Make your QEMU faster (2022)

I&#x27;ve recently deep-dived into QEMU performance especially in order to cut down boot&#x2F;bootstrapping times for quickly running tests and I&#x27;d like to share some tricks:<p>- Templating: <a href="https:&#x2F;&#x2F;qemu.readthedocs.io&#x2F;en&#x2F;v9.1.0&#x2F;system&#x2F;vm-templating.html" rel="nofollow">https:&#x2F;&#x2F;qemu.readthedocs.io&#x2F;en&#x2F;v9.1.0&#x2F;system&#x2F;vm-templating.h...</a> - virtio-fs: <a href="https:&#x2F;&#x2F;virtio-fs.gitlab.io&#x2F;" rel="nofollow">https:&#x2F;&#x2F;virtio-fs.gitlab.io&#x2F;</a> - Overlay storage images: <a href="https:&#x2F;&#x2F;wiki.archlinux.org&#x2F;title&#x2F;QEMU#Overlay_storage_images" rel="nofollow">https:&#x2F;&#x2F;wiki.archlinux.org&#x2F;title&#x2F;QEMU#Overlay_storage_images</a>

Is there a reason to use 9p over virtiofs in this case? It looks to me as though virtiofs was made exactly with the idea to deal good performance.

For better convenience there is now <a href="https:&#x2F;&#x2F;github.com&#x2F;KDAB&#x2F;hotspot">https:&#x2F;&#x2F;github.com&#x2F;KDAB&#x2F;hotspot</a> to render the flame graph from your perf data. You can also use online <a href="https:&#x2F;&#x2F;profiler.firefox.com&#x2F;" rel="nofollow">https:&#x2F;&#x2F;profiler.firefox.com&#x2F;</a>.

Discussed at the time:<p><i>Digging into a QEMU problem of slow data copying</i> - <a href="https:&#x2F;&#x2F;news.ycombinator.com&#x2F;item?id=33192152">https:&#x2F;&#x2F;news.ycombinator.com&#x2F;item?id=33192152</a> - Oct 2022 (31 comments)

wow, clickbait title, then 9p and nix in the first couple paragraphs firmly set the hook.

This was interesting, but by the end of it, I was still expecting some weird trick to work around the problem without the patch.

They delivered on the clickbait title to engagement time metric, as the title promises one weird trick and the content has no TLDR.

Tl;dr: file descriptors for the guest os are stored as linked lists by qemu, and changing that sped qemu file copying up by a ton.