when use libvirt0.9.4 ,then memStats() just output a '{ }'.

Generally, it means your qemu returns empty result for
monitor command query-balloon. Could you check the debug
log to find out what the qemu monitor command returns
actually?

libvirt > 0.9.4 memStats() can get the memory stats,but it seems that the value does not match guest memory usage:

virsh # version

Compiled against library: libvir 0.9.11

Using library: libvir 0.9.11

Using API: QEMU 0.9.11

Running hypervisor: QEMU 1.0.94

virsh # dommemstat centos

actual 524288

rss 276252

while in the 'centos' guest:

[root@centos-guest ~]# free -k

total used free shared buffers cached

Mem: 511148 84784 426364 0 5708 32664

-/+ buffers/cache: 46412 464736

Swap: 2064376 0 2064376

what does the 'rss' mean? thanks.