Bad TCP performance with large MTU on 7-stable.

[Pyun YongHyeon]

On Thu, May 29, 2008 at 04:25:33PM +0200, Arnaud Houdelette wrote:
 > 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 ?
 > 

It seems that handling jumbo frame in re(4) is one of trickiest
feature. Would you show me the following information?
 - Captured tcpdump data on receiving side(i.e. host with re(4))
    in your test.
 - dmesg output of re(4)
 - MTU size you've configured
 - output of "netstat -nd -I re0"
 - output of "ifconfig re0"

-- 
Regards,
Pyun YongHyeon