Hitting wall at 2048 IMAP connections

[Steffen Kaiser]

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On Tue, 3 Feb 2015, Ron Cleven wrote:

> We are gradually rolling out Dovecot (IMAP only, no POP3) to our customer 
> base.  We are replicating between a pair of CentOS 7 boxes.  All has been 
> working wonderfully.  However, to be sure our rollout continues to go 
> smoothly, we put together a simple benchmark client program to fire up X 
> persistent IMAP connections (using hundreds of mailboxes) that login, list 
> the folders, select the INBOX, periodically FETCH an email from the INBOX, 
> and otherwise sit in an IDLE loop.  As long as we keep the number of 
> concurrent IMAP connections under 2k, everything works fine.  2,000 processes 
> generate a minimal load on the server.  However, 2048 is a "hard limit" for 
> number of IMAP processes on a box the way things are configured right now. 
> The number (2048) is a bit too magical to be anything but a misconfiguration 
> of either some kernel limit or a dovecot configuration limit.  I have been 
> unable to figure out where else to look.  I have pasted in my dovecot config 
> below along with the kernel  setting for max_user_instances.
>
> Hoping someone can tell me what stupid mistake I have made or what else to 
> check.
>
> Just an aside, we front-end this with a separate set of proxy servers that 
> provide an SSL front-end, hence, no SSL in the dovecot config.
>
> /proc/sys/fs/inotify/max_user_instances = 4096

Do you use virtual users? Then this setting is way too low.
There should be entries in kernel log or Dovecot.

- -- 
Steffen Kaiser
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