dovecot - Jul 2011 - POP3 vs. IMAP Load/Memory usage in Dovecot 1.0.15

Hi all,

We've just provisioned a new cluster of dovecot nodes running Centos  
and Dovecot 1.0.15 (we needed to match the original configuration,  
we're upgrading to 1.2 next week!).

The nodes are currently equally allocated (50/50 split) to IMAP and  
POP3, with the intention to move them into a single cluster hosting  
both services in the next month.

All the servers are of identical spec (24 cores, 24G RAM) and are  
configured to load the indices, control files and maildirs via NFS.

We have noticed that the IMAP servers appear to be under much less  
load and utilising drastically less RAM than the POP3 servers and I'm  
wondering if there is a reason for this as we have seen some swapping  
onto disk yet we are only handling 500 concurrent POP3 connections to  
each server at any given time compared with over 600 IMAP connections.

I'm wondering if we've missed a config flag somewhere or (better  
still!) this issue will go away when we upgrade to 1.2.

If anyone can shed any light on this, that would be much appreciated.

Thanks in advance,

Matt

On Fri, 2011-07-08 at 10:48 +0100, lists at truthisfreedom.org.uk
wrote:
> We have noticed that the IMAP servers appear to be under much less  
> load and utilising drastically less RAM than the POP3 servers and I'm  
> wondering if there is a reason for this as we have seen some swapping  
> onto disk yet we are only handling 500 concurrent POP3 connections to  
> each server at any given time compared with over 600 IMAP connections.
Am I to take it that this is expected behaviour?

If anyone can shed more light on this I'd be very grateful.

Thanks,

Matt

At 01:59 -0500 12/7/11, Stan Hoeppner wrote:
>On 7/12/2011 1:12 AM, Rainer Frey wrote:
>>
>>  On 11.07.2011, at 17:03, Stan Hoeppner wrote:
>>
>>>  The R410 is a two socket Xeon box with max 2 x 6 core CPUs.  The
24 CPUs
>>>  you see is the result of HyperThreading being enabled.  I'd
disable HT
>>>  if I were you, or those boxen mine.
>>
>>  Why?
>
>It's a troubleshooting step.  HT can cause weird problems with some
>systems/kernels.  It can also decrease performance with some workloads.
>  As with anything, if it doesn't provide benefit, turn if off to reduce
>complexity and potential problems.
Or, another way, HT only helps when having twice as many (apparent) 
processors running at half the (effective) speed is good for your 
specific workload. Just because it's a band-waggon it doesn't mean 
you have to jump onto it.

David


-- 
David Ledger - Freelance Unix Sysadmin in the UK.
HP-UX specialist of hpUG technical user group (www.hpug.org.uk)
david.ledger at ivdcs.co.uk
www.ivdcs.co.uk