Re: I/O performance of VirtIO

[Jan Kiszka <jan.kiszka@web.de>]

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Michael Tokarev wrote:
> René Pfeiffer wrote:
>> Hello!
>>
>> I just tested qemu-kvm-0.11.0 with the KVM module of kernel 2.6.31.1. I
>> noticed that the I/O performance of an unattended stock Debian Lenny
>> install dropped somehow. The test machines ran with kvm-88 and 2.6.30.x
>> before. The difference is very noticeable (went from about 5 minutes up
>> to 15-25 minutes). The two test machines have different CPUs (one is an
>> Intel Core2 CPU, the other runs with an AMD Athlon 64 X2 Dual).
>>
>> Is this the effect of added code regarding caching/data integrity to the
>> VirtIO block layer or somewhere else? The qemu-system-x86_64 seems to
>> hang a lot more in heavy I/O (showing 'D' in top/htop).
>>
>> The command line is quite straight-forward:
>> qemu-system-x86_64 -drive file=debian.qcow2,if=virtio,boot=on -cdrom \
>> /srv/isos/debian-502-i386-netinst.iso -smp 2 -boot d -m 512 -net nic \
>> -net user -usb
>   ^^^^^^^^^
> 
> Care to try with something more real than user-level networking?
> You're using netinstall which - apparently - tries to use some
> networking d/loading components etc, and userlevel networking is
> known to be very very slow....

It can be particularly slow if you use in-kernel irqchips and the
default NIC emulation (up to 10 times slower), some effect I always
wanted to understand on a rainy day. So, when you actually want -net
user, try -no-kvm-irqchip.

Jan


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