[erlang-questions] Please suggest why these repeated timings increase

Isaac Gouy <>
Sun Aug 28 00:50:09 CEST 2011


I found my mistake ;-)

I'd put this in to debug, and then removed the handler 


   MainPid ! {details,[Id,Previous,self(),Next]}

so messages were just accumulating...





----- Forwarded Message -----
> From: Isaac Gouy <>
> To: "" <>
> Cc: 
> Sent: Saturday, August 27, 2011 11:17 AM
> Subject: Please suggest why these repeated timings increase
> 
> Here's a tiny (somewhat silly) program - a circle of processes, removed one 
> at a time until there's only one left.
> 
> I expect there's a simple explanation - I expect I've done something 
> basic wrong - but repeatedly creating and reducing such a circle of processes 
> seems to slowdown, and I'd like to understand why.
> 
> There doesn't seem to be any accumulation of zombie processes, so I'm a 
> little puzzled what else might be going on.
> 
> 
> 
> $ /usr/local/src/otp_src_R14B02/bin/erlc josephus.erl
> 
> $ /usr/local/src/otp_src_R14B02/bin/erl -noshell -run josephus main
> 
> 943157
> 2219531
> 3490628
> 4814178
> 6334408
> 7419798
> 8615281
> 9778484
> 11364821
> 12440720
> 
> 
> -module(josephus).
> -export([main/0,soldier/3,timedloop/1]).
> 
> soldier( PreviousPid, NextPid, Id ) ->
>    receive
>       {MainPid,countoff,Kth,K} -> 
>          countoff(PreviousPid,NextPid,Id,MainPid,Kth,K);
>          
>       {set_previous,Pid} ->
>          soldier(Pid,NextPid,Id);
> 
>       {set_next,Pid} ->
>          soldier(PreviousPid,Pid,Id)
>    end.
> 
> 
> countoff( Previous, Next, Id, MainPid, Kth, K ) when Kth == K -> 
>    MainPid ! {details,[Id,Previous,self(),Next]},
>    if 
>       Previous == self() -> 
>          MainPid ! {countoff_ack,Id};
> 
>       true -> 
>          Previous ! {set_next,Next},
>          Next ! {set_previous,Previous},
>          Next ! {MainPid,countoff,Kth,1}
>    end;
> 
> countoff( Previous, Next, Id, MainPid, Kth, K ) ->    
>    Next ! {MainPid,countoff,Kth,K+1},
>    soldier(Previous,Next,Id).
> 
> 
> ring(N) -> 
>    ring(N,1,nil,[]).
> 
> ring(N,I,P,L) when I =< N -> 
>    Pid = spawn(josephus,soldier,[P,nil,I]),
>    ring(N,I+1,Pid,[Pid|L]);
> 
> ring(_,_,Last,L) ->    
>    First = ring_set_next(nil,L),  
>    Last ! {set_next,First},
>    First ! {set_previous,Last},
>    First.
>  
> ring_set_next(Last,[]) -> Last;
> ring_set_next(P,[Pid|T]) -> 
>    Pid ! {set_next,P},
>    ring_set_next(Pid,T).
> 
> 
> countoffSoldiers(N,Kth) ->
>    Soldier = ring(N),
>    %Soldier ! {self(),countoff,Kth,1}, % standard Josephus problem
>    Soldier ! {self(),countoff,Kth,Kth},
>    receive {countoff_ack,I} -> I end.
> 
>  
> %-----------------------------------------------------------------------------
> 
> timedloop(0) -> ok;
> timedloop(N) ->
>     countoffSoldiers(40,3),
>     timedloop(N-1).
> 
> repeat(_,0) -> ok;
> repeat(N, Repeat) ->
>     {MicroSecs,I} = timer:tc(josephus,timedloop,[N]),
>     io:format("~w~n",[MicroSecs]),
>     repeat(N,Repeat-1).
> 
> main() ->
>     repeat(1000,10),
>     halt(0).
>



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