On 05/19/2011 05:46 PM, Rick Jones wrote:

On Thu, 2011-05-19 at 17:37 -0700, Ben Greear wrote:

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On 05/19/2011 05:24 PM, Rick Jones wrote:

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[root@i7-965-1 igb]# netstat -an|grep tcp|grep 8.1.1
tcp        0      0 8.1.1.1:33038               0.0.0.0:*                   LISTEN
tcp        0      0 8.1.1.1:33040               0.0.0.0:*                   LISTEN
tcp        0      0 8.1.1.1:33042               0.0.0.0:*                   LISTEN
tcp        0 9328612 8.1.1.2:33039               8.1.1.1:33040               ESTABLISHED
tcp        0 17083176 8.1.1.1:33038               8.1.1.2:33037               ESTABLISHED
tcp        0 9437340 8.1.1.2:33037               8.1.1.1:33038               ESTABLISHED
tcp        0 17024620 8.1.1.1:33040               8.1.1.2:33039               ESTABLISHED
tcp        0 19557040 8.1.1.1:33042               8.1.1.2:33041               ESTABLISHED
tcp        0 9416600 8.1.1.2:33041               8.1.1.1:33042               ESTABLISHED

I take it your system has higher values for the tcp_wmem value:

net.ipv4.tcp_wmem = 4096 16384 4194304

Yes:
[root@i7-965-1 igb]# cat /proc/sys/net/ipv4/tcp_wmem
4096	16384	50000000

Why?!?  Are you trying to get link-rate to Mars or something?  (I assume
tcp_rmem is similarly set...)  If you are indeed doing one 1 GbE, and no
more than 100ms then the default (?) of 4194304 should have been more
than sufficient.

Well, we occasionally do tests over emulated links that have several
seconds of delay and may be running multiple Gbps.  Either way,
I'd hope that offering extra RAM to a subsystem wouldn't cause it
to go nuts.

It has been my experience that the autotuning tends to grow things
beyond the bandwidthXdelay product.

Seems a likely culprit, or somehow it's not detecting round-trip-time
correctly, or maybe the timestamp is calculated when the pkt goes into
the send queue, and not when it's actually sent to the NIC?

As for several seconds of delay and multiple Gbps - unless you are
shooting the Moon, sounds like bufferbloat?-)

We try to test our stuff in all sorts of strange cases.  Maybe
some users really are emulating lunar traffic, or even beyond.
We also can emulate buffer bloat..but in this particular case,
real round-trip time is about 1-2ms, so if the socket is queuing up
a second's worth of bytes on the xmit buffer, then it's not
the network's fault...it's the sender.

quoted

Assuming this isn't some magical 1Gbps issue, you
could probably hit the same problem with a wifi link and
default tcp_wmem settings...

Do you also increase tx queue's for the NIC(s)?

No, they are at the default (1000, I think).  That's only
a few ms at 1Gbps speed, so the problem is mostly higher
in the stack.

Thanks,
Ben

rick

-- 
Ben Greear [off-list ref]
Candela Technologies Inc  http://www.candelatech.com