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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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[–] eager_eagle@lemmy.world 10 points 5 months ago (1 children)

if my system touches SWAP at all, it's run out of memory

That's a swap myth. Swap is not an emergency memory, it's about creating a memory reclamation space on disk for anonymous pages (pages that are not file-backed) so that the OS can more efficiently use the main memory.

The swapping algorithm does take into account the higher cost of putting pages in swap. Touching swap may just mean that a lot of system files are being cached, but that's reclaimable space and it doesn't mean the system is running out of memory.

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

From what I can tell, my system isn’t currently using swap at all but it does have 8GB of available swap if needed.

To make sure I’m following what you are saying, if I upgraded my system to 64GB and changed nothing else, and let’s assume ZFS didn’t trying caching more stuff, would there still be a potential for my system to use swap just because the system wanted to even if it wasn’t memory constrained?

[–] eager_eagle@lemmy.world 2 points 5 months ago (1 children)

yes, the system will likely use some swap if available even when there's plenty of free RAM left:

The casual reader1 may think that with a sufficient amount of memory, swap is unnecessary but this brings us to the second reason. A significant number of the pages referenced by a process early in its life may only be used for initialisation and then never used again. It is better to swap out those pages and create more disk buffers than leave them resident and unused.

Src: https://www.kernel.org/doc/gorman/html/understand/understand014.html

In my recently booted system with 32GB and half of that free (not even "available"), I can already see 10s of MB of swap used.

As rule of thumb, it's only a concern or indication that the system is/was starved of memory if a significant share of swap is in use. But even then, it might just be some cached pages hanging around because the kernel decided to keep instead of evicting them.

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

TIL. Thanks for the information