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

Stian Øvrevåge sovrevage at gmail.com
Fri Apr 13 13:00:36 CEST 2012

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 :-)

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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