Using Oracle VM 3 with some changes booting dom0 (additional parameters at the boot prompt) and with UEK2 as a guest kernel in my VM, I can make use of autoballooning. What you see in the graph below is very simple : it's a timeline (horizontal)of how much actual memory the VM is using/needing. I created 3 16GB VMs that I wanted to run on a 36GB Oracle VM server (so more VM memory than we have physically available in the server). When I start a 16GB VM (vertical) the Linux guest immediately balloons down to about 700Mb in size. It automatically releases pages to the hypervisor that are not needed, it's free/idle memory otherwise. Then I start a database with a 4GB SGA, as you can see, the second I start the DB, the VM grows to just over 4GB in size. Then I start swingbench runs, 25, 50, 100, 500, 1000 users. Every time such a run starts, you can see memory use/grab go up, when swingbench stops it goes down. In the end after the last run with 1000 users I also shut down the database instance and memory drops all the way to 700Mb.
I ran 3 guests with swingbench and the database in each and through dynamic ballooning and the guests cooperatively working with the hypervisor, I was able to start all 3 16GB VMs and there was no performance impact. When there was free memory in the hypervisor, cleancache kicked in and guests made use of those pages, including deduping and compression of the pages.
If you want to play with this yourself, you can run this command in dom0 to get decent statistics out of the setup : xm tmem-list --long --all | /usr/sbin/xen-tmem-list-parse. It will show you the compression ratio, the cache ratios etc. I used those statistics to generate the chart below. This yet another example of how, when one can control both the hypervisor and the guest operating system and have things work together, you get better and more interesting results than just black box VM management.
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