[Dovecot] load increase after upgrade to 2.0.8

[Mark Moseley]

On Wed, Dec 8, 2010 at 3:03 PM, Timo Sirainen <tss at iki.fi> wrote:
> On 8.12.2010, at 22.52, Cor Bosman wrote:
>
>> 1 server with service_count = 0, and src/imap/main.c patch
>
> By this you mean service_count=0 for both service imap-login and service imap blocks, right?
>
>

Speaking from my own experience, the system loads on our dovecot boxes
went up *substantially* when we upgraded kernels from 2.6.32.x and
2.6.33.x to newer ones (late 2.6.35.x and 2.6.36 -- haven't tried
2.6.36.1 yet). But I also saw loads on all sort of other types of
boxes grow when moved to 2.6.35.x and 2.6.36, so it's not necessarily
dovecot-related. Though you've got plenty to choose from between
2.6.27.x and up.

Getting 'imap-login' and 'pop3-login' set to service_count=0 and
'pop3' and 'imap' set to service_count=1000 (as per Timo's suggestion)
helped keep the boxes from spinning into oblivion. To reduce the
enormous amount of context switches, I've got 'pop3's client_limit set
to 4. I played around with 'imap's client_limit between 1 and 5 but
haven't quite found the sweet spot yet. pop3 with client_limit 4 seems
to work pretty good. That brought context switches down from
10,000-30,000 to sub-10,000. These are somewhat crummy Dell 850s (2
cores, 2 gigs) but lots of them, dedicated to just POP/IMAP, storage
on NFS (indexes on local disk though) running 2.0.7 across the board
(60 boxes or so).