Kernel Poll on ejabberd

Submitted by badlop on Fri, 2005-09-02 18:54

Support in Erlang/OTP

Several implementations of Kernel Poll are supported since Erlang/OTP R11B-3:

FreeBSD (kqueue)
Linux 2.4 (/dev/kpoll)
Linux 2.6 (/dev/epoll)
Solaris (/dev/poll)

How to enable Kernel Poll on Erlang/OTP

Short answer:

$ erl +K true
Erlang (BEAM) emulator version 5.6.2 [source] [smp:2] [async-threads:0] [kernel-poll:true]

Eshell V5.6.2  (abort with ^G)
1>

If Erlang or your system does not support Kernel Poll, this would report an error.

Check the performance tuning page for more details.

How much does it improve performance?

Kernel poll reduces ejabberd's CPU usage when it has hundreds (or more) network connections. It does not affect memory consumption or latency, so if you plan to support great amounts of simultaneous connected users, you really want to have a Jabber server with Kernel Poll support.

An experiment was ran to compare ejabberd performance with and without Kernel Poll support. The environment, tools, process and results are available on the Benchmark page, Test 3.

The next screenshot was taken during the experiment. It shows the ejabberd debug window, Jabsimul window and top. There are 3000 concurrent online users chatting and making roster changes. After 47 minutes of constant stress the ejabberd system process (beam) takes around 5% of CPU usage and nearly 200 megs of RAM.

Debian Etch and kernel poll in Erlang R11B2

Submitted by kostix on Mon, 2008-06-02 22:44.

Unfortunately, Erlang R11B2 distributed with Debian Etch contains a bug in epoll code that usually results in repeated failures to initiate outgoing s2s connection, such as:

=INFO REPORT==== 2008-06-01 16:22:51 ===
I(<0.446.0>:ejabberd_s2s:362) : New s2s connection started <0.668.0>

=INFO REPORT==== 2008-06-01 16:22:51 ===
I(<0.668.0>:ejabberd_s2s_out:1010) : Trying to open s2s connection: 007spb.ru
  -> tach.tclers.tk

=INFO REPORT==== 2008-06-01 16:22:52 ===
I(<0.668.0>:ejabberd_s2s_out:306) : Closing s2s connection: 007spb.ru
  -> tach.tclers.tk (close in wait_for_stream)

...

This problem was fixed in Erlang R11B3 (look for "OTP-6344" tag in its changelog).

To fix the problem either disable kernel polling (set the "POLL" option to false in /etc/default/ejabberd) or if you can't live without kernel polling build and install more recept version of Erlang. For instance, R11B3 from the snapshot archive will work. Note that you will also have to rebuild Ejabberd itself with the new erlang-dev package installed.