Date: Wed, 28 May 2003 11:41:41 -0700 From: Steve Francis <steve@expertcity.com> To: freebsd-net@freebsd.org Subject: Side effect of net.inet.tcp.rexmit_min? Message-ID: <3ED502E5.1060001@expertcity.com>
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On testing a 4.8 Free BSD system for short connections (basically a
single HTTP get, then close of the connection), we found big differences
from 4.6.
The 4.6 BSD system would comfortably do over 6000 connections per
second- the 4.8 system was only doing about 4000/sec, and also
fluctuating wildly in its rate, even though not CPU or NIC loaded. (The
4.8 is a faster machine with gig nics, cf the 4.6 which was 100Mbps.)
The behaviour of the 4.8 machine was made equal to the 4.6 machine (over
6000 connections/sec, limited by our test bed) by changing
net.inet.tcp.rexmit_min from default 1000 to 10.
However, all testing is on a LAN, and packet loss, AFAIK, is zero.
(Flood ping shows no loss; switch ports show no errors; netstat -sptcp
show a very small number of retransmissions (See below); a tcptrace ran
against tcpdump while net.inet.tcp.rexmit_min was 1000 reports no
retranmissions.)
So why should changing net.inet.tcp.rexmit_min have any impact on a
lossless LAN?
Any help appreciated as to what further info I can collect to diagnose
this..
Thanks
netstat -sptcp
tcp:
205678066 packets sent
51416973 data packets (3553577611 bytes)
474 data packets (30344 bytes) retransmitted
0 resends initiated by MTU discovery
102843980 ack-only packets (75 delayed)
0 URG only packets
0 window probe packets
0 window update packets
51416642 control packets
287817734 packets received
133565842 acks (for 3656410968 bytes)
14930 duplicate acks
0 acks for unsent data
102833595 packets (3826892727 bytes) received in-sequence
1177 completely duplicate packets (2370 bytes)
0 old duplicate packets
0 packets with some dup. data (0 bytes duped)
0 out-of-order packets (0 bytes)
51416434 packets (0 bytes) of data after window
0 window probes
51403368 window update packets
0 packets received after close
0 discarded for bad checksums
0 discarded for bad header offset fields
0 discarded because packet too short
12 connection requests
51416620 connection accepts
0 bad connection attempts
0 listen queue overflows
51416629 connections established (including accepts)
51424590 connections closed (including 0 drops)
5 connections updated cached RTT on close
5 connections updated cached RTT variance on close
0 connections updated cached ssthresh on close
0 embryonic connections dropped
133565841 segments updated rtt (of 51417199 attempts)
9788 retransmit timeouts
0 connections dropped by rexmit timeout
0 persist timeouts
0 connections dropped by persist timeout
9 keepalive timeouts
9 keepalive probes sent
0 connections dropped by keepalive
9 correct ACK header predictions
51416725 correct data packet header predictions
51416620 syncache entries added
191 retransmitted
122 dupsyn
0 dropped
51416620 completed
0 bucket overflow
0 cache overflow
0 reset
0 stale
0 aborted
0 badack
0 unreach
0 zone failures
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