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Help with understanding memory usage discrepancy (lemmy.world)

submitted 12 hours ago* (last edited 10 hours ago) by a_fancy_kiwi@lemmy.world to c/selfhosted@lemmy.world

30 comments fedilink hide all child comments

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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[–] a_fancy_kiwi@lemmy.world 1 points 11 hours ago* (last edited 10 hours ago) (2 children)

You're an angel. ~~I don't know what the fuck htop is doing showing 8GB in use~~ ~~Based on another user comment in this thread, htop is showing a misleading number~~. For anyone else who comes across this, this is what I have. ~~This makes the situation seem a little more grim.~~ I have ~2GB free, ~28GB in use ~~, and of that ~28GB only ~3GB is cache that can be closed~~. For reference, I'm using ZFS and roughly 27 docker containers. ~~It doesn't seem like there is much room for future services to selfhost.~~

MemTotal:           30.5838      GB
MemFree:            1.85291      GB
MemAvailable:       4.63831      GB
Buffers:            0.00760269   GB
Cached:             3.05407      GB

[–] Voroxpete@sh.itjust.works 2 points 6 hours ago (1 children)

Most of those containers are probably grabbing more memory than they actually need. Consider applying some resource constraints to some of them.

Dozzle is an excellent addition to your docker setup, giving you live performance graphs for all your containers. It can help a lot with fine tuning your setup.

[–] irmadlad@lemmy.world 1 points 21 minutes ago

Most of those containers are probably grabbing more memory than they actually need.

It took a bit of trial and error with Portainer, but under Runtime & Resources, you can adjust the amount of needed resources:

Shared memory size
Memory reservation (MB)
Memory limit (MB)
Maximum CPU usage

[–] rtxn@lemmy.world 3 points 10 hours ago (1 children)

You should also look at which processes use the largest amount of memory. ZFS is weird and might allocate its cache memory as "used" instead of "cached". See here to set its limits: https://forum.proxmox.com/threads/limit-zfs-memory.140803/

[–] a_fancy_kiwi@lemmy.world 1 points 10 hours ago* (last edited 10 hours ago)

Assuming the info in this link is correct, ZFS is using ~20GB for caching which makes htop's ~8GB of in use memory make sense when compared with the results from cat /proc/meminfo. This is great news.

My results after running cat /proc/spl/kstat/zfs/arcstats:

c                               4    19268150979
c_min                           4    1026222848
c_max                           4    31765389312
size                            4    19251112856