[PATCH 0/8] tcp: Fix throughput issues with non-local peers

[Stefano Brivio <sbrivio@redhat.com>]

Patch 1/8 is the most relevant fix here, as we currently advertise a
window that might be too big for what we can write to the socket,
causing retransmissions right away and occasional high latency on
short transfers to non-local peers.

Mostly as a consequence of fixing that, we now need several
improvements and small fixes, including, most notably, an adaptive
approach to pick the interval between checks for socket-side ACKs
(patch 2/8), and several tricks to reliably trigger TCP buffer size
auto-tuning as implemented by the Linux kernel (patches 4/8 and 6/8).

These changes make some existing issues more relevant, fixed by the
other patches.

With this series, I'm getting the expected (wirespeed) throughput for
transfers between peers with varying non-local RTTs: I checked
different guests bridged on the same machine (~600 us) and hosts with
increasing distance (approximately 100 to 600 km, ~4 to ~35 ms), using
iperf3 as well as HTTP transfers.

For short transfers, we strictly stick to the available sending buffer
size to (almost) make sure we avoid local retransmissions, and
significantly decrease transfer time as a result: from 1.2 s to 60 ms
for a 5 MB HTTP transfer from a container hosted in a virtual machine
to another guest.

Stefano Brivio (8):
  tcp: Limit advertised window to available, not total sending buffer
    size
  tcp: Adaptive interval based on RTT for socket-side acknowledgement
    checks
  tcp: Don't clear ACK_TO_TAP_DUE if we're advertising a zero-sized
    window
  tcp: Acknowledge everything if sending buffer is less than SNDBUF_BIG
  tcp: Don't limit window to less-than-MSS values, use zero instead
  tcp: Allow exceeding the available sending buffer size in window
    advertisements
  tcp: Send a duplicate ACK also on complete sendmsg() failure
  tcp: Skip redundant ACK on partial sendmsg() failure

 README.md  |  2 +-
 tcp.c      | 85 ++++++++++++++++++++++++++++++++++++++++++------------
 tcp_conn.h |  9 ++++++
 util.c     | 14 +++++++++
 util.h     |  1 +
 5 files changed, 92 insertions(+), 19 deletions(-)

-- 
2.43.0