Linux – Why is the KVM, RHEL6.2 server swapping

I have a RHEL6.2 server that I'm using to run KVM virtual machines.

The server itself has 16 GB of RAM. I want to see the biggest VM I can run on it and not let the qemu-kvm process swap. The VM ram is ~15GB. (Yes, I realize that this is pushing the limit, but read to the end before answering with something like "15GB is too much".)

[root@xxx libvirt]# virsh dumpxml VM2 | grep -i memory
  <memory>15000000</memory>
  <currentMemory>15000000</currentMemory>

[root@xxx libvirt]# ps -ef | grep kvm
root       872     1 16 10:55 ?        00:03:00 /usr/libexec/qemu-kvm [...] -m 14649 -name VM2 [...]


[root@xxx libvirt]# free -k
             total       used       free     shared    buffers     cached
Mem:      16332640   16194440     138200          0       1544      15700
-/+ buffers/cache:   16177196     155444
Swap:     35651568    7583432   28068136

But the RSS of the KVM process is only 880 MB (column 6 below). I expect it to be more like 12-14 GB.

[root@xxx libvirt]# ps -eF | grep kvm
root       872     1 14 4534221 882916 7 10:55 ?       00:03:11 /usr/libexec/qemu-kvm 

And, if I add up the RSS of all processes, its only ~ 1gig.

[root@xxx libvirt]# ps -eF | awk '{print $6}' | grep '[0-9]' | tr '\n' '+' | sed 's/+$/\n/' | bc
1004064

Here are the processes that are the biggest memory users (RSS, column 6).

root      5188 22329  0 27572  1192   4 11:19 pts/0    00:00:00 ps -eF
root     31461     1  0 10746  1236   7 Jul25 ?        00:06:22 [...]
root      6339  6275  0 272676 3288   4 Jul27 ?        00:13:38 [...]
root      2059     1  1 443909 13352  7 Jul17 ?        05:29:11 libvirtd --daemon
root       872     1 13 4534221 928300 2 10:55 ?       00:03:24 /usr/libexec/qemu-kvm [...]

I expect that about 300-500 MB of memory should still be available for the VM.

UPDATE

After rebooting the machine, I cannot reproduce this anymore. The system now works as I expect it to. Here are these expected numbers.

Oh, I should mention. There is a memory intensive program running in the VM. It allocates 80% of the total memory and continually writes random numbers to it.

RSS of the VM:

[root@hb05b15 ~]# ps -eF | grep kvm
root      7330     1 97 4520362 12483728 2 11:59 ?     00:39:55 /usr/libexec/qemu-kvm [...]

Mem and swap numbers:

[root@hb05b15 ~]# free -k
             total       used       free     shared    buffers     cached
Mem:      16332640   13277468    3055172          0      21064     215196
-/+ buffers/cache:   13041208    3291432
Swap:     35651568          0   35651568

Sum up RSS for all processes:

[root@hb05b15 ~]# ps -eF | awk '{print $6}' | grep '[0-9]' | tr '\n' '+' | sed 's/+$/\n/' | bc
12607180