What is kernel_task on a Mac, and why does it use so much CPU?

kernel_task is the core of macOS, the part of the system that manages the processor, memory and every piece of hardware. When it shows a high CPU figure, it usually does not mean kernel_task is the problem. It often means the Mac has got hot and macOS is holding the processor back to cool it, and kernel_task is the process that takes up that time. The way to bring the number down is to find and remove whatever is making the Mac hot. You cannot end kernel_task, and you should not try.

What kernel_task is

Every program on a Mac is a process, and kernel_task is the one that stands for the kernel, the lowest level of macOS. The kernel decides which program gets the processor and for how long, hands out memory, and talks to the disk, the network and the display. Without it nothing else can run, which is why it is always there and always has the process ID of 0.

In Foreman it shows as “macOS kernel”, with the real name kernel_task beside it in grey. Foreman’s one-line description is: the core of macOS, and high CPU here often means the Mac is slowing itself down to stay cool.

Why its CPU figure can be so high

Processors produce heat when they work, and every Mac has a limit above which it is not safe to keep going at full speed. When the Mac approaches that limit, macOS has to cut back how much work the processor does. One way it does this is by giving a share of the processor to kernel_task, which does nothing useful with it. That share is time taken from the apps, so they run slower, and the processor makes less heat.

So a high kernel_task figure is usually a symptom of a Mac that is too warm, not a cause of slowness in itself. Think of it as the Mac putting its own foot on the brake. The more you see, the harder it is braking. macOS also cools a hot Mac by lowering the processor’s speed, which does not show under kernel_task at all, so a quiet kernel_task does not prove the Mac is cool. You may also see thermalmonitord in the list, which Foreman calls “Temperature monitor”: it watches temperatures and slows the Mac down when it gets hot.

A few notes on what you may see.

• Brief spikes are normal, especially at start up, when waking from sleep or when plugging in a display. The kernel does real work for the hardware too, so heavy disk, network or display activity adds some.
• Long periods of high kernel_task, with the Mac hot to the touch (and the fans running hard, on a Mac that has fans), are the sign of a thermal problem.
• It can happen on any Mac, Intel or Apple silicon, laptop or desktop. A hot room, blocked vents or a long heavy job is enough.

Why you cannot end kernel_task

You may be tempted to force it to quit. There is no point, and you cannot. Ending the kernel would stop the Mac, so macOS does not allow it, and Foreman never offers to end kernel_task or launchd.

That is a good thing. Even if you could stop the process, you would be removing the safety brake, and the heat would stay. The right approach is to deal with the heat.

Find what is making the Mac hot

Work through these in order, starting with the two most common.

1. Find the busy app. Something is using the processor hard. Open Foreman with Control-Shift-Escape, go to the Processes page and click the CPU column to sort the heaviest to the top. A browser tab, a video editor, a game, a runaway helper or a backup can all do it. How to see which app is using the most CPU on a Mac walks through it in detail. If an app is using far more than you expect, use End Task, or Efficiency Mode if you want it open but quiet.
2. Check the airflow. Vents blocked by a bed, a sofa, a lap or a thick case make a Mac hot quickly. Put it on a hard flat surface with space around it. A hot room or direct sun matters too.
3. Look at what is plugged in. A hub, a dock or an external display adds heat and load. Unplug accessories one at a time and watch whether the figure falls.
4. Charging. A Mac that is charging at full rate while working hard is warmer. Try the Mac on battery or plugged in at a cooler time to see if it matters.
5. Dust and age. An older Mac with dust in its fans and vents cannot shed heat as well. Cleaning inside it is a job for a repair shop, not a setting.

The related post why your MacBook gets hot and how to find what’s heating it goes further on this. If the fans are loud as well, read what to do when your Mac fans get loud.

Foreman does not show temperatures or fan speeds, so it cannot tell you how hot the Mac is. What it can show is which apps are keeping the processor busy, which is nearly always the answer to why the Mac is hot in the first place. Watching the CPU total above the CPU column fall after you end or slow an app tells you that you found the cause.

A simple test

To check whether the heat really is the cause, close the heavy app or two, leave the Mac on a hard surface for five minutes with nothing running, and watch kernel_task. If its CPU figure drops back to a few percent and stays there, the Mac was too hot, and you know where to look next time. If it stays high even with nothing running and the Mac cool to the touch, restart the Mac, because a restart clears a lot of odd states. Unplug accessories too, since a misbehaving device or driver can keep the kernel busy. If it still happens after that, it is worth a visit to an Apple Store or authorised repairer, because there may be a fault with a sensor or a fan that macOS cannot work around.

Is it a virus?

No. kernel_task is part of macOS and is not malware. If you see it using a lot of CPU, that is the system doing its job, not something that has crept in. The one thing to be wary of is a different process that has been given a name that looks similar. The real kernel_task always has process ID 0, which Foreman shows if you add the PID column from the header’s menu, so a lookalike with another ID is not the real thing.

Does it use memory?

You may notice kernel_task holding a good deal of memory as well. That is also expected: the kernel keeps its own data in memory, along with memory set aside for drivers and hardware. It is not something to chase unless the Mac is also short of memory. Memory pressure on a Mac explains how to tell whether it is.

Summary

kernel_task is the core of macOS. High CPU there often means the Mac is slowing itself to cool down. Find the app doing the heavy work, give the Mac air, and unplug accessories to see if one is adding heat. You cannot and should not end it. Foreman is $12 once, is free for 24 hours and makes the first step, finding the busy app, a matter of one sort.