As of #12449, the all extras package group does not automatically install all of the optional dependencies. Specifically, the cache_memory extras group was no longer installed, which meant that without explicitly installing it, it was missing in both the Debian and Docker images we distribute.

The cache_memory extras group installs the pympler package, which is needed to test the caches.track_memory_usage experimental config option. While this is indeed experimental, deployments using the docker images or debian packages can't test it if this dependency is missing.

systemd is another optional dependency which is not installed via all. It enables the systemd.journal.JournalHandler handler as configured in the log config. In the past we have explicitly chosen to exclude this, as it is only needed on systems which use systemd.

However, despite systemd-less versions of Debian in existence, it is needed to configure the above handler on Debian-based Linux distributions with systemd. I'm unsure whether it should be installed on the Docker images (would it be useful to log to systemd on the docker images? I assume they are meant to simply log to stdout?).

(This issue originated from discussion in #12640 (comment)).