Higher CPU load when running containerd with root on tmpfs

[agners]

We've noticed higher CPU load of containerd (3-4% on idle) after moving to a new Docker/containerd version. It turns out that the version wasn't really relevant, but due to configuration changes the root = was located on a tmpfs (instead of a ext4 file system as it was before). It seems that this causes order of magnitude higher amount of futex, nanosleep and epoll_pwait calls (e.g. 79 vs 28779 futex calls).

We've reverted back to use a location on an ext4 file system for root = to resolve the problem.

But out of curiosity: What causes containerd behaving so much different on these two file system types?

On my Arch system, this is reproducible by launching containerd with the following config:

version = 2

root = "/tmp/containerd"

This issue has noticed on Home Assistant OS, see the issue and discussions around it home-assistant/operating-system#2476, and home-assistant/operating-system#2476 (comment) has some details what exactly changed.