[PATCH 6/8] tcp: Allow exceeding the available sending buffer size in window advertisements

[Stefano Brivio <sbrivio@redhat.com>]

...under two conditions:

- the remote peer is advertising a bigger value to us, meaning that a
  bigger sending buffer is likely to benefit throughput, AND

- this is not a short-lived connection, where the latency cost of
  retransmissions would be otherwise unacceptable.

By doing this, we can reliably trigger TCP buffer size auto-tuning (as
long as it's available) on bulk data transfers.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
---
 tcp.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/tcp.c b/tcp.c
index 2220059..454df69 100644
--- a/tcp.c
+++ b/tcp.c
@@ -353,6 +353,13 @@ enum {
 #define LOW_RTT_TABLE_SIZE		8
 #define LOW_RTT_THRESHOLD		10 /* us */
 
+/* Try to avoid retransmissions to improve latency on short-lived connections */
+#define SHORT_CONN_BYTES		(16ULL * 1024 * 1024)
+
+/* Temporarily exceed available sending buffer to force TCP auto-tuning */
+#define SNDBUF_BOOST_FACTOR		150 /* % */
+#define SNDBUF_BOOST(x)			((x) * SNDBUF_BOOST_FACTOR / 100)
+
 #define ACK_IF_NEEDED	0		/* See tcp_send_flag() */
 
 #define CONN_IS_CLOSING(conn)						\
@@ -1137,6 +1144,9 @@ int tcp_update_seqack_wnd(const struct ctx *c, struct tcp_tap_conn *conn,
 
 		if ((int)sendq > SNDBUF_GET(conn)) /* Due to memory pressure? */
 			limit = 0;
+		else if ((int)tinfo->tcpi_snd_wnd > SNDBUF_GET(conn) &&
+			 tinfo->tcpi_bytes_acked > SHORT_CONN_BYTES)
+			limit = SNDBUF_BOOST(SNDBUF_GET(conn)) - (int)sendq;
 		else
 			limit = SNDBUF_GET(conn) - (int)sendq;
 
-- 
2.43.0