Bill Moran wrote:
This one has got me pretty befuddled.
We're seeing some really odd behaviour with FreeBSD ignoring SYN packets.
I've been trying to diagnose this for a couple of weeks now, and my current
guess is that there's something wrong with the em driver. Here's a narrowed
down list of what I've ruled out:
*) I've done my best to eliminate other network components as the problem.
My theory at this point is that it can't possibly be any other network
hardware, based on the tcpdump show below.
*) The problem occurred on both FreeBSD 6.1 and FreeBSD 6.2-p3.
*) The problem does not appear to be tied to CPU usage -- the CPU is nearly
idle when the problem occurs.
*) I can now reproduce it pretty easily, so I'll know when it's fixed.
*) The system exhibiting the problem is running 15 jails, but they are
idle 95% of the time. The problem initially occurred inside one of
the jails, but I just recreated it outside the jail (on the host) and
it's _easier_ to reproduce outside the jail.
*) The problem occurred with both GENERIC, and the SMP kernel (this is a
dual-CPU, hyperthreaded system)
*) I've tested and the behavior occurs both with a dynamically generated
file (from PHP) or from a static file.
The nature of the beast is that we've got a SOAP application running under
Apache and PHP. This application is subject to many requests in rapid
succession, such that load can be simulated by the following loop:
while true; do fetch http://192.168.121.250/test.php; done
The problem is that occasionally, the Apache server machine just ignores
SYN packets. Take the following tcpdump output for example:
13:34:17.312296 IP web04-v100.cust00.pitbpa1.priv.collaborativefusion.com.54808 >
anchor-is00.is.pitbpa1.priv.collaborativefusion.com.http: S 2645061726:2645061726(0) win
65535 <mss 1380,nop,wscale 1,nop,nop,timestamp 2690201156 0,sackOK,eol>
13:34:20.312398 IP web04-v100.cust00.pitbpa1.priv.collaborativefusion.com.54808 >
anchor-is00.is.pitbpa1.priv.collaborativefusion.com.http: S 2645061726:2645061726(0) win
65535 <mss 1380,nop,wscale 1,nop,nop,timestamp 2690204156 0,sackOK,eol>
13:34:23.512626 IP web04-v100.cust00.pitbpa1.priv.collaborativefusion.com.54808 >
anchor-is00.is.pitbpa1.priv.collaborativefusion.com.http: S 2645061726:2645061726(0) win
65535 <mss 1380,nop,wscale 1,nop,nop,timestamp 2690207356 0,sackOK,eol>
This is the _only_ traffic on port 80 during the test. It looks like the
kernel has ignored the initial syn packet and two duplicates. I've seen it
take as long as 45 seconds to establish a connection, and this causes
ugly performance problems, as well as frequent timeouts on the client end.
The only clue I've found so far is this output from netstat -s.
153099 syncache entries added
6184 retransmitted
6491 dupsyn
0 dropped
150923 completed
0 bucket overflow
0 cache overflow
235 reset
1941 stale
0 aborted
0 badack
0 unreach
0 zone failures
Unfortunately, I've been unable to determine how to fix the problem. Any
advice is welcome.
Before we go into more detail:
a) the em(4) driver is most likely totally unrelated to this
b) you may run out of socket on the client side and reuse them
too fast. Try to lower net.inet.ip.portrange.first to 30,000
or so.
c) related to (b) you may have a lot of connections in TIME_WAIT
on the server catching not really stray packets. Try it with
net.inet.tcp.nolocaltimewait=1
d) if the above didn't help then it'd be very helpful to test
against a server with FreeBSD-current (the future 7.0) on
it. In -current we've got detailed logging of LISTEN socket
failures that allow rapid analysis of the problem.
--
Andre
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