Re: [RFC PATCH] tcp: Replace send buffer boost with EPOLLOUT monitoring

[Stefano Brivio <sbrivio@redhat.com>]

On Fri, 20 Mar 2026 18:32:14 +0800
Yumei Huang <yuhuang@redhat.com> wrote:

> Currently we use the SNDBUF boost mechanism to force TCP auto-tuning.
> However, it doesn't always work, and sometimes causes a lot of
> retransmissions. As a result, the throughput suffers.
> 
> This patch replaces it with monitoring EPOLLOUT when sendmsg() failure
> (with EAGAIN and EWOULDBLOCK) and partial sends occur.
> 
> Tested with iperf3 inside pasta: throughput is now comparable to running
> iperf3 directly on the host without pasta. However, retransmissions can
> still be elevated when RTT >= 50ms. For example, when RTT is between
> 200ms and 500ms, retransmission count varies from 30 to 120 in roughly
> 80% of test runs.
> 
> Link: https://bugs.passt.top/show_bug.cgi?id=138
> Suggested-by: Stefano Brivio <sbrivio@redhat.com>
> Signed-off-by: Yumei Huang <yuhuang@redhat.com>
> ---
>  tcp.c | 57 +++++++++++++++++----------------------------------------
>  1 file changed, 17 insertions(+), 40 deletions(-)

Thanks a lot! This is definitely a massive improvement, and a much
needed simplification over the original, so I applied this as it is (I
also tested it quite thoroughly).

I'm still looking into how we can replace the 75% to 100% linearly
scaled usage factor from tcp_get_sndbuf() with a more accurate
calculation (assuming it's doable), as a follow-up change, but even
once/if we do that, properly reacting on EPOLLOUT as this patch adds is
something we'll need anyway.

-- 
Stefano