[Dovecot] A large number of imap processes

[Timo Sirainen]

On Fri, 2008-04-18 at 14:14 +0200, Claude Frantz wrote:
> When a large number of messages for many different users arrive nearly 
> at the same time (e.g. a message to all the users), a large number of 
> sleeping processes become active, the CPU usage increases to more than 
> 80 or 90 % and the LA can be many hundreds for a long time. The machine 
> is not more really operational from user's point of view. NFS is 
> probably a problem here, but some NFS tests have not allowed to discover 
> problems at this level. Now, I'm trying to find a "solution" in the way 
> to delay the delivery of the messages to the maildirs. I think, it is 
> not really a nice solution.
> 
> I think that the large number of now running processes is the rebuilding 
> of the index. I am right here, Timo ? 

Index updates typically don't take long. Although if client fetches some
metadata that isn't in cache file, Dovecot needs to open the messages
and parse them. That takes some I/O. This can be avoided with v1.1 by
using deliver, since it updates the cache file immediately.

Although I'm still not sure if that would help, because a lot of clients
just download the entire message body automatically, which in any case
causes I/O.

> At the present time, the mailfilter language of 
> maildrop (and Courier) is used here in place of procmail used in the 
> past. I ignore if using dovecot's LDA could be a solution in this 
> environment. But the mailfilter language could be a problem. A migration 
> to sieve is not really a choice, in my opinion. I have never seen a good 
> sieve script testing software.

Can you tell maildrop to deliver the message to a specific mailbox using
an external command (i.e. calling deliver) instead of delivering itself?
Probably wouldn't be a difficult change to the code if it doesn't
support it.
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