Re: [libvirt] Qemu/KVM is 3x slower under libvirt (due to vhost=on)

[Reeted <reeted@shiftmail.org>]

On 09/28/11 16:51, Reeted wrote:
> On 09/28/11 14:56, Richard W.M. Jones wrote:
>> On Wed, Sep 28, 2011 at 12:19:09PM +0200, Reeted wrote:
>>> Ok that seems to work: it removes the vhost part in the virsh launch
>>> hence cutting down 12secs of boot time.
>>>
>>> If nobody comes out with an explanation of why, I will open another
>>> thread on the kvm list for this. I would probably need to test disk
>>> performance on vhost=on to see if it degrades or it's for another
>>> reason that boot time is increased.
>> Is it using CPU during this time, or is the qemu-kvm process idle?
>>
>> It wouldn't be the first time that a network option ROM sat around
>> waiting for an imaginary console user to press a key.
>>
>> Rich.
>
> Of the two qemu-kvm processes (threads?) which I see consuming CPU for 
> that VM, one is at about 20%, the other at about 10%. I think it's 
> doing something but maybe not much, or maybe it's really I/O bound and 
> the I/O is slow (as I originarily thought). I will perform some disk 
> benchmarks and follow up, but I can't do that right now...
> Thank you

Ok still didn't do benchmarks but I am now quite a lot convinced that 
it's either a disk performance problem or cpu problem with vhostnet on. 
Not a network performance problem or idle wait.

Because I have installed another virtual machine now, which is a fedora 
core 6 (old!),  but with a debian natty kernel vmlinuz + initrd so that 
it supports virtio devices. The initrd part from Ubuntu is extremely 
short so finishes immediately, but then the fedora core 6 boot is much 
longer than with my previous ubuntu-barebone virtual machine, and with 
more messages, and I can see the various daemons being brought up one by 
one, and I can tell you such boot (and also the teardown of services 
during shutdown) is very much faster with vhostnet disabled.

With vhostnet disabled it takes 30seconds to come up (since after grub), 
and 28 seconds to shutdown.
With vhostnet enabled it takes 1m19sec to come up (since after grub), 
and 1m04sec to shutdown.


I have some ideas about disk benchmarking, that would be fio or simple 
dd. What could I use for CPU benchmarking? Would "openssl speed" be too 
simple?

Thank you