[Dovecot] POP3 vs. IMAP Load/Memory usage in Dovecot 1.0.15

[Stan Hoeppner]

On 7/11/2011 4:28 AM, lists at truthisfreedom.org.uk wrote:
> Quoting Stan Hoeppner <stan at hardwarefreak.com>:
> 
>> On 7/11/2011 1:24 AM, Matthew Macdonald-Wallace wrote:
>>> 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.
>>
>> More specific information would be helpful.  Load as shown through top
>> doesn't really tell anything.  Are you simply seeing memory pressure?
>> Is all that RAM being used for block device cache or actually eaten by
>> the pop servers?
> 
> Hi Stan,
> 
> Thanks for getting back to me.
> 
> The Load average comparisons are taken from Munin graphs and based upon
> the servers being in production for five days between Monday and Friday.

This still doesn't provide us with the necessary information to give you
an intelligent answer to your question.  You've told us you have a
Mustang and a Camaro and that one burned more gas in a week than the
other.  You didn't tell us the driving conditions of each, whether both
city driving, or one city and one highway, winter or summer, or if
grandma was driving one and Mario Andretti driving the other.  The
details matter.

> The vast majority of the RAM usage is cache, however there is still a
> discrepancy between the IMAP servers and the POP3 servers.

A discrepancy where?  RAM usage by the pop and imap processes?  Is there
any reason why you didn't post the actual data?

> I guess all I'm really after knowing is if there is a reason why this is
> the case so I can put my mind (and those of my team!) at ease before we
> start making other changes to the infrastructure - the last thing I want
> to do is increase the load on these nodes and watch them die because
> they didn't have enough resources.

You still have not demonstrated what resources, if any, these nodes are
lacking.  The only thing you have mentioned is memory consumption.  All
Unices today will dump cache pages if a process needs memory space and
will instantly reallocate it.  If the bulk of the RAM on these systems
is consumed by disk cache, you don't have a problem.  If the "load" you
mentioned is caused by something other then memory usage, then can you
please show detail of such?  Could you at least provide a snapshot of
top output from one pop and one imap machine?

I feel like I'm pulling teeth here.  You've made two posts about this
issue and provided zero technical detail in either.  Make this easier on
both of us, and post some darn detail.

-- 
Stan