[Dovecot] POP3 vs. IMAP Load/Memory usage in Dovecot 1.0.15
lists at truthisfreedom.org.uk
lists at truthisfreedom.org.uk
Mon Jul 11 19:22:45 EEST 2011
>> * All the servers are made by the same manufacturer (Dell)
>> * They are all the same model (R410)
>> * The have the same engine (24 cores, 24G RAM, SAS Drives)
>
> 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.
OK, I'll take a look at this, thanks.
>> * The motorway is exactly the same for all servers (NFS to a NetApp 6080
>> and a RAMSAN)
>> * The weather is almost exactly the same (Same Datacentre, different
>> rooms/racks)
>> * The Driver is exactly the same (Dovecot 1.0.15)
>
> What operating system? Linux or *BSD? If Linux, what kernel version?
> 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.
Linux, Centos 5.6 and (yup, you've guessed it...) 2.6.18 again, I'll
take a look at this, thanks.
>> 1) Load Average
>
> On Linux, load average strictly shows total system CPU usage in
> intervals, nothing else. Neither memory, disk, nor network or anything
> else affects load average. Thus, with a 12 core system, until you see a
> load average above 12 you have absolutely nothing to worry about. With
> HT enabled load averages pretty much go out the window as half the
> "CPUs" are merely glorified duplicate register file phantoms.
>
> 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. This assuming you
> run a newer kernel and with HT disabled. In other words, no more than 4
> cores worth of CPU time will ever be eaten by your workload. What
> number do your Munin graphs show for load average for each set of boxes?
> Do they even come close to 4?
They're showing as between 20 and 24 for the POP3 servers and 1.4 for
the IMAP servers.
> Also note that TCP stack processing on the pop nodes will be greater
> than that of the imap boxes, eating more CPU cycles. More data sent
> over the wire means more packets, more packets means more CPU time in
> both code/data processing and interrupts. If you're running iptables
> rules on each host that bumps up network processing cycles a bit more yet.
OK, I'll take a look at that as well
>> 2) RAM Usage (particularly in regard to cache)
>
>> In both cases, the value for each area is higher on the three nodes
>> running POP3 than the nodes running IMAP.
>
> Almost all the memory consumption on both systems is buffer cache. Thus
> you don't have a memory issue on either host. The kernel will free and
> immediately reassign pages from cache to application processes as
> needed. I don't see evidence of the pop machine using more memory, in
> fact the imap processes are using more. Both boxes are just under 24GB
> total usage and both using right at 20GB of cache. Looks like a default
> config Linux kernel based on the ultra aggressive caching and eating up
> nearly all memory.
So a kernel update is more than sensible...
> It may have been. I'll know when you post your load numbers from those
> top secret graphs. ;)
LOL, see above.
Thanks again,
Matt
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