The virtio balloon device allows KVM guests to reduce their memory size (thus relinquishing memory to the host) and to increase it back (thus taking memory from the host).
This feature is mainly intended to support over-committing memory on KVM hosts. That is, hosts that are running VMs whose total memory size is greater than what the host has physically available. For example, a 2G host running two VMs each with 2G would be over-committed.
The balloon device is important to support memory over-commitment because it allows for reducing a guest's memory size if needed. Suppose, in the previous example, that one of the guests is using 1G and its other 1G is free. We could use the balloon device to reduce this guest's size from 2G to 1G, this would free 1G in the host allowing the other VM to use it. Of course, if the reduced guest wants to run an application that consumes more than the 1G it currently has, it has to grow again.
That's the problem with the current balloon device, it's entirely manual. Someone (or some tool) is supposed to be watching the pressure in the host and guest and then operate the balloon accordingly. This doesn't work well in practice (if doable at all).
The balloon has to be automatic in order to be really useful. It could like this: when the host is under pressure, it asks guests to relinquish some megas if they can. When/if a guest gets into memory pressure, it gets some megas back from the host. That's what the automatic ballooning series is about.
KVM Forum 2013 presentation slides
They can be found here, but note that much has changed since this talk.
Patches and Git trees
Latest RFC version posted upstream
- More performance tests
- Make it dynamic to enable/disable automatic ballooning
- Do kernel work required in order to have non-cgroups code using in-kernel memory pressure notification
- Install kernel 3.10 or higher in your host. Make sure the kernel options CONFIG_CGROUPS and CONFIG_MEMCG are enabled
- Build QEMU from the Git trees section
- Build and install the guest kernel from the Git trees section
Then start qemu with:
# qemu [...] -device virtio-balloon,automatic=true
To see automatic ballooning in action, do the following:
- Generate pressure in your host (like running several VMs in parallel); or connect to QMP and issue the balloon-inject-host-pressure command
- You can monitor automatic ballooning activity by issuing info balloon in QEMU's monitor (also, I have debug code enabled that will dump a lot of info to stdout when "info balloon" is issued)
- After the guest has shrinked a bit, you can generate pressure in the guest to see it increasing its size again