libvirt users - May 2022 - Slow VM start/revert, when trying to start/revert dozens of VMs in parallel

Hi,

my problem can be described simply: libvirt can't handle starting dozens of
VMs at the same time.

(technically, it can, but it's really slow.)

We have an AMD machine with 256 logical cores and 1.5T ram.
On that machine there is roughly 200 VMs.
Each VM is the same: 8GB of RAM, 4 VCPUs. Half of them is Win7 x86, the other
half is Win7 x64.
VMs are using qcow2 as the disk image. These images reside in the ramdisk
(tmpfs).

We use these machines for automatic malware analysis, so our scenario consists
of this cycle:
- reverting VM to a running state
- execute sample inside of the VM for ~1-2 minutes
- shutdown the VM

Of course, this results in multiple VMs trying to start at the same time.
At first, reverts/starts are really fast - second or two.
After about a minute, the "revertToSnapshot" suddenly takes 10-15
seconds, which is really unacceptable.
For comparison, we're running the same scenarion on Proxmox, where the
revertToSnapshot usually takes 2 seconds.

Few notes:
- Because of this fast cycle (~2-3 minutes) and because of VMs taking 10-15
seconds to start, there is barely more than 25-30 VMs running at once.
  We would really love to utilise the whole potential of such beast machine of
ours, and have at least ~100 VMs running at any given time.
- During the time running, the avg. CPU load isn't higher than 25%. Also,
there's only about 280 GB of RAM used. Therefore, it's not limitation of
our resources.
- When the framwork is running and libvirt is making its best to start our VMs,
I noticed that every libvirt operation is suddenly very slow.
  Even simple "virsh list [--all]" takes few seconds to complete, even
though it finishes instantly when no VM is running/starting.

I was trying to search for this issue, but didn't really find anything
besides this presentation:
https://events19.linuxfoundation.org/wp-content/uploads/2017/12/Scalability-and-Stability-of-libvirt-Experiences-with-Very-Large-Hosts-Marc-Hartmayer-IBM-1.pdf

However, I couldn't find those commits in your upstream.

Is this a known issue? Or is there some setting I don't know of which would
magically make the VMs start faster?

As for steps to reproduce - I don't think there is anything special needed.
Just try to start/destroy several VMs in a loop.
There is even provided one-liner for that in the presentation above.

```
# For multiple domains:
# while virsh start $vm && virsh destroy $vm; do : ; done
# ? ~30s hang ups of the libvirtd main loop
```

Best Regards,
Petr

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I forgot to mention:

```
$ lsb_release -a
Distributor ID: Ubuntu
Description:    Ubuntu 22.04 LTS
Release:        22.04
Codename:       jammy

$ virsh version
Compiled against library: libvirt 8.0.0
Using library: libvirt 8.0.0
Using API: QEMU 8.0.0
Running hypervisor: QEMU 6.2.0
```

________________________________
Od: Petr Bene? <w.benny at outlook.com>
Odesl?no: pond?l? 9. kv?tna 2022 20:52
Komu: libvirt-users at redhat.com <libvirt-users at redhat.com>
P?edm?t: Slow VM start/revert, when trying to start/revert dozens of VMs in
parallel

Hi,

my problem can be described simply: libvirt can't handle starting dozens of
VMs at the same time.

(technically, it can, but it's really slow.)

We have an AMD machine with 256 logical cores and 1.5T ram.
On that machine there is roughly 200 VMs.
Each VM is the same: 8GB of RAM, 4 VCPUs. Half of them is Win7 x86, the other
half is Win7 x64.
VMs are using qcow2 as the disk image. These images reside in the ramdisk
(tmpfs).

We use these machines for automatic malware analysis, so our scenario consists
of this cycle:
- reverting VM to a running state
- execute sample inside of the VM for ~1-2 minutes
- shutdown the VM

Of course, this results in multiple VMs trying to start at the same time.
At first, reverts/starts are really fast - second or two.
After about a minute, the "revertToSnapshot" suddenly takes 10-15
seconds, which is really unacceptable.
For comparison, we're running the same scenarion on Proxmox, where the
revertToSnapshot usually takes 2 seconds.

Few notes:
- Because of this fast cycle (~2-3 minutes) and because of VMs taking 10-15
seconds to start, there is barely more than 25-30 VMs running at once.
  We would really love to utilise the whole potential of such beast machine of
ours, and have at least ~100 VMs running at any given time.
- During the time running, the avg. CPU load isn't higher than 25%. Also,
there's only about 280 GB of RAM used. Therefore, it's not limitation of
our resources.
- When the framwork is running and libvirt is making its best to start our VMs,
I noticed that every libvirt operation is suddenly very slow.
  Even simple "virsh list [--all]" takes few seconds to complete, even
though it finishes instantly when no VM is running/starting.

I was trying to search for this issue, but didn't really find anything
besides this presentation:
https://events19.linuxfoundation.org/wp-content/uploads/2017/12/Scalability-and-Stability-of-libvirt-Experiences-with-Very-Large-Hosts-Marc-Hartmayer-IBM-1.pdf

However, I couldn't find those commits in your upstream.

Is this a known issue? Or is there some setting I don't know of which would
magically make the VMs start faster?

As for steps to reproduce - I don't think there is anything special needed.
Just try to start/destroy several VMs in a loop.
There is even provided one-liner for that in the presentation above.

```
# For multiple domains:
# while virsh start $vm && virsh destroy $vm; do : ; done
# ? ~30s hang ups of the libvirtd main loop
```

Best Regards,
Petr

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On Mon, May 09, 2022 at 06:52:32PM +0000, Petr Bene?
wrote:
> Hi,
> 
> my problem can be described simply: libvirt can't handle starting
dozens of VMs at the same time.
> 
> (technically, it can, but it's really slow.)
> 
> We have an AMD machine with 256 logical cores and 1.5T ram.
> On that machine there is roughly 200 VMs.
> Each VM is the same: 8GB of RAM, 4 VCPUs. Half of them is Win7 x86, the
other half is Win7 x64.
> VMs are using qcow2 as the disk image. These images reside in the ramdisk
(tmpfs).
> 
> We use these machines for automatic malware analysis, so our scenario
consists of this cycle:
> - reverting VM to a running state
> - execute sample inside of the VM for ~1-2 minutes
> - shutdown the VM
> 
> Of course, this results in multiple VMs trying to start at the same time.
> At first, reverts/starts are really fast - second or two.
> After about a minute, the "revertToSnapshot" suddenly takes 10-15
seconds, which is really unacceptable.
> For comparison, we're running the same scenarion on Proxmox, where the
revertToSnapshot usually takes 2 seconds.
Can you share the XML configuration of one of your guests - assuming
they all have the same basic configuration.

As a gut feeling it sounds to me like it could be initially fast due to
utilization of host I/O cache, but then slows down due to having to
flush data to disk / read fresh from disk. This could be the case if
the disk configuration cache mode is set to certain values, so the XML
config will show us this info.

With regards,
Daniel
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