[mrtg-developers] Periodic hangs on snmp requests

Larry Fahnoe fahnoe at FahnoeTech.com
Tue Jan 25 17:44:22 MET 2005


>From time to time I'm finding that mrtg will hang on snmp requests.
The processes will never die (until I manually kill them) and if the
processes are not killed, they will eventually collect to the point
that virtual memory is exhausted.  This is happening on Red Hat
Enterprise Linux release 3 which is kept current with patches from Red
Hat.  mrtg is 2.11.0, rrdtool is 1.0.49, and perl is 5.8.0.

I've been seeing this problem almost exclusively with a bunch of
Nortel and Cisco switches, the routers do not cause the problem.  I
have not (yet) isolated down to a particular switch, but I don't think
it is just one switch that is causing the problem.  What I typically
see is three mrtg processes in a group that are hung.  Here is a
recent example, using strace to see what the parent, child, and
grandchild processes are doing:

# strace -v -p 14914   [parent process]
wait4(-1,  <unfinished ...>

# strace -v -p 14916   [child process]
select(16, [4], NULL, [4], NULL <unfinished ...>

# strace -v -p 15051   [grandchild process]
recvfrom(4,  <unfinished ...>

# netstat -anp | grep 15051
udp        0      0 *

Upon killing the grandchild, I get the following in the log:

ERROR: Bailout after SIG TERM
ERROR: fork 0 has died ahead of time ...
Command exited with non-zero status 29
16.80user 0.65system 21:55:23elapsed 0%CPU (0avgtext+0avgdata
0inputs+0outputs (719major+35652minor)pagefaults 0swaps

I have been seeing this off and on for several months with different
versions of mrtg and perl.  My thought is that the snmp request
timeouts are not being honored but beyond that I'm stumped.  Any
insight into what might be happening here?


Larry Fahnoe, Fahnoe Technology Consulting, fahnoe at FahnoeTech.com
952/925-0744      Minneapolis, Minnesota       www.FahnoeTech.com 

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