Severe problems establishing IMAP connections

[Aidan Evans]

On Mon, 9 Sep 2002 at 16:04 Aidan Evans wrote to info-cyrus at andrew.cmu.edu...

>On Fri, 6 Sep 2002 at 12:24 Aidan Evans wrote to info-cyrus at andrew.cmu.edu...
>
>>  We are experiencing severe but inconsistent problems with establishing
>>IMAP connections.  If I do a "telnet imap.server 143" it just sits at
>>
>>	Trying...
>>	Connected to imap.UCIS.dal.ca.
>>	Escape character is '^]'.
>>
>>Eventually I may get a response from imapd, but other times I never get
>>through.  Curiously, POP connections have similar problems, but much less
>>severe; often they get the pop3d banner just about right away, but not
>>always.
...
>  On the weekend Cyrus was updated to version 2.1.9.  Coincidentally the
>connections sitting in SYN_RECV state, or ESTABLISHED but no process, hgave
>gone away.  However, we still have connection problems.
>
>  What I see now is connections that are established, and have been
>accepted by an imapd, but are not getting any response (the IMAP client
>says).  Looking at the imapd with strace shows something like
>
>	select(0, NULL, NULL, NULL, [ I've lost the rest of the call ]
>
>being repeated over and over.  The imapd is one that has been reused
>several times.  I suspect a simular thing is happening with re-used lmtpd
>processes because eventually sendmail-s stop getting connected and begin
>to accumulate (over 1,500 at one point).
>
>  On the theory that something is getting confused over the multiple
>sessions, I've changed MAX_USE in master/service.h from 250 to 1 in order
>to get a new process for each connection, re-compiled, and replaced imapd,
>pop3d, and lmtpd.

  Performance has been much improved since changing MAX_USE to 1, but this
morning we had another incident of connection problems.  We had a sustained
load of 60 to 90 connections that were ESTABLISHED (netstat) but not yet
accepted by an imapd.  It would take 2 - 4 minutes to get the initial imapd
banner.  It may be coincidence, but during this time a user with a mailbox
with over 3,000 smallish messages (about 1,500 bytes each) was being
migrated from a UW imap server.  The problem went away after the migration
finished.  It seems odd that a high rate of new message file creations
would have an impact on the ability to accept new connections.

Aidan Evans   | Networks & Systems
(902)494-3332 | University Computing & Information Services
              | Dalhousie University, Halifax, N.S., Canada