[c-nsp] Centos upload speed slower on 1000m than 100m over WAN links

[Joe Loiacono]

OK I'm jumping in on this thread late as I just got back from some 
vacation, don't know if this particluar observation has been discussed, 
but ...

We've seen this problem a lot when moving up to new local connection 
speeds. The problem for us has been that unless the entire path can 
support the new speed (e.g., 1G) switches down the path that connect to 
slower speeds (e.g. 100M) will overflow and put your data transfer into 
TCP slow-start recovery. As soon as the sending NIC is 'downgraded' (e.g., 
back to 100 M) the overflows disappear, slow-start is avoided, and 
performance improves. Bitterly ironic.

Joe



From:
Paul <paul at gtcomm.net>
To:
cisco-nsp at puck.nether.net
Date:
06/27/2010 03:08 AM
Subject:
[c-nsp] Centos upload speed slower on 1000m than 100m over WAN links



I'm not even sure this is the right forum but since we use mainly Cisco 
equipment I'll give this a shot. :)
I have tried several centos based servers and compiled various kernels 
and the results have been extremely weird.
90% of the cases the remote hosts can download from a server at 
1-5megabytes per second, and most of these are over
the internet ranging from 30-200ms away.  Local (1ms or less) is super 
fast 100MB/s for example. 
Ok that sounds normal since it's going over the internet, etc.  But 
here's the )(!@*! part..
If I set the port speed to 100 megabits full duplex on the switch and 
server , the clients that get 1-5MB/s now get 11MB/s which is
approximately the limit of the 100mbit port. 
Totally stumped here, tried different nics, servers, even 4 different 
switches.  Is a very interesting problem and I'm probing to see
if anyone else has encountered it. 
So far the only OS i have tried is centos, but different versions and 
kernels and hardware.
All the switches/routers are Cisco based, but I seriously doubt that has 
anything to do with this. :P

-- 
GloboTech Communications
Phone: 1-514-907-0050 x 215
Toll Free: 1-(888)-GTCOMM1
Fax: 1-(514)-907-0750
paul at gtcomm.net
http://www.gtcomm.net 

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