kyle-qmail at memoryhole
Jun 20, 2008, 1:42 PM
Post #2 of 6
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On Friday, June 20 at 10:09 PM, quoth Oliver Block:
> can anyone tell what's determining the maximum number of qmail-smtpd
> processes (incoming messages)?
So far? I'm gonna guess divine intervention. ;)
> I am using qmail with xinetd and the value for instances is
> UNLIMITED there. But I assume that is not the real limit.
You *assume*? What makes you think that?
Well, with xinetd, if your connections are unlimited, then it will
*attempt* to accept every connection it can. You will, however, run
out of resources at some point, and how that manifests itself depends
on your system and which resource you've run out of first. For
example, if you run out of RAM first, that *may* manifest itself as
looking like qmail-smtpd crashed, or it may manifest itself as
qmail-smtpd failed to load. Or it may manifest itself as an exec()
error. Or it may manifest itself within xinetd as a failure to perform
some intrinsic task (such as fork() or maybe even something as basic
as pipe() will fail). This assumes, of course, that xinetd is behaving
itself and is responsibly checking all of these syscalls for errors.
If it isn't, then you may get much stranger-looking errors, probably
from xinetd, as things it tries to do that relied on previous actions
inevitably fail. Or, depending on your OS, you may trigger an
OOMKiller, which will cause random processes to suddenly die
(including, qmail-send, init, or even xinetd itself).
But that's just assuming that RAM is your limiting factor (which would
be pretty typical). It's also possible that you'll run out of open
file descriptors first (your limit depends on your OS and how its been
configured). Or, you may run out of PIDs (your limit depends on your
OS). Or you might run out of connection entries in your OS's TCP
stack. And how those things manifest themselves, and which one bites
you first, depends entirely on your system, your hardware, xinetd, and
how they interact. For example, I'm assuming that xinetd and your OS
are bugless under load; they probably aren't.
But, let's be honest, you've asked a pretty open-ended question here.
What are you trying to find out, *really*?
~Kyle
--
I have an existential map; it has 'you are here' written all over it.
-- Steven Wright
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