AF_XDP not transmitting frames immediately

Hi Magnus and Bjørn,

I'm coding on an AF_XDP program[1] that need to send (a bulk of packets)
in a short time-window (related to Time-Triggered Ethernet).

My observations are that AF_XDP doesn't send the frames immediately.
And yes, I do call sendto() to trigger a TX kick.
In zero-copy mode this is particular bad.  My program want to send 4
packets in a burst, but I'm observing 8 packets grouped together on the
receiving host.

Is the a known property of AF_XDP?

How can I get AF_XDP to "flush" TX packets when calling sendto()?
Should we add another flag than the current MSG_DONTWAIT?

--Jesper

Hint, I'm using tcpdump hardware timestamping on receiving hist via cmdline:

  tcpdump -vv -s0 -ni eth1 -j adapter_unsynced 
--time-stamp-precision=nano  -w af_xdp_tx_cyclic.dump42

Notice[1] on specific branch:
  [1] 
https://github.com/xdp-project/bpf-examples/tree/vestas03_AF_XDP_example/AF_XDP-interaction