[Dovecot] Degeneration of CPU Performance

[Curtis Maloney]

Thiago Monaco Papageorgiou wrote:
>    We have 2 server running with the same load, one with our old pop3 
> solution (out of date) and other with Dovecot. We realized that dovecot 
> are comsuming more CPU, and this consumption is growing day by day. When 
> we starts dovecot, it runs between 40%-45% of CPU consumption and our 
> old solution runs on 30-35%. This is quite acceptable, so no problem 
> here. The problem is one day after it jumps to 45%-55% of cpu 
> comsunption while the old pop3 solution runs on the same CPU consumption 
> of one day before (30%-35). I attached a graph with this information.

Are your using leaving mail on the server?  If not, you may find it 
advantageous to disable the indexing, since it's of no real use for "drive 
by collect" mail.

> In this graph the green line is dovecot CPU consumption and the blue 
> line is the old solution.
> 
> In this graph we restart dovecot at monday morning. This restart is 
> noticed as a big fall of CPU consumption(green line) in the graph. After 
> that, it stay between 40%-45%, one day after it runs on 45%-55% and next 
> day, it reaches 50%-60%! You can notice that the blue line (old pop3 
> solution) runs with the same behavior, so we have the same load. We have 
> other monitoring informations that confirm it.

This, to me, is consistent with Dovecot spending a lot of time indexing the 
overnight deliveries as everyone logs on in the morning.

Are you using dovecot deliver as your LDA?

> Is there any dovecot problem or wrong configuration that could cause 
> this degeneration of CPU performance? Any sugestion about what can be 
> the cause of this?

It could be that Dovecot is doing work you don't need it to.  By which I 
mean building indices.

> I attached dovecot -n output. Two important information: we store the 
> mailboxes in NFS and we are only using pop3 protocol.

As I said above, if your users are using this service ONLY to collect mail, 
not to store it, then the indexes Dovecot tries to maintain are a waste of 
effort.

You can read more about POP3 configuration in the wiki at: 
http://wiki.dovecot.org/POP3Server

The MailLocation page also has some notes about index file placement, including:

"""
If you really want to, you can also disable the index files completely by 
appending :INDEX=MEMORY.
"""

-- 
Curtis Maloney
cmaloney at cardgate.net