Make your QEMU faster (2022)(linus.schreibt.jetzt)

150 pointsby tripdout2 anni fa22 comments

linus.schreibt.jetzt

Make your QEMU faster (2022)

https://linus.schreibt.jetzt/posts/qemu-9p-performance.html

24 comments

I've recently deep-dived into QEMU performance especially in order to cut down boot/bootstrapping times for quickly running tests and I'd like to share some tricks:

- Templating: https://qemu.readthedocs.io/en/v9.1.0/system/vm-templating.h... - virtio-fs: https://virtio-fs.gitlab.io/ - Overlay storage images: https://wiki.archlinux.org/title/QEMU#Overlay_storage_images

nijave2 anni fa

Cloud init will handle unique IDs although might add some boot time

remram2 anni fa

You need to get it to re-run, which I'm not sure there is a standard way to do during runtime. Can it even do all its job without a reboot?

nijave2 anni fa

Maybe I misunderstood, is this skipping boot as well?

I was thinking the VM would still follow the normal boot process (in which case you can reset cloud init before powering down)

remram2 anni fa

Yes they are saving memory state to a file.

Svenstaro2 anni fa

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.

zimbatm2 anni fa

NixOS has switched to virtiofs 4 days ago: https://github.com/NixOS/nixpkgs/pull/362081

Not all use-cases are faster with virtiofs.

remram2 anni fa

> Not all use-cases are faster with virtiofs

Aren't they though? Do you have a counter example?

stuaxo2 anni fa

Interesting and may be worth writing a qemu issue about.

Svenstaro2 anni fa

Oh cool!

hackernudes2 anni fa

The author commented on that in 2022 https://news.ycombinator.com/item?id=33200731

reginald782 anni fa

Is 9p compatible with saving VM states? I recently experimented with virtiofs and was disappointed to find it was incompatible so it didn't really work for my use case.

I don't even really need performance, I only really want to be able to grab driver files from a common location and ditch the samba shares.

mordae2 anni fa

For better convenience there is now https://github.com/KDAB/hotspot to render the flame graph from your perf data. You can also use online https://profiler.firefox.com/.

dang2 anni fa

Discussed at the time:

Digging into a QEMU problem of slow data copying - https://news.ycombinator.com/item?id=33192152 - Oct 2022 (31 comments)

m4632 anni fa

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

m4632 anni fa

title was "Make your QEMU 10 times faster with this one weird trick"

glandium2 anni fa

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

[deleted]2 anni fa

ryan_lane2 anni fa

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

dmitrygr2 anni fa

I've got you:

hashtables faster than linked lists in places where lookups are frequent & item count is high

eru2 anni fa

Well, the article is fairly short, so a TLDR is perhaps not necessary.

mannyv2 anni fa

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.

tuukkah2 anni fa

And the commit here: https://gitlab.com/qemu-project/qemu/-/commit/f5265c8f917ea8...

Included in QEMU version 7.2.0 from 2022-12-14.

752963e642 anni fa