I ran into an issue where after migrating a bunch of VMs off of one of my hosts, the remaining VMs on it refused to turn on. Every time I tried the command would hang for a while and eventually error out with this message
TASK ERROR: start failed: command '/usr/bin/kvm -id <truncated>... ' failed: got timeout
I suspected this might be due to RAM use and sure enough the usage was too high for a system that didn’t have any VMs running on it. I found here that I could run a command to flush the cache:
echo 3 > /proc/sys/vm/drop_caches
That caused the RAM usage to go down but the symptom of the VM not starting remained. I then saw the KSM sharing still had some memory in it. I decided to restart the KSM sharing service:
sudo systemctl restart ksmtuned
After running that the VM started!