Memory pressure and swap on a Mac, explained properly

Almost everyone reads Mac memory wrong, because the number macOS shows most prominently is the one that matters least. Here is what to look at instead.

The short answer: memory used tells you almost nothing. Memory pressure tells you whether your Mac is struggling, and swap used tells you how long it has been struggling. Those are the two numbers worth watching.

Why "15 GB of 16 GB used" is not a problem

macOS treats unused memory as wasted memory. If you have spare RAM it will fill it with file caches, so that the next time you open something it is already there. That is why a healthy Mac sits near its memory limit almost all the time.

The moment an app genuinely needs memory, macOS drops those caches instantly. Nothing is lost and nothing slows down. So a nearly full memory bar on its own is not evidence of anything at all.

Memory pressure, the number that counts

Open Activity Monitor, go to the Memory tab, and look at the graph in the bottom left.

• Green means macOS is comfortable. Whatever the used figure says, you are fine.
• Amber means it has started compressing memory and moving things to disk to cope.
• Red means it is genuinely short and everything is suffering.

Green with a full memory bar is normal. Amber with a half empty bar is a real problem. The colour beats the number every time.

What swap actually is

Swap is disk space used as pretend memory. When real memory runs short, macOS writes the least recently used pages out to the drive and reads them back when needed.

Disk is enormously slower than memory, even a fast SSD. So when your Mac is swapping heavily, everything develops a lag: switching apps, scrolling, typing. This is the specific sensation people mean by "it just got slow".

Some swap is completely normal. macOS will use a few hundred megabytes without any consequence you would notice. Several gigabytes, with amber or red pressure, means the machine is thrashing.

The part that catches people out

Swap does not clear itself while the apps holding that memory are still running. It builds up over days of closing the lid rather than shutting down. This is why a Mac that has been awake for three weeks feels worse than the same Mac after a restart, and why the old advice to reboot genuinely does work for this one case.

Finding the app responsible

Sort the Memory tab by the Memory column and look at the top of the list. The question to ask is not "what is using the most", because a browser or a video editor legitimately uses a lot. The question is "what is using far more than it should be".

Background helpers are the usual suspects, because nobody thinks to check them. Something with "helper" or "agent" in the name sitting at several gigabytes is almost certainly leaking. Quitting and reopening its parent application gives the memory straight back.

When more RAM is the answer, and when it is not

If pressure is amber during ordinary work, with nothing obviously misbehaving, you have genuinely outgrown the machine and more memory would help.

If pressure is green, or if one specific process is holding an absurd amount, more RAM just delays the same problem. A leak will happily fill 32 GB as readily as 16 GB. Fix the leak instead.

The short version

Ignore memory used. Watch the pressure graph, and watch swap. Green pressure with full memory is a healthy Mac doing exactly what it was designed to do. Amber pressure with gigabytes of swap is a Mac asking for help, and usually one specific app is the reason.

If yours is slow but memory looks fine, the other three usual causes are worth ruling out next.

Keep reading

• Why is my Mac slow all of a sudden?: the four usual causes, and how to tell them apart.
• What is safe to delete on a Mac, and what absolutely is not: what regenerates, and what never comes back.