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

[David Gibson <david@gibson.dropbear.id.au>]

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On Fri, Dec 05, 2025 at 02:20:12AM +0100, Stefano Brivio wrote:
> 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.

Ok.  Adding that context to the commit message might be useful.
Otherwise,

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

> 
> > 
> > > 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
> 

-- 
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

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