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Help with understanding memory usage discrepancy (lemmy.world)

submitted 5 months ago* (last edited 5 months ago) by a_fancy_kiwi@lemmy.world to c/selfhosted@lemmy.world

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I recently noticed that htop displays a much lower 'memory in use' number than free -h, top, or fastfetch on my Ubuntu 25.04 server.

I am using ZFS on this server and I've read that ZFS will use a lot of RAM. I also read a forum where someone commented that htop doesn't show caching used by the kernel but I'm not sure how to confirm ZFS is what's causing the discrepancy.

I'm also running a bunch of docker containers and am concerned about stability since I don't know what number I should be looking at. I either have a usable ~22GB of available memory left, ~4GB, or ~1GB depending on what tool I'm using. Is htop the better metric to use when my concern is available memory for new docker containers or are the other tools better?

Server Memory Usage:

htop = 8.35G / 30.6G
free -h =

               total        used        free      shared  buff/cache   available
Mem:            30Gi        26Gi       1.3Gi       730Mi       4.2Gi       4.0Gi

top = MiB Mem : 31317.8 total, 1241.8 free, 27297.2 used, 4355.9 buff/cache
fastfetch = 26.54GiB / 30.6GiB

EDIT:

Answer

My Results

tldr: all the tools are showing correct numbers. Htop seems to be ignoring ZFS cache. For the purposes of ensuring there is enough RAM for more docker containers in the future, htop seems to be the tool that shows the most useful number with my setup.

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[–] vk6flab@lemmy.radio 2 points 5 months ago

This is the job for the OS.

You can run most Linux systems with stupid amounts of swap and the only thing you'll notice is that stuff starts slowing down.

In my experience, only in extremely rare cases are you smarter than the OS, and in 25+ years of using Linux daily I've seen it exactly once, where oomkiller killed running mysqld processes, which would have been fine if the developer had used transactions. Suffice to say, they did not.

I used a 1 minute cron job to reprioritize the process, problem "solved" .. for a system that hadn't been updated for 12 years but was still live while we documented what it was doing and what was required to upgrade it.