[collectd] Max number of UDP sockets per collectd-process (NOT the file descriptor limit)

[Teet Talviste]

Depends, if you use mostly snmp polling with 5min interval, the performance 
impact should be negligible, if any. I use it with 6600+ switches and the 
bottleneck is still IO.
Timing the threads would be rather difficult. Colllectd actually uses a read 
thread perl host polled... So, be sure to increase the read-thread variable if 
you have slow snmp hosts...
> Hi Teet,
> 
> That's a nice little patch!
> 
> What is the performance impact to adding all of those open/close
> sessions, per device poll?
> 
> Hi Stian,
> 
> Does this work for you, without breaking up the collection into
> smaller polling groups?
> 
> Can you "time" the multiple threads and "time" the single unified
> thread, so we can see the user/real/sys time of each scenario?
> 
> Thanks - Dave
> http://netmgt.blogspot.com/
> 
> On 4/14/12, Teet Talviste <teet.talviste at elion.ee> wrote:
> > You can take a look at this, maybe it helps you
> >
> > 
https://github.com/frogmaster/collectd/commit/67c4863e0aaadaa103ee07e49a17a1510e8d4eaf
> >
> >> Found this handy anecdote on
> >> http://collectd.org/wiki/index.php/Plugin:SNMP
> >>
> >> "Maximum number of hosts
> >> While collectd and the SNMP plugin don't have any limitation on the
> >> number of hosts you can configure, the library used by the SNMP
> >> plugin, libnetsnmp, uses the select(2) system call. This system call
> >> uses a fixed-size bitfield to hold file descriptors. On many systems
> >> this limits the number of hosts you can query with the SNMP plugin to
> >> 1024 (for example when using the GNU libc).
> >>
> >> To solve this issue, the netsnmp library must be changed. A solution
> >> would be to switch to the poll(2) system call which doesn't have a
> >> static limit on the largest file descriptor it can handle."
> >>
> >> So my current work-around and using several collectd processes seems
> >> to be a permanent one :-)
> >>
> >> Brgds
> >> Stian Øvrevåge
> >>
> >> On Thu, Apr 12, 2012 at 11:21, Stian Øvrevåge <sovrevage at gmail.com> 
wrote:
> >> > Hi list,
> >> >
> >> > Banging my head against the wall for weeks now trying to get a working
> >> > medium scale collectd-installation working...
> >> >
> >> > I thought I had fixed the max number of sockets/connecting when tuning
> >> > /etc/security/limits.conf. It now reads:
> >> >
> >> >    ulimit -n
> >> >    32768
> >> >
> >> > I have the instances of collectd now. One of them is set to poll 2300
> >> > hosts. Of which an unknown number is offline at any time. I'm watching
> >> > strace as well as netstat and everything seems fine and "netstat -anop
> >> > udp|wc -l" counts the number of udp sockets created until the number
> >> > hits about 1092. Here it stalls and syslog logs thousand lines of
> >> >
> >> >    "Apr 12 11:07:41 collectd-new collectd[1488]: snmp plugin: host
> >> > x.y.z: snmp_sess_synch_response failed:"
> >> >
> >> > within a few seconds. Number of UDP sockets from then on are stable.
> >> >
> >> > If I also start the other two instances the number of sockets grows to
> >> > 1292. Which leads me to believe that there is a per-process(or
> >> > thread?) limit somewhere.
> >> >
> >> > Information on the internet on the issue is scarce other than the file
> >> > descriptor limit which I believe is unrelated.
> >> >
> >> > Regards,
> >> > Stian Øvrevåge
> 
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