How do I set qemu guest CPU speed?

szatox · Advocate Joined: 27 Aug 2013 Posts: 3650

Long story short I'm trying to run an untrusted, resource-hungry application in a VM and it kinda looks single-threaded to me.
So, is there any way to instruct qemu not to limit passed-through CPU's speed? My qemu guest only sees 1,8GHz, while the host tends to reliably put out 2,4GHz on every core (out of 3,9GHz advertised).
I control my VMs with virt-manager; the host is Intel Core i5, in case it makes any difference.
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eccerr0r · Posted: Mon Apr 28, 2025 4:07 am Post subject:

Odd, my qemu/virt-manager guests passes bogomips and cpu MHz in /proc/cpuinfo pretty much straight through. Your vm shouldn't be changing these things if kvm is working... also kvm is working pretty close to native/baremetal speeds as well and will slow down only if the host machine is bogged down... (at least this seems to be the case for my C2Q and i5/i7/Xeons with and without HT.) Note all my machines have vmx but I'd suspect your machine has vmx if it's 3.9GHz and iX series...

Sure KVM is working properly?

Another observation is that when you run two processes in parallel in qemu it seems to run the processes as two threads on the host which gets parallelized by the host kernel... not sure what you're seeing?

Incidentally I don't know if it's possible to "slow down" guests when using KVM (other than limiting number of cores and setting nice to the qemu process). So if there are timing related software problems they are hard to emulate a slower machine other than perhaps using full emulation...
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szatox · Advocate Joined: 27 Aug 2013 Posts: 3650

> Sure KVM is working properly?

It certainly is slower than native, but it doesn't feel 'kvm not working'-slow... Well, I'll double check it to be on the safe side. It would actually be hilarious too.

> Another observation is that when you run two processes in parallel in qemu it seems to run the processes as two threads on the host which gets parallelized by the host kernel... not sure what you're seeing?

I see a VM with 2-3 cores and load avg around 1, when application inside can't keep up with crunching through the data stream thrown at it.
So I think it just doesn't scale out, even though it is "technically" multithreaded.
Other applications can use multiple cores at the same time. However, now that you mentioned it, I've seen some emerge jobs take much longer than they should, with load average also equal to 1. I shrugged it at the time, but maybe it is actually worth a second glance.
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Anon-E-moose · Posted: Mon Apr 28, 2025 10:02 am Post subject:

You might try pinning cores/threads to your vm instance,
it might help, especially if this is one of the processors with e-cores,
but even if just regular cores, it might make a difference.

I know libvirt should have an option for tagging cores but not sure what it's called,
with cmd line qemu, I would use taskset "taskset -c 4,5,6,7,12,13,14,15"
and set smp to some sane value "-smp 8,cores=4,threads=2"

Edit to add: https://wiki.archlinux.org/title/PCI_passthrough_via_OVMF#Performance_tuning
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szatox · Advocate Joined: 27 Aug 2013 Posts: 3650

KVM is enabled and in use.

lscpu from guest:

Anon-E-moose · Posted: Mon Apr 28, 2025 12:22 pm Post subject:

what parameters are you using for "cpu" libvirt-wise

Edit to add: "host" or a particular model type?
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eccerr0r · Posted: Mon Apr 28, 2025 1:55 pm Post subject:

hmm maybe something i don't have enabled right, I've always thought my qemu client exposed too much information about the host.

szatox · Advocate Joined: 27 Aug 2013 Posts: 3650

> what parameters are you using for "cpu" libvirt-wise

I'm using host cpu (pass-through mode).

> As for HT, (...)

I actually disabled it because there's not enough cooling to run full throttle anyway. Like you said, "it could" doesn't mean "it would".
Welll, it does look a bit better today, with HT enabled and cpu changed to host-model. Still, it's not good enough.

Anyway, I'll try to convert it to an LXC. Maybe qemu just isn't the right tool for the job...
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Anon-E-moose · Posted: Tue Apr 29, 2025 12:44 pm Post subject:

A container or chroot is sometimes a better choice, all depending on what you're trying to do.
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blu3bird

szatox · Advocate Joined: 27 Aug 2013 Posts: 3650

Yes, it is just a visual bug. Thanks for pointing that out.
I've run some number crunching in the VM and on the host, and the difference it raw power is like 5%... And yet the application performance difference (vm vs host) is huge. But it must be caused by something else. _Could_ have something to do with usb rediretion via spice (input lag), but I find load average = 1 to be suspicious. It being almost exactly 1 when a multi-threaded application is not getting enough CPU power on a multicore system is just weird.

I eventually ended up restricting the app with a different user and firewall (access to my files was already somewhat restricted compared to defaults). Well, I guess that's about as good as it gets for now.
Oh well. I'll leave it for now.
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eccerr0r · Posted: Wed Apr 30, 2025 8:28 pm Post subject:

system calls including I/O have always been an Achilles heel to virtualization, it's bad enough as it is on baremetal, worse on virtual, perhaps that's why my VM emerges so slow compared to host, granted it's not an apples to apples comparison (32 vs 64 bit).

Still strange why my machine copies bogomips and MHz straight to the VM as far as I can see (I suppose it doesn't matter on my Core2Quad which does not have turboboost, but still see the same behavior on my turboboost capable machines too.)
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