Hi,
I found another regression with Jan's TCP diff that sends less ACK
packets. relayd run-args-http-slow-consumer.pl fails on i386 due
to his commit. This test writes a lot of data from the http server,
but blocks receive for 2 seconds in the client. Relayd between
these machines should handle the delay. The socket buffer size is
very small to trigger the situation reliably.
The current TCP stack does not recover after the delay. Packets
are sent very slowly and the regress test runs in a timeout. When
I backout the change, the test passes quickly.
Ususally the test runs on localhost loopback. There the problem
is not triggered. Only my i386 regress setup uses a remote machine.
As Jan's change send less ACKs that have to be handled, throughput
increases when TCP/IP stack is CPU bound. This can give a 20%
increase.
Due to the maxburst limitation, throughput may also drop by 70% on
a single connection. I have removed this limitation, so it does
not happen if the other side is a current OpenBSD. But TCP connection
between -current and 6.8 can run into this problem.
Now I have found another corner case in a regress test where we get
only 10 TCP packets per second. But that may be the result of
unrealistic settings of the socket buffer size.
Should we back it out for release? Diff below.
bluhm
Index: netinet/tcp_input.c
===================================================================
RCS file: /data/mirror/openbsd/cvs/src/sys/netinet/tcp_input.c,v
retrieving revision 1.366
retrieving revision 1.365
diff -u -p -r1.366 -r1.365
--- netinet/tcp_input.c 3 Feb 2021 13:40:06 -0000 1.366
+++ netinet/tcp_input.c 19 Jun 2020 22:47:22 -0000 1.365
@@ -165,8 +165,8 @@ do { \
#endif
/*
- * Macro to compute ACK transmission behavior. Delay the ACK until
- * a read from the socket buffer or the delayed ACK timer causes one.
+ * Macro to compute ACK transmission behavior. Delay the ACK unless
+ * we have already delayed an ACK (must send an ACK every two segments).
* We also ACK immediately if we received a PUSH and the ACK-on-PUSH
* option is enabled or when the packet is coming from a loopback
* interface.
@@ -176,7 +176,8 @@ do { \
struct ifnet *ifp = NULL; \
if (m && (m->m_flags & M_PKTHDR)) \
ifp = if_get(m->m_pkthdr.ph_ifidx); \
- if ((tcp_ack_on_push && (tiflags) & TH_PUSH) || \
+ if (TCP_TIMER_ISARMED(tp, TCPT_DELACK) || \
+ (tcp_ack_on_push && (tiflags) & TH_PUSH) || \
(ifp && (ifp->if_flags & IFF_LOOPBACK))) \
tp->t_flags |= TF_ACKNOW; \
else \
