read() returns ETIMEDOUT on steady TCP connection
mark at pogo.org.uk
Sat Apr 19 14:27:30 UTC 2008
I'm are having a trouble with TCP connections being dropped with "read:
Operation timed out". What is unusual is that this is happening right in
the middle of sending a steady stream of data with no network congestion.
The system is FreeBSD 7 and a bespoke streaming server with 1Gbit
connection. The server receives a 192kbps inbound stream over TCP, and
broadcasts it over a large number of TCP streams.
With no visible or obvious pattern, the inbound read() fails with
ETIMEDOUT. The likelihood of this happening seems to increase as the
number of audience connections increases. It's happens every few minutes
even with a small audience (eg. 300 outbound connections and about
It doesn't cough and splutter -- steady data is coming in, then it just
drops the connection.
systat doesn't show problems inbound; all packets received are delivered
to the upper layer. But on outbound, there is consistent 'output drops':
7028 total packets sent
7028 - generated locally
314 - output drops
As the number of outbound connections increases, the 'output drops'
increases to around 10% of the total packets sent and maintains that
ratio. There's no problems with network capacity.
I've tried different servers, different network interfaces (bge, em),
different kernel (7-RELEASE, 7-STABLE). Have also checked dev.bge.0.stats
and dev.em.0.stats for CRC errors etc. which show no problems. 'netstat
-m' doesn't show any reaching of mbuf and sbuf limits. The problem is seen
in a dedicated, uncontended test environment.
Can anyone explain why the packets are being dropped outbound, and how
this could affect inbound TCP data in such an abrupt way? What can I do to
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