[erlang-questions] Message-sending performance
Alex Arnon
alex.arnon@REDACTED
Mon Sep 10 14:20:11 CEST 2007
What are the numbers on an SMP VM with >2 schedulers?
On 9/10/07, David King <dking@REDACTED> wrote:
>
> > The problem here is that you are insisting on receiving messages by
> > a given order: {msg, X} with X <- [1..N]. Messages sent become
> > interleaved in the mailbox; when you try to remove, say {msg,
> > 40000} from the 1st process, there are already 10000s of messages
> > in the mailbox from the other process before that one, that must be
> > scanned; receive becomes very slow.
>
> Ah, I can see that, and I can see why, as now it is the messages out
> of order, so expecting to be able to receive them in order is silly
>
> > This is one of the problems one must be keeping an eye on: beware
> > the size of the mailbox. In your problem you will see that if you
> > replace receive by
> > receive {msg, _} -> ok end
> > the program will execute extremely fast, as with 1 process.
>
> Yes, it does, now sending with two processes is just as fast as
> sending with one (and when I actually have work to do as I receive
> them, it will be faster). And in my use-case, receiving the messages
> out-of-order isn't a problem at all.
>
> Thank you for the advice.
>
>
> > David King wrote:
> >> I've noticed that I can send 100,000 messages from one process to
> >> another very quickly (less than a second), but if I have two
> >> process send 50,000 messages each to a given process, it receives
> >> them very slowly (in my test, 36s). I found this using the
> >> mapreduce implementation in Joe's book where potentially
> >> thousands of processes are spawned and many (potentially very
> >> small) messages are sent.
> >> I assume that this is because the message queue is locked while
> >> sending and receiving messages. Is there any way to work around
> >> this so that having multiple processes sending many messages each
> >> isn't so slow? In my case I'd like the sender and receiver to be
> >> working simultaneously, so having each sender process send one
> >> large list isn't quite as efficient
> >> Here it is with one process:
> >> (nodename@REDACTED)11> message_streamer:stream_messages(100000).
> >> Receiving 10000 at 63356401790
> >> Receiving 20000 at 63356401790
> >> Receiving 30000 at 63356401790
> >> Receiving 40000 at 63356401790
> >> Receiving 50000 at 63356401790
> >> Receiving 60000 at 63356401790
> >> Receiving 70000 at 63356401790
> >> Receiving 80000 at 63356401790
> >> Receiving 90000 at 63356401790
> >> Done sending 100000 messages
> >> Receiving 100000 at 63356401790
> >> 100000 messages in 1s, 1.00000e+5 msg/s
> >> And here it is with two processes:
> >> (nodename@REDACTED)9> message_streamer:stream_messages(100000).
> >> Receiving 10000 at 63356401639
> >> Receiving 20000 at 63356401643
> >> Receiving 30000 at 63356401650
> >> Receiving 40000 at 63356401660
> >> Receiving 50000 at 63356401673
> >> Done sending 50000 messages
> >> Receiving 60000 at 63356401673
> >> Receiving 70000 at 63356401673
> >> Receiving 80000 at 63356401673
> >> Receiving 90000 at 63356401673
> >> Receiving 100000 at 63356401673
> >> Done sending 50000 messages
> >> 100000 messages in 36s, 2777.78 msg/s
> >> In the multiple-process case, it actually slows down as more
> >> messages are sent until one of the processes completes, and then
> >> it receives them all very quickly. Here's the code:
> >> --- code begins ---
> >> -module(message_streamer).
> >> -compile(export_all).
> >> stream_messages(N) ->
> >> Self=self(),
> >> Start=myapp_util:now(),
> >> List=lists:seq(1,N),
> >> Split_List=split_list(List,erlang:system_info(schedulers)),
> >> lists:foreach(fun(Sublist) ->
> >> spawn_link(fun() ->
> >> lists:foreach(fun(Which) ->
> >> Self ! {msg,Which}
> >> end,
> >> Sublist),
> >> io:format("Done sending ~p messages~n",
> >> [length(Sublist)])
> >> end)
> >> end, Split_List),
> >> lists:foreach(fun(Which) ->
> >> case Which rem (N div 10) of
> >> 0 ->
> >> io:format("Receiving ~p at ~p ~n",
> >> [Which,myapp_util:now()]);
> >> _ -> ok
> >> end,
> >> receive {msg,Which} -> ok end
> >> end,
> >> List),
> >> Time=case myapp_util:now()-Start of
> >> 0 -> 1;
> >> X -> X
> >> end,
> >> io:format("~p messages in ~ps, ~p msg/s~n",[N,Time,N/Time]).
> >> %% splits a list into equal parts
> >> %% split_list([1,2,3,4,5,6],2) -> [[1,2,3],[4,5,6]]
> >> split_list(List,Pieces) ->
> >> Length=length(List),
> >> lists:reverse(split_list(List,Length div Pieces,Length,[])).
> >> split_list([], _Per_Piece, 0, Acc) ->
> >> Acc;
> >> split_list(List, Per_Piece, Length, Acc) when Length>=Per_Piece ->
> >> {Short,Long} = lists:split(Per_Piece,List),
> >> split_list(Long, Per_Piece, Length-Per_Piece, [ Short | Acc ]);
> >> split_list(List, Per_Piece, Length, Acc) when Length <Per_Piece ->
> >> {Short,Long} = lists:split(Length, List),
> >> split_list(Long, Per_Piece, Length-Length, [ Short | Acc ]).
> >> --- code ends ---
> >> myapp_util:now() just looks like:
> >> now() ->
> >> calendar:datetime_to_gregorian_seconds(calendar:universal_time()).
> >> _______________________________________________
> >> erlang-questions mailing list
> >> erlang-questions@REDACTED
> >> http://www.erlang.org/mailman/listinfo/erlang-questions
> >
> >
> > --
> > Paulo Sérgio Almeida Email:
> > psa@REDACTED
> > Dep. Informatica - Universidade do Minho Phone: +351 253
> > 604451
> > Campus de Gualtar - 4710-057 Braga - PORTUGAL Fax : +351 253
> > 604471
>
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