Last Week on My Mac: Solo or CoreDuet?

When I was first starting to visit pubs, an older and wiser friend advised me never to discuss politics or religion there, as they would only lead to standup rows and punch-ups. It seems that we’ve now entered similar ground with our computers. Instead of programming languages or architectures being taboo, it’s topics like battery management and internal temperature that can’t be discussed in the open.

They have features in common with politics and religion, in that they’re often founded on systems of belief. When you start digging deeper, what’s often claimed as rule and fact proves more intangible. For example, the oft-quoted 20-80 rule for battery operation isn’t universal by any means. This is usually quoted as stating that you’ll get best lifespan from a lithium-based battery if it’s only operated within a range of 20% to 80% of its maximum charge capacity.

Yet look closer at different ‘expert’ sources and you’ll see different figures. Some prefer a bottom end of 25%, and the top end could be anywhere between 80-95%. One thing I do know, though, is that the curves used to justify 20-80 or 25-95 rules aren’t those for the batteries fitted in current Mac models. They’re generalised, and lack both data points and error bars. There’s also a puzzling similarity with the Pareto 80/20 rule, which has a completely different mathematical basis.

These beliefs come even closer to those of politics or religion when you suggest following Apple’s recommendations. Some rely instead on almost anyone else who appears to share a similar belief, picking and choosing carefully to confirm their own opinions.

If that doesn’t work, they dismiss anything provided by Apple as part of a conspiracy to increase its income by replacing batteries, often under either warranty or AppleCare. I haven’t yet had the heart to challenge the arithmetic of making a lower-quality premium product, or deliberately providing misleading advice, so you have to perform more free repairs. But there are some corporations that have fired engineers who have drawn attention to defects, so maybe in the world of conspiracy theories, arithmetic is also part of the grand conspiracy.

Where so much of this, be it battery management or internal temperature, comes unstuck is in what I have alluded to but not spelled out: CoreDuet.

Every Mac has its own System Management Controller, the SMC that you occasionally reset when you don’t know of anything more specific to fix an odd problem. In Apple silicon Macs there’s even more, at least one whole Arm core, one that you can’t reset so easily. I’m sure you’re also aware that your Mac contains multiple temperature sensors embedded in different components, most also have cooling fans, and every Mac put under thermal stress will hand over a lot of its CPU to running kernel_task.

Those are the more visible parts of the systems monitored and managed by CoreDuet. Every second that your Mac is running, CoreDuet is keeping a watch on its internal environment. It then manages internal systems to ensure that components remain within their design temperature envelopes, that any battery gets charged and discharged appropriately, and that background tasks are run when the resources they use are available. The latter are despatched by DAS, Duet Activity Scheduler, note its name. From Spotlight indexing to Time Machine backups and iCloud syncs, CoreDuet and your Mac’s internal environment are constant influences.

Outside Apple, none of us knows much about CoreDuet, nor how it works. Look for a control system diagram and you’ll find none.

Yet I often see reports of users who have disabled their Mac’s cooling fans, because of the noise that they make, or overridden parts of CoreDuet without knowing how that might affect the rest of this control system. ‘Look – in Activity Monitor kernel_task is slowing my Mac down, let’s try killing it’, they say. Not content with gambling with their own Mac, they encourage others to do the same.

In my apparently so controversial advice about battery management, I mentioned the management of another complex system, our blood sugar level, and asked whether you’d want to take that on and try managing that using your own blood sugar levels. Little did I realise that too has become a fad, and there’s now at least one company who will charge you around £300 a year to do just that. I wonder whether they inform subscribers that blood sugar levels are critical to certain body systems, particularly the brain and nervous system. Get your blood sugar wrong and you can suffer permanent brain damage: ask any diabetic about that risk.

At least blundering around with CoreDuet could only cost you your Mac, so I suppose it’s a safer gamble to take.