Zero copy doesn't work with UDP

[undergroundSimplex]

Context

• Version of iperf3: 3.10.1

• Hardware: Intel(R) Atom(TM) Processor E3950 @ 1.60GHz

• Operating system (and distribution, if any): Debian GNU/Linux 9 (stretch)

Bug Report

• Expected Behavior
  Zero copy should reduce kernel call to copy_user_enhanced_fast_string.

• Actual Behavior
  When zero copy is enabled during UDP benchmark. The kernel call copy_user_enhanced_fast_string takes up 51% of cpu time when perf record -g -e cpu-clock is used to benchmark.
  However, when TCP is used, cpu time of copy_user_enhanced_fast_string is significantly reduced as expected.

• Steps to Reproduce
  iperf3 -s
sudo perf record -g -e cpu-clock iperf3 -c 127.0.0.1 -Z -u -b 30g -l 64000
sudo perf report --call-graph --no-children

• Possible Solution

[davidBar-On]

iperf3 is supporting zero-copy for TCP using sendfile(). It seems that it is not supported for UDP. See https://stackoverflow.com/questions/7450001/is-zero-copy-udp-packing-receiving-possibly-on-linux.

Comment 3 in the above suggests using sendmmsg/recvmmsg for UDP. There is already an open PR #1034 with a suggested enhancement to add sendmmsg/recvmmsg support to iperf3.