[erlang-questions] eep: multiple patterns

[Christopher Atkins]

Hello, I tried (poorly--I'm a complete novice) to implement a benchmark from
your earlier statement.  I didn't do the same thing (load up the message
mailbox before consuming them), but what I did write led to a perplexing (to
me) discovery.  If I uncomment the line in [loop1/0] below, performance for
that loop degrades by an order of magnitude.  Why is that?

-module(test_receive).
-compile(export_all).

start() ->
        statistics(runtime),
        statistics(wall_clock),
        PidLoop1 = spawn(?MODULE, loop1,[]),
        sender(PidLoop1, 10000000),
        {_, Loop1Time1} = statistics(runtime),
        {_, Loop1Time2} = statistics(wall_clock),
        io:format("Sent ~p messages in ~p /~p~n", [100000, Loop1Time1,
Loop1Time2]),
        statistics(runtime),
        statistics(wall_clock),
        PidLoop2 = spawn(?MODULE, loop2,[]),
        sender(PidLoop2, 10000000),
        {_, Loop2Time1} = statistics(runtime),
        {_, Loop2Time2} = statistics(wall_clock),
        io:format("Sent ~p messages in ~p /~p~n", [100000, Loop2Time1,
Loop2Time2]).

sender(_, 0) -> void;
sender(Pid, N) ->
        if
          N rem 2 =:= 2 ->
                Pid ! test2;
          true ->
                Pid ! test1
        end,
        sender(Pid, N - 1).

proc1(F) ->
        receive
                start -> spawn_link(F)
        end.

loop1() ->
        receive
                %%test1 -> loop1();
                test2 -> loop1()
        end.

loop2() ->
        receive
                _ -> loop2()
        end.


------------------------------------------------------------------------------------------------------------------------------
Message: 2
Date: Fri, 30 May 2008 18:07:18 +0200
From: "Per Melin" <per.melin@REDACTED>
Subject: Re: [erlang-questions] eep: multiple patterns
To: "Sean Hinde" <sean.hinde@REDACTED>
Cc: Erlang Questions <erlang-questions@REDACTED>
Message-ID:
       <d57f8dd90805300907u44fe7316x2421ab28632cf633@REDACTED>
Content-Type: text/plain; charset=ISO-8859-1

2008/5/30 Per Melin <per.melin@REDACTED>:
> If I send 100k 'foo' messages and then 100k 'bar' messages to a
> process, and then do a catch-all receive until there are no messages
> left, that takes 0.03 seconds.
>
> If I do a selective receive of only the 'bar' messages, it takes 90
seconds.

I found my old test code:

-module(selective).

-export([a/2, c/2]).

a(Atom, N) ->
   spawn(fun() -> b(Atom, N) end).

b(Atom, N) ->
   spam_me(foo, N),
   spam_me(bar, N),
   R = timer:tc(?MODULE, c, [Atom, N]),
   io:format("TC: ~p~n", [R]).

c(Atom, N) ->
   receive
       Atom -> c(Atom, N - 1)
   after 0 ->
       N
   end.

spam_me(Msg, Copies) ->
   lists:foreach(fun(_) -> self() ! Msg end, lists:duplicate(Copies, 0)).

---

2> selective:a(bar, 100000).
<0.38.0>
TC: {124130689,0}
3> selective:a(foo, 100000).
<0.40.0>
TC: {23176,0}
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