Re: massive performance drop after a while when using virtio nics

[Fabian Deutsch <fabian.deutsch@gmx.de>]

Am Montag, den 08.09.2008, 14:07 +0200 schrieb Yann Dupont:
> Fabian Deutsch a écrit :
> > Hey.
> >
> > When running a guest in the following setup, I observ a "massive"
> > performance decrease after a couple of gigabytes of transfrerred data.
> >
> > other guests keep running fine (no regressions) and nothing unusual is
> > observed on the host.
> >
> > fedora 8 host, kvm-72 (user and module)
> >
> > fedora 9 guest, all updates.
> >         nic 0, vtio:
> >         subnet a
> >         samba listening
> >
> >         nic 1, vtio:
> >         subnet b
> >         iscsi initiator running
> >
> >         samba shares a mount point, pointing to an iscsi target.
> >
> >
> > now pulling/pushing about 80GB over samba results in a drop from about
> > 60/70Mbit to about 1/2Mbit of troughput.
> >
> > The problem is reproducible.
> > The problem is solved after a reboot of the guest. Just restarting
> > networking doesn't help.
> > Guest's dmesg doesn't say anything about problems. No errors in
> > ifconfig.
> >
> > Has someone observed something similar?
> >
> >
> > Greetings
> > - fabian
> >
> > --
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> >
> >   
> We just saw the very same issue.
> guest & host :  2.6.25.3, debian etch 64 bits
> 
> KVM 70 on this machine.
> 
> As fabian said,  rebooting the  kernel doesn't help, but rebooting the 
> whole kvm guest  does.
> 
> No error messages as far as I can tell. The kvm guest is a clonezilla 
> server and send lots of data.
> It began to crawl after 2/3 weeks of usage.

Are you also using model=vortio?
If so, you might want to switch to e1000. Less performance but stable.

The issues seems to be related to virtio/tap.
And a little bit more to the tap itnerface/implementation, because of
the reboot-observation.

fabian