On Wed, 30 Jan 2008 08:01:46 -0600 (CST)
Bruce Allen <ballen@gravity.phys.uwm.edu> wrote:

quoted text
> Hi David,
> 
> Thanks for your note.
> 
> >> (The performance of a full duplex stream should be close to 1Gb/s in
> >> both directions.)
> >
> > This is not a reasonable expectation.
> >
> > ACKs take up space on the link in the opposite direction of the
> > transfer.
> >
> > So the link usage in the opposite direction of the transfer is
> > very far from zero.
> 
> Indeed, we are not asking to see 1000 Mb/s.  We'd be happy to see 900 
> Mb/s.
> 
> Netperf is trasmitting a large buffer in MTU-sized packets (min 1500 
> bytes).  Since the acks are only about 60 bytes in size, they should be 
> around 4% of the total traffic.  Hence we would not expect to see more 
> than 960 Mb/s.
> 
> We have run these same tests on older kernels (with Broadcomm NICS) and 
> gotten above 900 Mb/s full duplex.
> 
> Cheers,
>      Bru

Don't forget the network overhead: http://sd.wareonearth.com/~phil/net/overhead/
 Max TCP Payload data rates over ethernet:
  (1500-40)/(38+1500) = 94.9285 %  IPv4, minimal headers
  (1500-52)/(38+1500) = 94.1482 %  IPv4, TCP timestamps

I believe what you are seeing is an effect that occurs when using
cubic on links with no other idle traffic. With two flows at high speed,
the first flow consumes most of the router buffer and backs off gradually,
and the second flow is not very aggressive.  It has been discussed
back and forth between TCP researchers with no agreement, one side
says that it is unfairness and the other side says it is not a problem in
the real world because of the presence of background traffic.

See:
  http://www.hamilton.ie/net/pfldnet2007_cubic_final.pdf
  http://www.csc.ncsu.edu/faculty/rhee/Rebuttal-LSM-new.pdf
   

-- 
Stephen Hemminger <stephen.hemminger@vyatta.com>


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