From: Michael Tokarev <mjt@tls.msk.ru>
To: Hans-Kristian Bakke <hkbakke@gmail.com>
Cc: kvm@vger.kernel.org
Subject: Re: Virtio network performance on Debian
Date: Tue, 17 Apr 2012 12:33:50 +0400 [thread overview]
Message-ID: <4F8D2AEE.6010308@msgid.tls.msk.ru> (raw)
In-Reply-To: <CAD_cGvHq8_F+b6Pprz1yLkTP4dKw+U80VDU=8Cp66VvVwWCQNA@mail.gmail.com>
On 12.04.2012 11:42, Hans-Kristian Bakke wrote:
> Hi
>
> For some reason I am not able to get good network performance using
> virtio/vhost-net on Debian KVM host (perhaps also valid for Ubuntu
> hosts then).
The issue has been identified, after Hans-Kristian gave me access
to his machine and I did alot of testing. And as usual, the root
cause was very stupid... ;)
In last release of debian qemu-kvm package I changed the way how
debian package version string propagates to build procedure -- before
it was a patch grabbing values from debian/version, but in last
release I used --with-pkgversion configure flag. This resulted in
the following change:
- QEMU emulator version 1.0 (qemu-kvm-1.0 Debian 1.0+dfsg-8), Copyright (c) 2003-2008 Fabrice Bellard
+ QEMU emulator version 1.0 (Debian qemu-kvm 1.0+dfsg-9), Copyright (c) 2003-2008 Fabrice Bellard
As it turns out, libvirt parses `qemu -version' output and looks for
" (qemu-kvm-" string in there, and if it is found, libvirt enables
some "extra" features. Important for us was setting IFF_VNET_HDR
flag for a tap device, -- apparently this flag makes a HUGE difference
in networking speed, especially when using vhost_net.
Obviously this is a change unique to debian, and I never thought
about such an.. "interesting" effect it may give us.
This is a libvirt bug actually, since support of vnet_hdr can be
determined by other means, and since upstream qemu now has almost
everything from qemu-kvm, and it wants to be fast too. But since
qemu[-kvm] has a long history of changing features, it is difficult
to blame libvirt that much...
Oh well.
Thanks!
/mjt
prev parent reply other threads:[~2012-04-17 8:33 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-04-12 7:42 Virtio network performance on Debian Hans-Kristian Bakke
2012-04-16 9:01 ` Stefan Pietsch
2012-04-16 9:28 ` Michael Tokarev
2012-04-16 11:29 ` Alexandre DERUMIER
2012-04-16 11:46 ` Hans-Kristian Bakke
2012-04-17 8:33 ` Michael Tokarev [this message]
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