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

[Miquel van Smoorenburg]

On 11-07-11 5:03 PM, Stan Hoeppner wrote:
> Given that you're running Dovecot 1.0.15 I'm guessing you're using
> CentOS or RHEL 5.x and thus have kernel 2.6.18-xxx.  2.6.18 is 5 years
> old now and not inappropriate for a modern 2 socket, 6 core
> HyperThreading box.  You need a much newer kernel, preferably in the
> 2.6.3x series.  2.6.18 could be reporting incorrect load numbers on
> these machines.

RHEL kernel version numbers do not say much. The redhat 2.6.18 is 2.6.18 
+ a boatload of "enterprise load" patches and backports from 2.6.2x. 
OTOH, dovecot 1.0.15 is ancient indeed :)

>> The discrepancies lie in two areas:
>>
>> 1) Load Average
>
> On Linux, load average strictly shows total system CPU usage in
> intervals, nothing else.

That would be FreeBSD, AFAIK. On linux, I/O does add to the load 
average. A process in state 'D' (Disk wait, could be NFS wait too btw) 
adds '1' to the load. If you have a broken NFS server and 2000 processes 
waiting on I/O, the reported load will go over 2000.

You get a better impression of system load by running 'top' and paying 
attention to the number on the 'cpu' line: us == time spent in user 
process, sy = kernel, id = idle, wa = I/O wait, si = interrupts

Press '1' while in top to expand the view to all CPUs seperately. Quite 
enlightening.

> Given that all mail apps are 100% IO bound, never CPU or memory bound,
> I'd guess you'll never see a load average over 4.00 on any of these
> machines with less than 1000 concurrent connections.

Well, see above. Also, if you have SSL enabled, the crypto will actually 
eat quite a bit of CPU if you have a lot of network traffic.

Mike.