IMAP hibernate and scalability in general

[Timo Sirainen]

> On 12 Apr 2017, at 3.33, Christian Balzer <chibi at gol.com> wrote:
> 
>> Should be. In general I haven't heard of installations hitting CPU limits in proxies. The problem so far has always been related to getting enough outgoing sockets without errors, which is a server-wide problem. 2.2.29 has one tweak that hopefully helps with that.
>> 
> Which would be? The delayed connection bit?

This one - we're getting these errors a few times per minute after proxy had ~30k sessions:

commit 2ba664518940f4cef7f7339719944f80d0a238ca
Author: Timo Sirainen <timo.sirainen at dovecot.fi>
Date:   Tue Apr 4 13:28:44 2017 +0300

    lib: Increase net_connect*() EADDRNOTAVAIL retries to 20

    4 is too little, since on busy systems it's sometimes failing. These calls
    should be pretty cheap, so lets try if 20 is enough.

    It would be nice if this was configurable, but the only practical way right
    now would be to use environment variable, which is a bit ugly. We could
    try it next if 20 is still not enough.