Re: [Qemu-devel] [PATCH] net: QEMU_NET_PACKET_FLAG_MORE introduced

[Stefan Hajnoczi]

On Mon, Dec 09, 2013 at 03:55:00PM +0200, Michael S. Tsirkin wrote:
> On Mon, Dec 09, 2013 at 01:42:30PM +0100, Stefan Hajnoczi wrote:
> > So the effect of batching should be relatively small on latency.  In
> > fact, it's almost like sendmmsg(2)/recvmmsg(2) but using a
> > one-packet-at-a-time interface.
> > 
> > Does this sound right?
> > 
> > Stefan
> 
> Why would it be small?  Consider a queue of 256 packets.
> You are sending out a single short packet, followed
> by a burst of 255 larger packets.
> the single packet is not transmitted until qemu completes
> processing 255 larger ones.

Seems like my intuition is wrong.  I figured QEMU processing those 255
packets is quick.  If zero-copy is possible then we just need to put the
address/length into a ring before we flush the packets to the host
kernel.

The netperf results do show a regression so we need to understand that
better.

Stefan