Re: Degrading Network performance as KVM/kernel version increases

[Brian Jackson <iggy@theiggy.com>]

  On 8/31/2010 6:00 PM, matthew.r.rohrer@L-3com.com wrote:
> I have been getting degrading network performance with newer versions of
> KVM and was wondering if this was expected?  It seems like a bug, but I
> am new to this and maybe I am doing something wrong so I thought I would
> ask.
>
> KVM Host OS: Fedora 12 x86_64
> KVM Guest OS Tiny Core Linux 2.6.33.3 kernel
>
> I have tried multiple host kernels 2.6.31.5, 2.6.31.6, 2.6.32.19 and
> 2.6.35.4 along with versions qemu-kvm 11.0 and qemu-system-x86_64 12.5
> compiled from from qemu-kvm repo.


I can't say anything about the kernel version making things worse. At 
least for the qemu-kvm version, you should be using -device and -netdev 
instead of -net nic -net tap (see 
*http://git.qemu.org/qemu.git/tree/docs/qdev-device-use.txt since it's 
not in the 0.12 tree).*


> Setup is: 2 hosts with 1 guest on each connected by 10 Gb nic.
>
> I am using virtio and have checked that hardware acceleration is
> working.
>
> Processor usage is less than 50% on host and guests.
>
> Here is what I am seeing, I will just include guest to guest statistics,
> I do have more (host to guest, etc.) if interested:

<snip results>

>
> My goal is to get as much bandwidth as I can between the 2 guests
> running on separate hosts.  The most I have been able to get is ~4 Gb/s
> running 4 threads on iperf from guest A to guest B.  I cannot seem to
> get much over 1.5Gb/s from guest to guest with a single iperf thread.
> Is there some sort of know send limit per thread?  Is it expected that
> the latest version of the kernel and modules perform worse than earlier
> versions in the area of network performance ( I am guessing not, am I
> doing something wrong?)?  I am using virtio and have checked that
> hardware acceleration is working.  4 iperf threads host to host yields
> ~9.5 Gb/s.  Any ideas on how I can get better performance with newer
> versions?  I have tried using vhost in 2.6.35 but I get the vhost could
> not be initialized error.  The only thing I could find on the vhost
> error is that selinux should be off which it is.
>
> I am looking for ideas on increasing the bandwidth between guests and
> thoughts on the degrading performance.


Vhost-net is probably your best bet for maximizing throughput. You might 
try a separate post just for the vhost error if nobody chimes in about 
it here.


> Thanks for your help! --Matt