Matthias Schneider:
> Hi Wietse,
> 
> 
> > I suspect that the bottleneck is on the receiving side.
> >
> > - Maybe the loopback stack does not like the 65535 block size. Try
> > using an ethernet interface address instead.
> 
> I tried switching from interface lo to eno1 - same issue.

What's the en01 MTU size? Postfix has code (in vstream_tweak.c
which is called by the Milter client and other Postfix code) to
automatically increase the VSTREAM buffer from 4096 to the maximal
segment size, to avoid Nagle delays, though that would not help if
the MTU is > 65535 (the Milter's 'packet' size).

I would therefore not expect Nagle delays on en01.

> I compared some tcpdumps with postfix as client and my milterclient 
> (which is fast) as client.
> 
> It seems that we run into nagle timeout (the ACK is always delayed).
> Setting TCP_QUICKACK on my milter socket doesnt help, is there a way to 
> set TCP_NODELAY on the milter connection in postfix?

You've got the source.

	Wietse