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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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[–] EarMaster@lemmy.world 6 points 5 months ago (1 children)

If that's the case you should look into your swappiness settings. You can set this to zero meaning the swap will only be used if you're actually out of memory, but as others have noted that is maybe not a healthy decision…

[–] a_fancy_kiwi@lemmy.world 1 points 5 months ago

I’m currently not in a situation where swap is being used so I think my system is doing fine right now. I’m not against swap, I get it’s better to have it than not but my intention was to figure out how close is my system getting to using swap. If it went from not using swap at all to using it constantly, I’d probably want to upgrade my ram, right? If nothing else just to avoid system slow downs and unneeded wear on my SSD