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

[Stefano Brivio <sbrivio@redhat.com>]

On Fri, 5 Dec 2025 11:08:06 +1100
David Gibson <david@gibson.dropbear.id.au> 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.

> 
> > Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
> > ---
> >  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