Bad TCP performance with large MTU on 7-stable.

[Arnaud Houdelette]

I got really poor performance when I try to upload files to the box (via 
pureFTP or Samba) when using jumbo frames somewhat  above  2k.

uname -a :
FreeBSD carenath.tzim.net 7.0-STABLE FreeBSD 7.0-STABLE #4: Wed May 28 
17:45:14 CEST 2008     
tzim at carenath.tzim.net:/usr/obj/usr/src/sys/CARENATH  amd64

It seems that TCP acks are delayed for about a second, so the upload 
rate nevers goes above 60kB/s. I can provide wireshark captures done on 
the box uploading the files if necessary.
The client is a Windows XP64 box, but I do not recall having the same 
problem on 6.2. I did got about 100 MB/s for similar file transfers, but 
I had to disable jumbo frames when I upgraded to 7.0-Release because of 
some bug on txcsum/rxcsum on re(4) so I only noticed the issue recently.

The txcsum bug doesn't seem to occur anymore, but now I have this 
strange rate issue, with or without checksum offloading disabled.
I also tried with tso disabled. Same results. Is it related to the re(4) 
driver ? Or to the TCP stack ?


Arnaud Houdelette