netdev - [PATCH net] tcp: reduce tcp_comp_sack_slack

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Message-ID: <20251114135141.3810964-1-edumazet@google.com>
Date: Fri, 14 Nov 2025 13:51:41 +0000
From: Eric Dumazet <edumazet@...gle.com>
To: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>
Cc: Simon Horman <horms@...nel.org>, Neal Cardwell <ncardwell@...gle.com>, 
	Kuniyuki Iwashima <kuniyu@...gle.com>, netdev@...r.kernel.org, eric.dumazet@...il.com, 
	Eric Dumazet <edumazet@...gle.com>
Subject: [PATCH net] tcp: reduce tcp_comp_sack_slack_ns default value to 10 usec

net.ipv4.tcp_comp_sack_slack_ns current default value is too high.

When a flow has many drops (1 % or more), and small RTT, adding 100 usec
before sending SACK stalls the sender relying on getting SACK
fast enough to keep the pipe busy.

Decrease the default to 10 usec.

This is orthogonal to Congestion Control heuristics to determine
if drops are caused by congestion or not.

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
 Documentation/networking/ip-sysctl.rst | 3 ++-
 net/ipv4/tcp_ipv4.c                    | 2 +-
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/Documentation/networking/ip-sysctl.rst b/Documentation/networking/ip-sysctl.rst
index 2bae61be18593a8111a83d9f034517e4646eb653..f4ad739a6b532914e4091c425828b329ee342bc6 100644
--- a/Documentation/networking/ip-sysctl.rst
+++ b/Documentation/networking/ip-sysctl.rst
@@ -875,8 +875,9 @@ tcp_comp_sack_slack_ns - LONG INTEGER
 	timer used by SACK compression. This gives extra time
 	for small RTT flows, and reduces system overhead by allowing
 	opportunistic reduction of timer interrupts.
+	Too big values might reduce goodput.
 
-	Default : 100,000 ns (100 us)
+	Default : 10,000 ns (10 us)
 
 tcp_comp_sack_nr - INTEGER
 	Max number of SACK that can be compressed.
diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index a7d9fec2950b915e24f0586b2cb964e0e68866ed..6fcaecb67284ecade97b623d955dbbe2cd02a831 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -3593,7 +3593,7 @@ static int __net_init tcp_sk_init(struct net *net)
 		       sizeof(init_net.ipv4.sysctl_tcp_wmem));
 	}
 	net->ipv4.sysctl_tcp_comp_sack_delay_ns = NSEC_PER_MSEC;
-	net->ipv4.sysctl_tcp_comp_sack_slack_ns = 100 * NSEC_PER_USEC;
+	net->ipv4.sysctl_tcp_comp_sack_slack_ns = 10 * NSEC_PER_USEC;
 	net->ipv4.sysctl_tcp_comp_sack_nr = 44;
 	net->ipv4.sysctl_tcp_comp_sack_rtt_percent = 33;
 	net->ipv4.sysctl_tcp_backlog_ack_defer = 1;
-- 
2.52.0.rc1.455.g30608eb744-goog