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 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.

> 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
> 

-- 
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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