[ejabberd] Strange Latency

Christopher Zorn christopher.zorn at gmail.com
Wed Sep 3 18:42:33 MSD 2008


    I have some scripts to measure latency on various points during a user
session in ejabberd, and using external components connected to ejabberd.
About 50%-60% of the time the latency is really great. An iq send and result
is about 9ms. But at some intervals that same test will be 1500ms. I have
been trying to figure out why this happens.

Here are some details. This is ejabberd-svn r1531 with some small
modifications. There are about 10 external components connected.

When latency goes up, CPU load is down, IO is down, and the database load is
low. Even other accounts/connections do not experience the latency. To try
and find out the problem I adjusted the c2s_shaper and the external
component shapers. This has helped but not rid me of the problem. I ran some
experiments with the shaper, with some debugging, and the shaper delay was
not being called when lag happened. Even with the shapers removed or set to
a really high value there was no delay due to the shapers but latency would
still be a bit high at times.

To elliminate network latency I measure on the local machine where ejabberd
is running.

My question is where else should I look? I thought it would be the shapers,
and those have helped, but I do not know where else to look. I can not
isolate where it is happening, it seems to be at random times and random
stanzas. I have looked at the time it takes to check offline messages and
privacy lists, these seem to be fast.

Does anyone have an idea where I should look next? Or what could be causing
this? I would like to get good measurement of the true latency on the server
and it seems right now I can not.

Thanks in advance.
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