On Tue, 2013-07-09 at 14:57 +0100, Sam Crawford wrote:
> Hi all,
> 
> This issue persists unfortunately. Attached is a log from an instrumented
> TCP server (the sender), logging CWND values and the retransmits. This has
> been run on two identical servers on the same switch - one at 100Mbit and
> the other at 1Gbit. You can see that a small amount of losses occur after
> 1-2 seconds with the 1Gbit setup, limiting the congestion window to ~200
> MSSs. The 100Mbit server is able to hit a CWND of 1092 stably. These
> results are highly repeatable.
> 
> TSO/GSO/GRO are disabled on all hosts. Packet captures from both ends are
> available upon request.
> 
> Any suggestions gratefully received!

What you are seeing is pretty normal, as the standard pfifo_fast qdisc
allows a queue of up to 1000 packets.

Using 100Mbit with such amount of queueing 'allows' RTT to grow at
insane levels. And since you are below the nominal WAN bandwidth, you
get no packet losses and 'optimal' tcp throughput.

As soon as you allow tcp sender to send more packets than real
bandwidth, you experiment packet losses, and if RTT is big, performance
sunks badly (Depending if SACK and/or tcp timestamps are enabled)

I advise you use a rate limiter, using HTB + fq_codel.

(Allow your LAN traffic to reach 1Gb, but shape the traffic meant for
WAN to 100Mbits)

BTW, 100ms RTT doesn't need 8MB of TCP buffers to fill the pipe. You
only add bufferbloat.

You theoretically need 1.25 MB  (10.000.000 bits)

And the ramp up should be much faster than 60 sec !



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