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

150 pointsby tripdoutvor 2 Jahren22 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

nijavevor 2 Jahren

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

remramvor 2 Jahren

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?

nijavevor 2 Jahren

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)

remramvor 2 Jahren

Yes they are saving memory state to a file.

Svenstarovor 2 Jahren

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.

zimbatmvor 2 Jahren

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

Not all use-cases are faster with virtiofs.

remramvor 2 Jahren

> Not all use-cases are faster with virtiofs

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

stuaxovor 2 Jahren

Interesting and may be worth writing a qemu issue about.

Svenstarovor 2 Jahren

Oh cool!

hackernudesvor 2 Jahren

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

reginald78vor 2 Jahren

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.

mordaevor 2 Jahren

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/.

dangvor 2 Jahren

Discussed at the time:

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

m463vor 2 Jahren

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

m463vor 2 Jahren

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

glandiumvor 2 Jahren

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]vor 2 Jahren

ryan_lanevor 2 Jahren

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

dmitrygrvor 2 Jahren

I've got you:

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

eruvor 2 Jahren

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

mannyvvor 2 Jahren

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.

tuukkahvor 2 Jahren

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

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

752963e64vor 2 Jahren