[erlang-questions] gen_tcp receive extremely high CPU

Anita Wong <>
Fri Oct 12 09:48:35 CEST 2012

Hi All,

Sorry that I'm an Erlang newbie and may make stupid question. But please
help me to solve the issue.

I have written an Erlang server to replace the one I'm using with Node.js,
which ate all my memory and I'm praying that Erlang could be a way out.
The server works properly under unit test and internal testing, but face a
high CPU usage in stress test.

After trimming down, I found that the CPU burst was due to the TCP receive
from clients.

*receiveClientPacket(Sock) ->*
*  inet:setopts(Sock, [{active, once}, {buffer, ?CLIENTHEARTBEATSIZE}]),*
*  receive*
*    {tcp, Sock, Data} ->*
*      {ok, Data}**;*
*    {tcp_closed, Sock} ->*
*      {error, closed}*
*      {error, timeout}*
*  end.*

I tried making the process sleep for 10 hours at the beginning of the
function (to prevent it from calling receive), the CPU didn't burst at all.
Therefore I conclude that the burst of CPU is due to TCP receive. (Please
correct me if I made any mistake)

Here are information about my stress test:

   1. start the Erlang server with:
      1. erl +zdbbl 2097151 -K true +A 128 +P 5000000
   2. connect 5000 clients to the Erlang server
   3. each connected client sends a 2 byte data to the server every 1 min
   4. after all the connections is done, (i.e. only the 2 byte data per min
   are performing), the CPU burst to ~30%sy (from "top")

I'm using an Amazon Linux AMI (large instance, 64-bit) for the Erlang
server. Is the burst due to the linux? As I have no idea how the system
will use up the CPU. Or is it my poor code's problem? (I believe so...)

In real situation, our servers don't only receive ping pong, but also
messages, which is a lot more loading... This is only the first step...

Millions of thanks to anyone who can save me.



Information about large instance (for reference):
7.5 GB memory
4 EC2 Compute Units (2 virtual cores with 2 EC2 Compute Units each)
850 GB instance storage
64-bit platform
I/O Performance: High
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://erlang.org/pipermail/erlang-questions/attachments/20121012/446fb410/attachment.html>

More information about the erlang-questions mailing list