Comment 4 for bug 1767315

this is not a but at least not as reported.

free_ram_mb is not ment to be equal to the free value of "free -m"

memory_mb is 4047 and memory_mb 4096

as a result the total amount of memery that you have acounted against guests is 49 mb more then the avalible ram

so free_ram_mb which is caluated as memory_mb - memory_mb_used
is -49 as you are oversubsibed by 49MBs

the default ram allocation ratio is 1.5 which is why you are allowed to allocate more ram then you have up to 6GiB in your
case.

not that memory_mb_used is calulated by suming the total ram of all vms scheduled to a node + the reserved_host_memory_mb config value which i would guess defaults to 4096 in triplo.

https://docs.openstack.org/nova/latest/configuration/config.html#DEFAULT.reserved_host_memory_mb