Andrew Morton wrote:
Begin forwarded message:
Date: Thu, 29 Apr 2004 14:53:36 -0700
From: Jan Olderdissen <jan@xxxxxxxxxxx>
To: "'linux-kernel@xxxxxxxxxxxxxxx'" <linux-kernel@xxxxxxxxxxxxxxx>
Subject: Stack sends SYN/ACKs even though accept queue is full
Attaching a patch which adds a sysctl to turn off this
behaviour. Could you test this, please? Patch against
2.6.5 vanilla kernel. If you need a 2.4 version, let me
know.
Because newly accepted connections are considered 'young', two such
connections put on the synq will cause additional SYNs to be dropped until
young connections age and additional connections are SYN/ACKed , etc. Since
the initial TCP timeout is three seconds, you would expect two additional
connections to be accepted every three seconds. However, experiments with
2.4.25 show that number to be two connections every four seconds for unclear
reasons.
Normally, I think the expected behaviour was that connections
would be short-lived. This is a reasonable expectation for most
web-servers etc. In which case, the accept queue would free
up frequently, and having the syn request right there would save
a full timeout and round trip over the Internet. i.e. useful in the
common case.
I don't think it is worthwhile for environments where connections
are long-lived and turnover is infrequent, added to a heavily
congested network. I'd prefer to have this be a tunable option.
thanks,
Nivedita
diff -urN linux-2.6.5/include/linux/sysctl.h
linux-2.6.5synq/include/linux/sysctl.h
--- linux-2.6.5/include/linux/sysctl.h 2004-04-03 19:37:23.000000000
-0800
+++ linux-2.6.5synq/include/linux/sysctl.h 2004-04-30 15:24:12.000000000
-0700
@@ -323,6 +323,7 @@
NET_IPV4_IPFRAG_SECRET_INTERVAL=94,
NET_TCP_WESTWOOD=95,
NET_IPV4_IGMP_MAX_MSF=96,
+ NET_TCP_PRELOAD_SYNQ=97,
};
enum {
diff -urN linux-2.6.5/include/net/tcp.h
linux-2.6.5synq/include/net/tcp.h
--- linux-2.6.5/include/net/tcp.h 2004-04-03 19:36:18.000000000 -0800
+++ linux-2.6.5synq/include/net/tcp.h 2004-04-30 13:55:18.000000000
-0700
@@ -583,6 +583,7 @@
extern int sysctl_tcp_frto;
extern int sysctl_tcp_low_latency;
extern int sysctl_tcp_westwood;
+extern int sysctl_tcp_preload_synq;
extern atomic_t tcp_memory_allocated;
extern atomic_t tcp_sockets_allocated;
diff -urN linux-2.6.5/net/ipv4/sysctl_net_ipv4.c
linux-2.6.5synq/net/ipv4/sysctl_net_ipv4.c
--- linux-2.6.5/net/ipv4/sysctl_net_ipv4.c 2004-04-03 19:37:37.000000000
-0800
+++ linux-2.6.5synq/net/ipv4/sysctl_net_ipv4.c 2004-04-30
16:16:57.000000000 -0700
@@ -601,6 +601,14 @@
.mode = 0644,
.proc_handler = &proc_dointvec,
},
+ {
+ .ctl_name = NET_TCP_PRELOAD_SYNQ,
+ .procname = "tcp_preload_synq",
+ .data = &sysctl_tcp_preload_synq,
+ .maxlen = sizeof(int),
+ .mode = 0644,
+ .proc_handler = &proc_dointvec,
+ },
{ .ctl_name = 0 }
};
diff -urN linux-2.6.5/net/ipv4/tcp_ipv4.c
linux-2.6.5synq/net/ipv4/tcp_ipv4.c
--- linux-2.6.5/net/ipv4/tcp_ipv4.c 2004-04-03 19:36:55.000000000 -0800
+++ linux-2.6.5synq/net/ipv4/tcp_ipv4.c 2004-04-30 15:51:38.000000000
-0700
@@ -78,6 +78,7 @@
extern int sysctl_ip_dynaddr;
int sysctl_tcp_tw_reuse;
int sysctl_tcp_low_latency;
+int sysctl_tcp_preload_synq = 1;
/* Check TCP sequence numbers in ICMP packets. */
#define ICMP_MIN_LENGTH 8
@@ -1442,8 +1443,11 @@
* clogging syn queue with openreqs with exponentially increasing
* timeout.
*/
- if (tcp_acceptq_is_full(sk) && tcp_synq_young(sk) > 1)
- goto drop;
+ if (tcp_acceptq_is_full(sk)) {
+ if (!sysctl_tcp_preload_synq ||
+ (sysctl_tcp_preload_synq && (tcp_synq_young(sk) > 1)))
+ goto drop;
+ }
req = tcp_openreq_alloc();
if (!req)
@@ -2683,4 +2687,5 @@
EXPORT_SYMBOL(sysctl_local_port_range);
EXPORT_SYMBOL(sysctl_max_syn_backlog);
EXPORT_SYMBOL(sysctl_tcp_low_latency);
+EXPORT_SYMBOL(sysctl_tcp_preload_synq);
#endif
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