[Dovecot] Ongoing performance issues with 2.0.x

[Ralf Hildebrandt]

* Stan Hoeppner <stan at hardwarefreak.com>:

> Does this machine have more than 4GB of RAM?  You do realize that merely
> utilizing PAE will cause an increase in context switching, whether on
> bare medal or in a VM guest.  It will probably actually be much higher
> with a VM guest running a PAE kernel.  Also, please tell me the ESX
> kernel you're running is native 64 bit, not 32 bit.  If the VMWare
> kernel itself is doing PAE, as well as the guest Linux kernel, this may
> fully explain the performance disaster you have on your hands, if it is
> indeed due to context switching.

It sure work with 1.2.x now, so that's not really the problem

> The bigger question is, why does this problem surface so readily while
> running Dovecot 2.0.x and not while running Dovecot 1.2.x?

EXACTLY

> Is 1.2.x merely tickling the dragon's chin, whereas 2.0.x is sticking
> it's head into the dragon's mouth?

I'd say the difference between 1.2 and 2.0 is so dramatic that it's
probably something else.

> This very well may be the case.  You need to also look at the CONFIG_HZ=
> value of the Linux kernel of the guest.  If it's a tickless kernel you
> should be fine.  If tickless, IIRC, you should see CONFIG_NO_HZ=y.

# fgrep HZ config-2.6.32-23-generic-pae
CONFIG_NO_HZ=y
# CONFIG_HZ_100 is not set
CONFIG_HZ_250=y
# CONFIG_HZ_300 is not set
# CONFIG_HZ_1000 is not set
CONFIG_HZ=250
CONFIG_MACHZ_WDT=m

-- 
Ralf Hildebrandt
  Geschäftsbereich IT | Abteilung Netzwerk
  Charité - Universitätsmedizin Berlin
  Campus Benjamin Franklin
  Hindenburgdamm 30 | D-12203 Berlin
  Tel. +49 30 450 570 155 | Fax: +49 30 450 570 962
  ralf.hildebrandt at charite.de | http://www.charite.de