performance: 0.770
graphic: 0.760
device: 0.732
other: 0.722
semantic: 0.667
PID: 0.505
boot: 0.489
vnc: 0.469
network: 0.462
permissions: 0.456
socket: 0.441
debug: 0.436
files: 0.334
KVM: 0.306

qemu release memory to system 

Qemu pre-allocates the maximum balloon amount which is inconvinient if all of the memory is used up and some other system needs to be added memory resource

eg:- I have 4GB RAM with 4 virtual systems to be run.
I want each of them to start with 1GB RAM with maximum 2GB possible. This is not achievable since qemu pre-allocates the maximum balloon amount which is 2GBx4 systems . So to start all 4 systems the system needs 8GB RAM rather than 4GB RAM to start with although I have told initial balloon amount to be 1GB.

Looking through old bug tickets... I think you should rather use hotpluggable memory here for the guests instead - or use a big swap partition on the host. Anyway, this is not a bug, so closing this ticket now.