It has been determined that the specific error message mentioned below is a symptom, not the root cause. The stability issues and kernel panics are caused by Hyper-V Dynamic Memory management interacting with the Ubuntu 26.04 kernel.
I get the following error on Ubuntu 26.04: Error: The repository ‚https://apt.symcon.de stable InRelease‘ is not signed.
How should I solve this?
Err:5 ``https://apt.symcon.de`` stable InRelease The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY 69FF6261F2D8B290 Warning: OpenPGP signature verification failed: ``https://apt.symcon.de`` stable InRelease: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY 69FF6261F2D8B290 Error: The repository ‚``https://apt.symcon.de`` stable InRelease‘ is not signed. Notice: Updating from such a repository can’t be done securely, and is therefore disabled by default. Notice: See apt-secure(8) manpage for repository creation and user configuration details. Notice: Missing Signed-By in the sources.list(5) entry for ‚``https://apt.symcon.de``‘
Well, the apt source issue is the least of the problems. Symcon doesn’t work at all on Ubuntu 26.04. It consumes all available memory until the system hits a kernel panic.
I upgraded Ubuntu 24.04 LTS twice and encountered the same issue both times. I assume that if I try a third time, I’ll run into the same problem again.
The issue occurs 15-30 minutes after a reboot (when upgraded to Ubuntu 26.04 LTS).
I’ve upgraded from Ubuntu 24.04 LTS to 26.04 LTS once again. To resolve the stability issues, I adjusted the dynamic memory settings of the VM: I changed the Minimum RAM from 2048 MB to 4096 MB and lowered the Maximum from 16384 MB to 8192 MB (keeping the Startup RAM at 4096 MB). This seems to be a reliable workaround. Interestingly, the system is currently reporting memory usage of less than 2048 MB, despite the higher minimum setting. Hopefully, this prevents the kernel panic stability issues from returning.
Please keep me posted. I’d love to know if we have a potential memory leak, or any other excess memory usage (without a direct leak) that we need to search.
Google Gemini gave the explanation below. If that’s accurate, then something in the memory‑management behavior must have changed between Ubuntu 24.04 LTS and Ubuntu 26.04 LTS. IP‑Symcon is still running
Why the Kernel killed it (The Virtual Memory Trap)
Since you don’t have systemd-oomd installed, we know the „Userspace Killer“ didn’t do it. Instead, the Kernel stepped in because of a rule called Overcommit Accounting.
The Promise: When symcon starts, it tells the Kernel: „I might need 3.1 GB of space eventually“ (that’s your VSZ).
The Check: The Kernel looks at: Physical RAM + Swap.
The Failure: When Hyper-V had your VM squeezed down to 2 GB, the Kernel did the math:
Result:„I cannot promise 3.1 GB if I only have 2 GB. To prevent a total system freeze, I must kill the requester.“
How your fix solved it without the tool
By raising the Hyper-V Minimum RAM to 4 GB, you changed the math for the Kernel:
Available: 4 GB RAM + 0 GB Swap.
Requested: 3.1 GB.
Result:„I have plenty of room to cover that 3.1 GB promise. Carry on.“
You don’t need oomctl because the „Safety Officer“ (the Kernel) is now satisfied with the amount of floor space you’ve provided. The error was legitimate, the Kernel truly believed it was out of „promisable“ memory because the virtual request was larger than the physical hardware being allocated by Hyper-V at that moment.
Just a heads-up for anyone using Hyper-V to host IP-Symcon on Ubuntu.
Microsoft recommends setting the Minimum RAM and Startup Memory to the same value for Linux VMs when using Dynamic Memory. While I didn’t experience issues with previous Ubuntu releases, I found that following this recommendation is crucial for stability on Ubuntu 26.04 LTS.
This recommendation is specific to Linux VMs (not Windows). If you are seeing strange behavior, check your Hyper-V memory settings. Matching the startup and minimum RAM solved these issues/kernel panics for me.
I probably just got lucky with previous Ubuntu versions, but it seems the new kernel in Ubuntu 26.04 LTS is more sensitive to this.
