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    <p>In your lists:foreach call in the second block, you are calling <code>RESULT=prime:is_prime(ProcId,N)</code>, which is doing a <code>receive</code> for the result. So, you're spawning a process and then waiting for it to finish until you spawn the next process. That's why it takes the same amount of time as the single-threaded approach: both ways, you're only doing one at a time.</p> <p>Instead, you need to spawn all the processes first (using something like <code>lists:map</code> to keep track of all the PIDs) and then wait for the results in a separate step. Note that means you'll have to split the <code>ProcessID ! {self(),Number}</code> part from the <code>receive</code> part so that can be done in the first step, otherwise you're just spawning a bunch of idle processes.</p> <p>So, something like:</p> <pre><code>Pids = lists:map( fun(N)-&gt; ProcId = prime:start(N), io:fwrite("Calculating : procId=~p N=~p ~n",[ProcId,N]), ProcId end, NUMS). lists:foreach( fun(ProcId) -&gt; {N,RESULT}=prime:is_prime(ProcId), io:fwrite("Result procId=~p N=~p Result=~p ~n", [ProcId,N,RESULT]) end, Pids). start(N)-&gt;spawn(?MODULE, forever_function, [N]). forever_function(Number)-&gt; Result = is_prime(self(),2,Number), receive { From, get_result } -&gt; From! {Number,Result}, % unnecessary since we never call this Pid again, but do it anyway :) forever_function() % could also add more cases here to set the number % or even do a one-shot function like before end. %multithreaded function is_prime(ProcessID)-&gt; ProcessID ! {self(),get_result}, receive RESULT-&gt;RESULT end. </code></pre> <p>Note: this is untested, so it may need some tweaking.</p>
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    1. COThank you very much ! I'm currently trying to implement your solution :-) The notation spawn(?MODULE, fun, args) was unknown to me. I'll validate your solution later.
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    2. CO?MODULE is a built-in macro for the module name as an atom. You could also use spawn(prime,Fun,Args) and it would be the same.
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    3. COIt is in some ways better to spawn a fun as you did originally as it saves you from exporting, and making visible, a function which is really only meant for internal use.
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