[Qemu-devel] VM CPU/Disk unexplained increase in resize time

[Chathura M. Sarathchandra Magurawalage]

Hi,

Does anyone know the reason for, VM resizing time to increase faster if you 
continuously increase CPU or DISK resources by +1 (e.g. 1-2, 2-3, 3-4, 4-5). 
Whereas, when you increase from 1 to any other (e.g. 1-2, 1-2, 1,3, 1-4, 1-5) 
it takes less time in comparison. Can anyone give an explanation for this? I 
have plotted two graphs.

https://www.dropbox.com/s/5e8xrrctu0rcwx3/CPU%20scaling%20%20-%20continuous%20vs%20increasing%20from%201.png?dl=0
https://www.dropbox.com/s/txpkb8k6mpyexv8/CPU%20scaling%20-%20increase%20from%201.png?dl=0

The first graph shows the VM CPU resize time (y axis) vs number of vCPUs (x 
axis) of continuous (blue) and resize from a VM with 1 vCPU (green) 
scenarios.The second graph shows the VM CPU resize time (y axis) vs number of 
vCPUs (x axis), when resized from a VM with 1 vCPU at each step (The green line 
in first graph). The error bars show the standard error of the gathered values 
at each step, as I did resize multiple times to get a mean value. I use 
openstack on x86 with KVM, although I have asked the openstack community I 
could not yet find an answer to this.

Thanks!