From: Wietse Venema (wietseporcupine.org)
Date: Wed Aug 01 2007 - 18:16:49 CDT

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Mark Martinec:
> > Postfix stable release 2.4.5 fixes a one-bit typing error that
> > slipped in during code cleanup for the Postfix 2.4.4 release,
> > making the loopback TCP performance workaround ineffective.
>
> > Loopback TCP performance workaround:
> > Some systems exhibited poor SMTP and Milter performance with
> > loopback (127.0.0.1) connections.
>
> Perfect! I tried with postfix-current-2.5.20070731 and transfer
> of a large mail body to a milter on a loopback interface is now
> more than 50 times faster (38 MB/s).
> Capture: http://www.ijs.si/~mark/tmp/dkim-pf-20070731.tcpdump.bz2

The workaround is version dependent (#ifdef VSTREAM_CTL_BUFSIZE).

- Postfix 2.5 uses "large" VSTREAM buffers on connections with
  "large" MSS.

- Postfix 2.[34] use the same old 4096 byte VSTREAM buffers and
  turn off Nagle delays on connections with "large" MSS.

The 2.5 VSTREAM code needs to prove itself first for some time.
It should make very little difference with today's loopback
interfaces.

        Wietse

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