From mboxrd@z Thu Jan 1 00:00:00 1970 Received: by passt.top (Postfix, from userid 1000) id D0D425A08B5; Thu, 04 Dec 2025 08:45:42 +0100 (CET) From: Stefano Brivio To: passt-dev@passt.top Subject: [PATCH 6/8] tcp: Allow exceeding the available sending buffer size in window advertisements Date: Thu, 4 Dec 2025 08:45:39 +0100 Message-ID: <20251204074542.2156548-7-sbrivio@redhat.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20251204074542.2156548-1-sbrivio@redhat.com> References: <20251204074542.2156548-1-sbrivio@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Message-ID-Hash: OJPCNTRUKAKUBGMK4I3IKNCZHW5OU4F6 X-Message-ID-Hash: OJPCNTRUKAKUBGMK4I3IKNCZHW5OU4F6 X-MailFrom: sbrivio@passt.top X-Mailman-Rule-Misses: dmarc-mitigation; no-senders; approved; emergency; loop; banned-address; member-moderation; nonmember-moderation; administrivia; implicit-dest; max-recipients; max-size; news-moderation; no-subject; digests; suspicious-header CC: Max Chernoff , David Gibson X-Mailman-Version: 3.3.8 Precedence: list List-Id: Development discussion and patches for passt Archived-At: Archived-At: List-Archive: List-Archive: List-Help: List-Owner: List-Post: List-Subscribe: List-Unsubscribe: ...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 --- 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