is tcp flow control broken in -current

[Frank Kardel <kardel%netbsd.org@localhost>]

Hi !

While running a java application on -current(9.99.60+70) with JDBC 
talking to a postgres11 database I observed the following:

The application runs and processes large number of queries via the tcp 
back-end connection via the loopback interface. After a while the 
communication

slows down significantly.

The database back-end get a single character of a query string once 
every five seconds and later on once per minute. At the same time 
netstat -an

shows that the send-queue has more than one character available.

#root@Alpine:/home/kardel 20:42 [28]# tcpdump -i lo0 -n port 5433 and 
port 65525
tcpdump: verbose output suppressed, use -v or -vv for full protocol 
decode21:45:24.330991 IP 127.0.0.1.5433 > 127.0.0.1.65525: Flags [P.], 
seq 1517533791:1517533826, ack 31819, win 4096, options [nop,nop,TS val 
1066 ecr 1066], length 35
21:45:24.525198 IP 127.0.0.1.65525 > 127.0.0.1.5433: Flags [.], ack 
1517533826, win 4096, options [nop,nop,TS val 1066 ecr 1066], length 0
21:45:31.723722 IP 127.0.0.1.65525 > 127.0.0.1.5433: Flags [P.], seq 
31819:31978, ack 1517533826, win 4096, options [nop,nop,TS val 1080 ecr 
1066], length 159
21:45:31.725197 IP 127.0.0.1.5433 > 127.0.0.1.65525: Flags [P.], seq 
1517533826:1517534277, ack 31978, win 4096, options [nop,nop,TS val 1080 
ecr 1080], length 451
21:45:31.725459 IP 127.0.0.1.65525 > 127.0.0.1.5433: Flags [P.], seq 
31978:32074, ack 1517534277, win 4096, options [nop,nop,TS val 1080 ecr 
1080], length 96
21:45:31.726212 IP 127.0.0.1.5433 > 127.0.0.1.65525: Flags [P.], seq 
1517534277:1517534718, ack 32074, win 4096, options [nop,nop,TS val 1080 
ecr 1080], length 441
21:45:31.726477 IP 127.0.0.1.65525 > 127.0.0.1.5433: Flags [P.], seq 
32074:32233, ack 1517534718, win 4096, options [nop,nop,TS val 1080 ecr 
1080], length 159
21:45:31.727340 IP 127.0.0.1.5433 > 127.0.0.1.65525: Flags [P.], seq 
1517534718:1517535152, ack 32233, win 4096, options [nop,nop,TS val 1080 
ecr 1080], length 434
21:45:31.727536 IP 127.0.0.1.65525 > 127.0.0.1.5433: Flags [P.], seq 
32233:32329, ack 1517535152, win 4096, options [nop,nop,TS val 1080 ecr 
1080], length 96
21:45:31.728592 IP 127.0.0.1.5433 > 127.0.0.1.65525: Flags [P.], seq 
1517535152:1517535600, ack 32329, win 4096, options [nop,nop,TS val 1080 
ecr 1080], length 448
21:45:31.729087 IP 127.0.0.1.65525 > 127.0.0.1.5433: Flags [P.], seq 
32329:32552, ack 1517535600, win 4096, options [nop,nop,TS val 1080 ecr 
1080], length 223
21:45:31.730461 IP 127.0.0.1.5433 > 127.0.0.1.65525: Flags [P.], seq 
1517535600:1517535664, ack 32552, win 4096, options [nop,nop,TS val 1080 
ecr 1080], length 64
21:45:31.730994 IP 127.0.0.1.65525 > 127.0.0.1.5433: Flags [P.], seq 
32552:32622, ack 1517535664, win 4096, options [nop,nop,TS val 1080 ecr 
1080], length 70
21:45:31.731107 IP 127.0.0.1.5433 > 127.0.0.1.65525: Flags [P.], seq 
1517535664:1517535700, ack 32622, win 4096, options [nop,nop,TS val 1080 
ecr 1080], length 36
21:45:31.731309 IP 127.0.0.1.65525 > 127.0.0.1.5433: Flags [P.], seq 
32622:32683, ack 1517535700, win 4096, options [nop,nop,TS val 1080 ecr 
1080], length 61
21:45:31.731444 IP 127.0.0.1.5433 > 127.0.0.1.65525: Flags [P.], seq 
1517535700:1517535785, ack 32683, win 4096, options [nop,nop,TS val 1080 
ecr 1080], length 85
21:45:31.925506 IP 127.0.0.1.65525 > 127.0.0.1.5433: Flags [.], seq 
32683:32769, ack 1517535785, win 4096, options [nop,nop,TS val 1081 ecr 
1080], length 86
21:45:32.125511 IP 127.0.0.1.5433 > 127.0.0.1.65525: Flags [.], ack 
32769, win 4096, options [nop,nop,TS val 1081 ecr 1081], length 0
21:45:37.125738 IP 127.0.0.1.65525 > 127.0.0.1.5433: Flags [.], seq 
32769:32770, ack 1517535785, win 4096, options [nop,nop,TS val 1091 ecr 
1081], length 1

