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List:       postfix-users
Subject:    Re: postfix milter body chunk length
From:       Matthias Schneider <matthias.schneider () rmail ! de>
Date:       2019-08-20 7:44:22
Message-ID: 1b4fbd7d-181b-2e6b-23d2-3bfed7736b88 () rmail ! de
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I want to share my research about my milter tcp speed issue:


I have turned on smtpd -v in master.cf to see the mss in the logfile:

On my ubuntu 18.04 the loopback device has a mtu of 65536 which results 
in a "vstream_tweak_tcp: TCP_MAXSEG 21845 "
This is slow and has the nagle timeout problem.

When I set the mtu to 65535 (or 65534) it still gets TCP_MAXSEG 21845 
(and is still slow).


Once i do "ip link set dev lo mtu 21845" i will get vstream_tweak_tcp: 
TCP_MAXSEG 21793 and the performance is great (only 1 second for 100mb 
body).


Installing the Milter on another machine (which is available throught 
the eno1 (1500mtu) interface i will get TCP_MAXSEG 1448 and still bad 
performance (43secs instead of 1.30m).


Since this is strange i prefer to switch to unixsocket.


I also researched where the delay happened in my milter: its while 
waiting for the 64k packet 
https://github.com/mschneider82/milter/blob/b4a6d7c78ac39b8a1d71b53b74bc08d95652d310/session.go#L101


Best regards

Matthias Schneider



Am 19.08.19 um 20:05 schrieb Wietse Venema:
> 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
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