Re: fix TCP roundtrip time update code
flat view
From: Martin Josefsson <hidden>
Date: 2003-06-03 17:27:46
Also in:
lkml
(trimmed CC line and added netdev) On Tue, 2003-06-03 at 17:52, David Mosberger wrote:
One of those very-hard-to-track-down, trivial-to-fix kind of problems: without this patch, TCP roundtrip time measurements will corrupt the routing cache's RTT estimates under heavy network load (the bug causes RTAX_RTT to go negative, but since its type is u32, you end up with a huge positive value...). From there on, later TCP connections quickly will go south. The typo was introduced 8 months ago in v1.29 of the file by the patch entitled "Cleanup DST metrics and abstrct MSS/PMTU further".
I tested this patch and it looks like it has cured my mysterious TCP
stalls.
without patch:
cache mtu 1500 rtt 479411ms rttvar 953813ms cwnd 46 advmss 1460
I see that before and during the stall if not using this patch.
(rtt is never above 20ms accoring to ping)
With the patch I see normal rtt and rttvar times.
Havn't seen a stall yet (~30 kernelcompiles with distcc over a sometimes
congested link), will continue testing.
quoted hunk
===== net/ipv4/tcp_input.c 1.36 vs edited =====--- 1.36/net/ipv4/tcp_input.c Mon Apr 28 09:27:57 2003 +++ edited/net/ipv4/tcp_input.c Tue Jun 3 08:19:36 2003@@ -556,8 +556,8 @@ if (m >= dst_metric(dst, RTAX_RTTVAR)) dst->metrics[RTAX_RTTVAR-1] = m; else - dst->metrics[RTAX_RTT-1] -= - (dst->metrics[RTAX_RTT-1] - m)>>2; + dst->metrics[RTAX_RTTVAR-1] -= + (dst->metrics[RTAX_RTTVAR-1] - m)>>2; } if (tp->snd_ssthresh >= 0xFFFF) {
-- /Martin