IMAP hibernate and scalability in general

[Christian Balzer]

Hello,

On Thu, 6 Apr 2017 22:13:07 +0300 Timo Sirainen wrote:

> On 6 Apr 2017, at 21.14, Mark Moseley <moseleymark at gmail.com> wrote:
> >   
> >> 
> >> imap-hibernate processes are similar to imap-login processes in that they
> >> should be able to handle thousands or even tens of thousands of connections
> >> per process.
> >>   
> > 
> > TL;DR: In a director/proxy setup, what's a good client_limit for
> > imap-login/pop3-login?  
> 
> You should have the same number of imap-login processes as the number of CPU cores, so they can use all the available CPU without doing unnecessary context switches. The client_limit is then large enough to handle all the concurrent connections you need, but not so large that it would bring down the whole system if it actually happens.
>
Also keep in mind that pop3 login processes deal with rather ephemeral
events, unlike IMAP with IDLE sessions lasting months.
So they're unlike to grow beyond their initial numbers even with a small
(few hundreds) client_limit.

On the actual mailbox servers, either login processes tend to use about
1% of one core, very lightweight.  

> > Would the same apply for imap-login when it's being used in proxy mode? I'm
> > moving us to a director setup (cf. my other email about director rings
> > getting wedged from a couple days ago) and, again, for the sake of starting
> > conservatively, I've got imap-login set to a client limit of 20, since I
> > figure that proxying is a lot more work than just doing IMAP logins. I'm
> > doing auth to mysql at both stages (at the proxy level and at the backend
> > level).  
> 
> Proxying isn't doing any disk IO or any other blocking operations. There's no benefit to having more processes. The only theoretical advantage would be if some client could trigger a lot of CPU work and cause delays to handling other clients, but I don't think that's possible (unless somehow via OpenSSL but I'd guess that would be a bug in it then).
> 
Indeed, in proxy mode you can go nuts, here I see pop3-logins being
busier, but still just 2-5% of a core as opposed to typically 1-2% for
imap-logins. 
That's with 500 pop3 sessions at any given time and 70k IMAP sessions per
node.
Or in other words, less than 1 core total typically.

> > Should I be able to handle a much higher client_limit for imap-login and
> > pop3-login than 20?  
> 
> Yeah.
> 
The above is with a 4k client_limit, I'm definitely going to crank that up
to 16k when the opportunity arise (quite disruptive on a proxy...).

Christian
-- 
Christian Balzer        Network/Systems Engineer                
chibi at gol.com   	Global OnLine Japan/Rakuten Communications
http://www.gol.com/