[libvirt] Memory Ballooning for VM Issue

Hi, I have some problems with qemu ballooning. I saw the qemu docs. I found the qemu monitor command "balloon" that can request VM to change its memory allocation to value(in MB). it requested qemu command line with "-balloon virtio", then I start qemu into monitor mode with memory 512MB. first show balloon information. (qemu) info balloon (qemu)balloon: actual=512 the command above if like this, I think the balloon device worked fine. (qemu) balloon 400 there was no error when I executed this command. but when I executed "info balloon", the result was still 512. Is there something wrong? please give me some ideas. thanks. qemu-kvm version: 1.0.1 linux kernel version: 3.1.6 xuanmao_001