tcp: Increase tcp_sosend_agglim from 2 to 3

This change makes better use of TSO.

When 2 is used, most of the large TCP segments' size are 2*MSS or 3*MSS.
When 3 is used, most of the large TCP segments' size are 4*MSS or 5*MSS.

The math is quite simple, given 1448B segment size:
2 == 4096B == 2 segments
3 == 6144B == 4 segments

Increasing it to a higher value improves single full speed stream's CPU
utilization, but does not have much effect on multiple streams'.

diff --git a/sys/netinet/tcp_input.c b/sys/netinet/tcp_input.c
index 81fb6fc..dd906b5 100644 (file)
--- a/sys/netinet/tcp_input.c
+++ b/sys/netinet/tcp_input.c
@@ -249,7 +249,7 @@ int tcp_autorcvbuf_max = 2*1024*1024;
 SYSCTL_INT(_net_inet_tcp, OID_AUTO, recvbuf_max, CTLFLAG_RW,
     &tcp_autorcvbuf_max, 0, "Max size of automatic receive buffer");
 
-int tcp_sosend_agglim = 2;
+int tcp_sosend_agglim = 3;
 SYSCTL_INT(_net_inet_tcp, OID_AUTO, sosend_agglim, CTLFLAG_RW,
     &tcp_sosend_agglim, 0, "TCP sosend mbuf aggregation limit");