Performance issues with KVM versus Virtualbox
I'm trying to use KVM on Ubuntu (5.8.0-59-generic #66~20.04.1-Ubuntu) and finding some serious performance issues. In order to quantify the problem, I tried a fresh install of Windows 7, both under KVM and Oracle Virtualbox. The latter was considerably faster. Overall, performance on the Virtualbox VM is pretty good (obviously not as good as native, but still respectable enough), but with KVM was terrible (screen refresh was slow, mouse pointer stuttered, IO was up to 20x slower etc). Settings on both installations were "out of the box" (I used virt-manager to set up the VM under KVM) (so I didn't use virtio with KVM, or Guest Additions with Virtualbox).
So my question is: is this just a case of KVM being "a bit rubbish" (as has been suggested), or is this a problem with my setup? Is it worth doing a bit more work to try and get it working properly, or am I wasting my time? I know that KVM is good for passthrough hardware support... is this why it's popular?
Thanks!
1 Answer
KVM is massively better than VirtualBox. Windows takes a lot of hand holding to get the best performance when virtualizing, and KVM probably doesn't care about pandering to it by default.
Make sure your timers are appropriately configured:
Regarding I/O, make sure you enable writeback caching. Pinning vCPUs to physical cores will also help.
For graphical performance, KVM doesn't support 3D acceleration (VirtualBox sort of almost does to a usable extent), but for 2D it is pretty decent if you install the paravirtualized QXL driver and use Spice instead of VNC protocol for accessing it.
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