Cause of the Limitation  

Proxmox VE may limit the number of vCPUs for performance reasons. The system can only use as many vCPUs as the number of physical cores available. However, when there is a specific requirement for more threads that the physical machine cannot meet, the limitation needs to be lifted.  

When to Lift the Limitation ?

When there is a need to create a virtual machine with a custom number of cores.  

Note: Over-allocating threads does not improve performance. For example, if a physical machine has only 4 threads but you allocate 8 threads, 4 threads will work hard while the other 4 wait in the queue.  

Solution  

1. Edit the File   

   vi /usr/share/perl5/PVE/QemuServer.pm  

2. Search for the following line and comment it out:  

   #die “MAX $allowed_vcpus vcpus allowed per VM on this node\n” if ($allowed_vcpus < $maxcpus);  

3. Restart the pvedaemon service:

   systemctl restart pvedaemon