Poor network performance for clients in 100MB to Gigabit environment

Paul paul at gtcomm.net
Tue Jul 1 20:19:02 UTC 2008

What options do you have enabled on the linux server?
sysctl -a | grep net.ipv4.tcp
and on the bsd
sysctl -a net.inet.tcp

It sounds like a problem with BSD not handing the dropped data or ack 
packets so what happens is it pushes a burst of
data out > 100mbit and the switch drops the packets and then BSD waits 
too long to recover and doesn't scale the transmission
back.  TCP is supposed to scale down the transmission speed until 
packets are not dropped to a point even without ECN.

Options such as 'reno' and 'sack' etc. are congestion control algorithms 
that use congestion windows.

David Kwan wrote:
> I have a couple of questions regarding the TCP Stack:
> I have a situation with clients on a 100MB network connecting to servers
> on a Gigabit network where the client read speeds are very slow from the
> FreeBSD server and fast from the Linux server; Write speeds from the
> clients to both servers are fast.  (Clients on the gigabit network work
> fine with blazing read and write speeds).  The network traces shows
> congestion packets for both servers when doing reads from the clients
> (dup acks and retransmissions), but the Linux server seem to handle the
> congestion better. ECN is not enabled on the network and I don't see any
> congestion windowing or clients window changing.  The 100MB/1G switch
> is dropping packets.   I double checked the network configuration and
> also swapped swithports for the servers to use the others to make sure
> the switch configuration are the same, and the Linux always does better
> than FreeBSD.  Assuming that the network configuration is a constant for
> all clients and servers (speed, duplex, and etc...), the only variable
> is the servers themselves (Linux and FreeBSD).  I have tried a couple of
> FreeBSD machines with 6.1 and 7.0 and they exhibit the same problem,
> with no luck matching the speed and network utilization of Linux (2
> years old).  The read speed test I'm referring is doing transferring of
> a 100MB file (cifs, nfs, and ftp), and the Linux server does it
> consistently in around 10 sec (line speed) with a constant network
> utilization chart, while the FreeBSD servers are magnitudes slower with
> erratic network utilization chart.  I've attempted to tweak some network
> sysctl  options on the FreeBSD, and the only ones that helped were
> disabling TSO and inflight; which leads me to think that the
> inter-packet gap was slightly increased to partially relieve congestion
> on the switch; not a long term solution.
> My questions are: 
>   1. Have you heard of this problem before with 100MB clients to Gigabit
> servers?
>   2. Are you aware of any Linux fix/patch in the TCP stack to better
> handling congestion than FreeBSD?  I'm looking to address this issue in
> the FreeBSD, but wondering if the Linux stack did something special that
> can help with the FreeBSD performance.
> David K.
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