libvirt

Stumbling across a 5% performance increase in OpenStack Kolla-Ansible CI

I went on a side quest a couple of weekends ago which seems like it might have ended up being a big win. That win likely applies to other people running both OpenStack, but also other cloud orchestration systems using libvirt like oVirt, Proxmox, Shaken Fist, and so forth.

By way of back story, I have a stack of patches which I maintain on top of OpenStack’s Kolla and Kolla-Ansible. Most of those patches continue to progress the new “spice-direct” console type in OpenStack Nova, but I also occasionally find little bugs or CI regressions along the way that I want to clean up. One example of such a bug was Kolla’s failure to setup virtlogd to provide instance console log rotation, which is currently being backported to various releases. Another one is the topic of this post.

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virtio-vsock: python examples of running the server in the guest

I’ve been using virtio-serial for communications between Linux hypervisors and guest virtual machines for ages. Lots of other people do it to — the qemu guest agent for example is implemented like this. In fact, I think that’s where I got my original thoughts on the matter from. However, virtio-serial is actually fairly terrible to write against as a programming model, because you’re left to do all the multiplexing of various requests down the channel and surely there’s something better?

Well… There is! virtio-vsock is basically the same concept, except it uses the socket interface. You can have more than one connection open and the sockets layer handles multiplexing by magic. This massively simplifies the programming model for supporting concurrent users down the channel. So that’s actually pretty cool. I should credit Kata Containers with noticing this quality of life improvement nearly a decade before I did, but I get there in the end.

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