Thermals page shows every temperature sensor, fan speeds and the thermal pressure that explains kernel_task's behavior, and Tend can alert you when the CPU has been hot for a minute, naming the busiest app at the time. A separate Pro warning covers a battery that stays warm while charging (Tend's help).

Questions

Related guides

Sources

• Apple, If kernel_task is using a large percentage of your Mac CPU
• Apple Developer, ProcessInfo.ThermalState
• Apple, Batteries – Maximizing Performance
• Apple, Reset the SMC of your Mac
• Apple, Start up your Mac in safe mode
• Apple, Use Apple Diagnostics to test your Mac
• macOS 27.0.1, man powermetrics

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