Kernel_task and Summer Heat

Macs don’t sweat in the heat like we do. Now those of us in the northern hemisphere are well into summer, we need to be more aware of the warning signs that we or our Macs are getting too hot.

One of the most serious warning signs of imminent heat stroke in humans may seem paradoxical, as they stop sweating, and their skin may become pale rather than red. This is because their circulation has finally collapsed, and if they aren’t cooled very quickly, they’re at real risk of serious injury or death.

Hot Macs have their own paradox: open Activity Monitor and top of the CPU % list will be kernel_task hogging the CPU cores. Kerb any temptation to kill it and hope it goes away, as the reason it has taken over your Mac is to let it cool.

Causes

Every Mac contain multiple temperature sensors that are monitored by its Core Duet sub-system, which together with the System Management Controller (SMC) manages its internal environment and services. When a process runs away and takes over the cores, or any of the internal thermal sensors registers an abnormally high temperature, Core Duet responds with a set of actions to make it cool. These include its active cooling system of fans, which run up to speed to blow cooling air over sensitive components, and reducing CPU load to lower the production of heat. Where possible that will slow processor speed, and block any runaway processes by occupying the CPU with kernel_task.

Some users have taken to meddling with this system by trying to stop kernel_task from doing its job. That’s a bit like stopping someone from drinking to replace their fluid loss in sweat, and only makes matters worse. When the fans get loud and kernel_task hogs your CPU, it’s time to do all you can to let your Mac cool down.

There are rare occasions when fans blow full on and kernel_task goes wild without any thermal problem. In Intel Macs, resetting the SMC is usually curative, but this could instead be the result of a fault in a thermal sensor, or in the SMC. Hardware diagnostics should be informative. By far the most common cause of persistent problems is dust and debris in the air ducts, and in the case of some Intel Mac notebooks overheating of the left USB-C ports. For more details, see Apple’s note about kernel_task, which will also confirm what I have written here.

Actions

If your Mac is showing early signs of thermal strain and still running, encourage heat loss by immediately:

• opening its lid wide if it’s in clamshell mode,
• ensuring all ventilation openings in its case are free of obstruction,
• clearing space around its entire case, including the underside, to aid convective cooling,
• if the Mac is in sunlight, moving it into the shade,
• if you have an external fan, turn that on to cool its case by forced convection,
• if air conditioning is available, move the Mac to a cooler area.

Reduce its heat production by immediately:

• quitting all open apps, if necessary force-quitting any that won’t cooperate,
• killing all non-essential processes that are taking more than 5-10% CPU in Activity Monitor except kernel_task,
• disconnecting any mains charging cable, provided there is sufficient charge in its battery.

If a Mac has cooling fans, it’s better to leave the Mac running, as shutting it down will also stop those fans from blowing cooling air inside it. Some like to put cold packs against the case, which can cool the outside down more quickly than an external cooling fan. If you wish to try that, ensure that condensation doesn’t enter the Mac, particularly through its keyboard or ventilation openings, and don’t obstruct either the keyboard or those cooling ducts.

High and Low Power Modes

Some Macs offer one or two energy modes in their Battery or Energy settings. Where available, these can be useful for controlling a Mac’s heat production and loss, but they work quite differently. Their labelling may also appear confusing: these are offered as the Energy Mode, but its menu choices should refer to High Power Mode and Low Power Mode.

Low Power Mode reduces heat production by limiting the frequencies of the CPU and GPU cores, so maximum heat production is lower, and the battery will run longer on a charge. This also allows any cooling fans to run quieter.

High Power Mode doesn’t do the opposite and increase heat production from normal. Instead, the Mac uses its fans more aggressively to keep it cooler for longer when undertaking long and demanding work.

Apple provides detailed information on these, and which models support each mode.

May you stay cool this summer!