I run Docker on my Mac through Colima, which sits on top of Lima and boots a small Linux VM using Apple’s Virtualization.framework (the vz backend). For a long time it just worked. Then I started noticing my Mac getting slower the longer my workday went on — and the cause turned out to be a bug worth writing down.

The symptom

Build a few images, run a memory-hungry container, do some real work. At some point the VM touches the memory ceiling I configured for it. Fine — that’s what the ceiling is for.

The problem is what happens after. The containers exit, the build finishes, the guest goes quiet — and the host process backing the VM stays pinned at that peak. In Activity Monitor it shows up as a process holding several gigabytes of RAM with nothing running inside it. It never comes back down. The only thing that frees the memory is restarting the VM.

So my “8 GB Docker VM” wasn’t an 8 GB budget. It was an 8 GB high-water mark, and once I hit it, those 8 GB were gone from the rest of the system until I remembered to bounce Colima.

What I expected

A fresh VM idles at maybe 800 MB–1 GB. My mental model was simple: memory usage should track what the guest is actually doing. Spike under load, settle back down when idle. That’s how a process is supposed to behave, and it’s how the VM’s guest behaves — Linux inside the VM frees the pages just fine.

The host side is what doesn’t let go.

Opening the issue

I went looking and found I wasn’t the first to hit this — there was already a discussion describing the exact behavior I was seeing. It matched closely enough that I opened a tracking issue on Lima to get it properly on the radar:

lima-vm/lima#2789 — Memory is not being freed on VZ

What I learned from the thread is the interesting part.

Why it happens

The first answer from a maintainer reframed it for me: memory you dedicate to a virtual machine being locked is, in a sense, normal. A VM isn’t an ordinary process; the host commits that memory to it. The mechanism that would let it shrink back has a name — memory ballooning — and it isn’t on by default.

The next answer was blunter, and the one that actually explained my Mac: this is a macOS bug. It’s not specific to Lima or Colima. Docker Desktop exhibits the same thing, because they all sit on the same Apple framework. The one tool that doesn’t — OrbStack — got there by building its own dynamic memory management.

OrbStack did write a blog post about it, and it’s worth reading — but notice what it doesn’t say. It explains that the VM’s footprint grows and shrinks on demand, and why that matters; it doesn’t really explain how they pulled it off on the very same Apple framework that leaves everyone else stuck. The post is about the result, not the recipe. Whatever they’re doing, they’re keeping it to themselves — which is fair, but it means the rest of the ecosystem can’t just copy it.

The balloon that won’t deflate

Apple’s framework does expose a balloon device (VZVirtioTraditionalMemoryBalloonDevice), and Lima even creates one. In theory you shrink the VM’s footprint by lowering its targetVirtualMachineMemorySize while it runs: the guest hands unused pages back, the host reclaims them.

In practice, on the vz backend, it doesn’t reclaim anything. My short contribution to the thread was exactly that observation — the device is available, it just doesn’t work.

That’s not me guessing anymore. Over a year later, someone posted an instrumented test on macOS 26: a tiny VM, a controlled allocation, host RSS sampled once a second. The guest cooperates fully and frees its memory; the host process’s memory footprint is monotonic across the whole run — it never decreases. They’ve since filed an Apple Feedback report. The issue is still open.

Where I landed

Knowing the root cause didn’t fix my Mac. The realistic options were:

  • Restart the VM periodically to reclaim memory — a chore, and easy to forget until the machine is already crawling.
  • Cap the VM’s memory low enough that the locked amount doesn’t hurt — which just trades one problem for another, since now heavy builds are starved.
  • Use the tool that solved it.

I switched to OrbStack. Its dynamic memory management is the entire feature I was missing: the footprint expands under load and actually contracts when the work is done. For the way I use containers on a laptop, that’s not a nice-to-have — it’s the whole point.

Takeaways

  1. “Locked” and “leaked” look identical in Activity Monitor. A VM holding its peak forever isn’t leaking; it’s a platform that never reclaims. Same symptom, different cause — and the fix is different too.
  2. A feature existing in the SDK doesn’t mean it works. Lima creates the balloon device; the balloon still doesn’t deflate. “Available” is not “functional.”
  3. It’s worth filing the issue even when it’s not the project’s fault. #2789 isn’t a Lima bug, strictly — but having a public, linkable thread is how the next person finds the explanation in twenty minutes instead of a week.

The RAM still doesn’t come back on vz. But at least now I know why — and I’m running something that doesn’t need it to.