does anyone run guests for more than 5 minutes? (virtio-net perf anomaly)

[Alex Williamson <alex.williamson@hp.com>]

It seems like something happens around the 5 minute uptime in the guest
that causes virtio-net throughput to plummet.  Here's the scenario:

guest started as:

        taskset -c 4 /usr/local/bin/qemu-system-x86_64 -hda /dev/sdb -m
        2048 -vnc :1 -net nic,macaddr=02:00:10:91:73:02,model=virtio
        -net tap,script=$HOME/bin/null-ifup -serial
        tcp::1234,server,nowait -mem-path /hugepages/

null-ifup looks like this:

        #!/bin/sh
        /sbin/ifconfig $1 192.168.0.1
        /sbin/route add -net 192.168.0.0/24 gw 192.168.0.1

The guest gets a static IP of 192.168.0.2.

netserver (part of netperf) in the host is pinned to CPU0, which shares
cache with CPU4 from the above taskset.

When the guest boots, I run:

        netperf -c -C -H 192.168.0.1 -t TCP_STREAM -- -m 64k

This results in ~13.5Gbps (note you won't get close to this if you don't
get the tasksets correct)

Wait 5 minutes, retry.  Now I get ~4Gbps.  The only way I can get
13.5Gbps again is by rebooting the guest within the same qemu context,
or of course restarting it completely.

Any guesses as to what might be going on?  Can anyone reproduce?  I'm
hoping that I'm doing something dumb, but can't figure out what it is.
The system is running v2.6.29-rc6-121-g64e7130 in the guest,
v2.6.29-rc6-123-gbd7b3b4 on the host, kvm module kvm-84-620-g5bffffc and
userspace kvm-84-95-gea1b668.  Thanks,

Alex