Comment 2 for bug 1831886

our downstream virt folks have started to do some more testing of different confguration
and on differnet acrchitecutres and it appears that 32 is not a good default.
on arch64 values over 24 prevent the guest form properly booting and intial indication
are that the memory use while initally low ~0.7m per port grows after the the guest os intiallies the pci device as such it looks like defaultign to 32 or 24 would cause excessive memory usage to be a sane default.

initial testing a sweet spot in terms of memory overhead may be 16 ports or less.
after the guest fully initializes the devices the overhead grows form about 0.7M per device to ~4M per device based on the average useage over 15 spawns of an instance but the deleta betwen 4 pcie ports and 16 is within 1 sandard devation.

this testing was done by dan berrangé using libvirt directly with
the following setup.
 - a Fedora 30 host
 - qemu 3.1.0
 - libvirt 5.1.0
 - A Fedora 28 x86_64 guest OS
 - 2000 MB of RAM allocated
 - 8 vCPUs, freely floating
 - Memory set to preallocated, and locked in physical RAM
 - One disk, as an qcow2 overlay on a master readonly f28 image
 - One network PCI device
 - One video PCI device
 - One memory balloon PCI device
 - VNC graphics, no client connected

graph or data todate
https://berrange.fedorapeople.org/q35-usage.png
note the yellow line uses the second y axis on the right
were as the magenta and blue axis use the y axis on the left.