Projects/auto-ballooning
Automatic Ballooning
Introduction
When a Linux host is running out of memory, the kernel will take action to reclaim memory. This action may be detrimental to KVM guests performace (eg. swapping) or even extreme to the point where the kernel may kill a VM or an important virt stack component.
To help avoiding this situation, a KVM guest could automatically return memory to the host when the host is facing memory pressure (automatic inflate). By doing so the guest may also get into memory pressure so we also need a way to allow the guest to automatically get memory back (automatic deflate).
That's what the automatic ballooning project is about. The next section will dive into ways of implementing it. The following section will provide instructions on how to try current patches.
Design
Automatic Inflate
Automatic inflate is performed by QEMU, that is, the KVM host. There are two ways I'm playing with automatic inflate, in both of them QEMU registers for memory pressure events to learn when the host is under memory pressure:
- QEMU registers for VMPRESSURE_MEDIUM event and inflates the balloon by an user specified value when that event is received. That value value could default to 16MB or 32MB. This is what the current patchset does
- QEMU registers for the three memory pressure events and inflates the balloon accordingly. Say, it inflates the balloon by 1MB on VMPRESSURE_LOW; 16MB on VMPRESSURE_MEDIUM and 128MB on VMPRESSURE_CRITICAL. This is not done yet because there's a small limitation in the memory pressure code which doesn't allow this. I'll fix it
Automatic Deflate
Automatic deflate is performed by the virtio-balloon driver in the guest. There's also two ways of implementing this:
- The virtio-balloon driver registers a callback with the shrinker API. That callback is called when the guest kernel is facing memory pressure. The number of pages to be returned to the kernel is passed to the callback, so that the callback can deflate the balloon by that amount. This is what the current patchset does
- The virtio-balloon driver registers for an in-kernel memory pressure event (not upstream yet) and deflates the balloon by some fixed amount (maybe the same used in automatic inflate)