[erlang-questions] Comet server broadcasting messages to 40kclients

wde <>
Wed Jan 21 18:09:39 CET 2009


does the latency increase with the number of connected users ? 
How the latency increase with the number of users ? linearly ?


wde


  
======= le 21/01/2009, 17:39:58 vous écriviez: =======

>On Wed, Jan 21, 2009 at 5:38 AM, Adriano Bonat <> wrote:
>> 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
>> connections.
>>
>> Any other ideas are welcome, and thanks for you reply :)
>
>Maybe it's a problem with your platform? What version of Erlang and
>which OS/distro are you running?
>
>I haven't seen anything like 5 sec latency with mochiweb even with a
>lot of sockets connected.
>
>-bob
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wde

21/01/2009



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