Re: [Qemu-devel] [PATCH] Tap: fix vcpu long time io blocking on tap

[Stefan Hajnoczi]

On Mon, Jul 14, 2014 at 10:44:58AM +0000, Wangkai (Kevin,C) wrote:
> Here the detail network:
> 
> +--------------------------------------------+
> | The host add tap1 and eth10 to bridge 'br1'|                     +--------+
> | +------------+                             |                     |  send  |
> | |   VM  eth1-+-tap1 --- bridge --- eth10 --+---------------------+ packets|
> | +------------+                             |                     |        |
> +--------------------------------------------+                     +--------+
> 
> Qemu start vm by virtio, use tap interface, option is:
> -net nic,vlan=101, model=virtio -net 
> tap,vlan=101,ifname=tap1,script=no,downscript=no

Use the newer -netdev/-device syntax to get offload support and slightly
better performance:

-netdev tap,id=tap0,ifname=tap1,script=no,downscript=no \
-device virtio-net-pci,netdev=tap0

> And add tap1 and eth10 to bridge br1 in the host:
> Brctl addif br1 tap1
> Brctl addif br1 eth10
> 
> total recv 505387 time 2000925 us:
> means call tap_send once dealing 505387 packets, the packet payload was 300 
> bytes, and
> time use for tap_send() was 2,000,925 micro-seconds, time was measured by 
> record time stamp
> at function tap_send() start and end.
> 
> We just test the performance of VM.

That is 150 MB of incoming packets in a single tap_send().  Network rx
queues are maybe a few 1000 packets so I wonder what is going on here.

Maybe more packets are arriving while QEMU is reading them and we keep
looping.  That's strange though because the virtio-net rx virtqueue
should fill up (it only has 256 entries).

Can you investigate more and find out exactly what is going on?  It's
not clear yet that adding a budget is the solution or just hiding a
deeper problem.

Stefan

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