### What did just happen?

21:45:37.325728 IP 127.0.0.1.5433 > 127.0.0.1.65525: Flags [.], ack 
32770, win 4096, options [nop,nop,TS val 1092 ecr 1091], length 0
21:45:42.126167 IP 127.0.0.1.65525 > 127.0.0.1.5433: Flags [.], seq 
32770:32771, ack 1517535785, win 4096, options [nop,nop,TS val 1101 ecr 
1092], length 1
21:45:42.326164 IP 127.0.0.1.5433 > 127.0.0.1.65525: Flags [.], ack 
32771, win 4096, options [nop,nop,TS val 1102 ecr 1101], length 0
21:45:47.126311 IP 127.0.0.1.65525 > 127.0.0.1.5433: Flags [.], seq 
32771:32772, ack 1517535785, win 4096, options [nop,nop,TS val 1111 ecr 
1102], length 1
21:45:47.326311 IP 127.0.0.1.5433 > 127.0.0.1.65525: Flags [.], ack 
32772, win 4096, options [nop,nop,TS val 1112 ecr 1111], length 0
21:45:52.126460 IP 127.0.0.1.65525 > 127.0.0.1.5433: Flags [.], seq 
32772:32773, ack 1517535785, win 4096, options [nop,nop,TS val 1121 ecr 
1112], length 1
21:45:52.326463 IP 127.0.0.1.5433 > 127.0.0.1.65525: Flags [.], ack 
32773, win 4096, options [nop,nop,TS val 1122 ecr 1121], length 0
21:45:57.126615 IP 127.0.0.1.65525 > 127.0.0.1.5433: Flags [.], seq 
32773:32774, ack 1517535785, win 4096, options [nop,nop,TS val 1131 ecr 
1122], length 1
21:45:57.326619 IP 127.0.0.1.5433 > 127.0.0.1.65525: Flags [.], ack 
32774, win 4096, options [nop,nop,TS val 1132 ecr 1131], length 0
21:46:02.126768 IP 127.0.0.1.65525 > 127.0.0.1.5433: Flags [.], seq 
32774:32775, ack 1517535785, win 4096, options [nop,nop,TS val 1141 ecr 
1132], length 1
21:46:02.326766 IP 127.0.0.1.5433 > 127.0.0.1.65525: Flags [.], ack 
32775, win 4096, options [nop,nop,TS val 1142 ecr 1141], length 0
21:46:07.126920 IP 127.0.0.1.65525 > 127.0.0.1.5433: Flags [.], seq 
32775:32776, ack 1517535785, win 4096, options [nop,nop,TS val 1151 ecr 
1142], length 1
21:46:07.326932 IP 127.0.0.1.5433 > 127.0.0.1.65525: Flags [.], ack 
32776, win 4096, options [nop,nop,TS val 1152 ecr 1151], length 0
21:46:12.127070 IP 127.0.0.1.65525 > 127.0.0.1.5433: Flags [.], seq 
32776:32777, ack 1517535785, win 4096, options [nop,nop,TS val 1161 ecr 
1152], length 1
21:46:12.327069 IP 127.0.0.1.5433 > 127.0.0.1.65525: Flags [.], ack 
32777, win 4096, options [nop,nop,TS val 1162 ecr 1161], length 0
21:46:17.127229 IP 127.0.0.1.65525 > 127.0.0.1.5433: Flags [.], seq 
32777:32778, ack 1517535785, win 4096, options [nop,nop,TS val 1171 ecr 
1162], length 1
21:46:17.327230 IP 127.0.0.1.5433 > 127.0.0.1.65525: Flags [.], ack 
32778, win 4096, options [nop,nop,TS val 1172 ecr 1171], length 0
21:46:22.127380 IP 127.0.0.1.65525 > 127.0.0.1.5433: Flags [.], seq 
32778:32779, ack 1517535785, win 4096, options [nop,nop,TS val 1181 ecr 
1172], length 1
21:46:22.327374 IP 127.0.0.1.5433 > 127.0.0.1.65525: Flags [.], ack 
32779, win 4096, options [nop,nop,TS val 1182 ecr 1181], length 0

#root@Alpine:/home/kardel 20:40 [27]# netstat -na | grep 5433

tcp        0      0  127.0.0.1.5433 127.0.0.1.65525        ESTABLISHED
tcp        0    506  127.0.0.1.65525        127.0.0.1.5433 ESTABLISHED

I suspect something triggered the slow sending and it seems to be stuck 
in that state even though

the window is 4096 and all packets are acked.

Any ideas?

Frank