[erlang-questions] Comet server broadcasting messages to 40k clients

Adriano Bonat <>
Wed Jan 21 14:38:18 CET 2009

Hi Bernard,

On Wed, Jan 21, 2009 at 2:52 AM, Bernard Duggan <> wrote:

> My first suggestion would be to check that you're using kernel polling
> (do you start erl with "+K true"?  You should see the string
> "[kernel-poll:true]" in the version line).  If not, that's probably a
> significant part of it, since for every packet that comes in the VM will
> be iterating over all 40k sockets to figure out which one it came from
> before the message even arrives at your code.

Yes, I ran the tests using kernel polling.

> Also, be aware that using the -- operator on a big list can be a very
> expensive operation (there was a thread about it last week, in fact), so
> using it to remove a single socket from a list that big could be a
> significant cost.

That's one of the Joe's advices on his book :)
But the latency for the message is not caused by that point...

> Finally, I'd suggest that, if you haven't already tried it, you spawn a
> separate thread (process, in erlang parlance) to handle each connection
> rather than trying to deal with them all in one thread.  While I don't
> think this is going to improve your performance here, it makes your code
> simpler and easier to follow.  Remember, erlang processes are not like C
> threads - they're cheap, plentiful and carry a much lower switching
> overhead.  This would involve a little more work for your case since you
> want to broadcast to everyone based on a signal from anyone, but it is
> more in keeping with "The Erlang Way(tm)".  (Actual followers of The
> Erlang Way are of course free to correct my views...).

Yes, that would go with the Erlang's way, but I wanted a way that
works faster. Before I implemented my own http server, I was using
mochiweb, and it uses a thread per connection and has the same
performance problem as using a single thread holding several

Any other ideas are welcome, and thanks for you reply :)


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