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Message-ID: <4DD5AAFC.8070509@candelatech.com>
Date: Thu, 19 May 2011 16:42:52 -0700
From: Ben Greear <greearb@...delatech.com>
To: Stephen Hemminger <shemminger@...tta.com>
CC: netdev <netdev@...r.kernel.org>
Subject: Re: TCP funny-ness when over-driving a 1Gbps link.
On 05/19/2011 04:20 PM, Ben Greear wrote:
> On 05/19/2011 04:18 PM, Stephen Hemminger wrote:
>> If you overdrive, TCP expects your network emulator to have
>> a some but limited queueing (like a real router).
>
> The emulator is fine, it's not being over-driven (and has limited
> queueing if it was
> being over-driven). The queues that are backing up are in the tcp
> sockets on the
> sending machine.
>
> But, just to make sure, I'll re-run the test with a looped back cable...
Well, with looped back cable, it isn't so bad. I still see a small drop
in aggregate throughput (around 900Mbps instead of 950Mbps), and
latency goes above 600ms, but it still performs better than when
going through the emulator.
At 950+Mbps, the emulator is going to impart 1-2 ms of latency
even when configured for wide-open.
If I use a bridge in place of the emulator, it seems to settle on
around 450Mbps in one direction and 945Mbps in the other (on the wire),
with round-trip latencies often over 5 seconds (user-space to user-space),
and a consistent large chunk of data in the socket send buffers:
[root@...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
Thanks,
Ben
--
Ben Greear <greearb@...delatech.com>
Candela Technologies Inc http://www.candelatech.com
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