Re: [PATCH 4/8] tcp: Acknowledge everything if sending buffer is less than SNDBUF_BIG

5 Dec 2025

On Fri, Dec 05, 2025 at 02:20:12AM +0100, Stefano Brivio wrote:
...
On Fri, 5 Dec 2025 11:08:06 +1100
David Gibson  wrote:
...
On Thu, Dec 04, 2025 at 08:45:37AM +0100, Stefano Brivio wrote:
...
...instead of checking if it's less than SNDBUF_SMALL, because this
isn't simply an optimisation to coalesce ACK segments: we rely on
having enough data at once from the sender to make the buffer grow
by means of TCP buffer size tuning implemented in the Linux kernel.
Use SNDBUF_BIG: above that, we don't need auto-tuning (even though
it might happen). SNDBUF_SMALL is too... small.
Do you have an idea of how often sndbuf exceeds SNDBUF_BIG?  I'm
wondering if by making this change we might have largely eliminated
the first branch in practice.
Before this series, or after 6/8 in this series, it happens quite
often. It depends on the bandwidth * delay product of course, but at 1
Gbps and 20 ms RTT we get there in a couple of seconds.
Maybe 1 MiB would make more sense for typical conditions, but I'd defer
this to a more adaptive implementation of the whole thing. I think it
should also depend on the RTT, ideally.
Ok.  Adding that context to the commit message might be useful.
Otherwise,

Reviewed-by: David Gibson 
...
...
...
Signed-off-by: Stefano Brivio 
---
 tcp.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tcp.c b/tcp.c
index e4c5a5b..fbf97a0 100644
--- a/tcp.c
+++ b/tcp.c
@@ -1079,7 +1079,7 @@ int tcp_update_seqack_wnd(const struct ctx *c, struct tcp_tap_conn *conn,
  if (bytes_acked_cap && !force_seq &&
      !CONN_IS_CLOSING(conn) &&
      !(conn->flags & LOCAL) && !tcp_rtt_dst_low(conn) &&
-	    (unsigned)SNDBUF_GET(conn) >= SNDBUF_SMALL) {
+	    (unsigned)SNDBUF_GET(conn) >= SNDBUF_BIG) {
      if (!tinfo) {
      	tinfo = &tinfo_new;
      	if (getsockopt(s, SOL_TCP, TCP_INFO, tinfo, &sl))
-- 
2.43.0
-- 
Stefano
-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

    