[erlang-questions] Low disk logging performance in SMP

Per Melin <>
Fri Apr 24 21:08:48 CEST 2009


Per Melin:
> Anders Nygren:
>> Per Melin:
>>> It is known that once you queue up a few tens of thousand messages
>>> into a mailbox your performance drops noticeably for doing a receive.
>>
>> I hope You meant when doing a SELECTIVE receive, right.
>
> Of course. I've been away from Erlang for too long (again). I'm really
> sorry for the misinformation.

At this point I should just be quiet, but I can't help myself.

The performance characteristics of this problem makes it look like
there's indeed a selective recieve in there somewhere. When opening
the file in raw mode I didn't expect that. But:

1) The file:write gets faster and faster as the size of the message
queue shrinks.

2) If you change the writer so that it waits for an ack from the
listener between each send the problem goes away.

I don't have time to learn the ins and outs of the prim_file module,
but I did a quick trace and I suspect prim_file:drv_get_response/1
(which does a selective receive) is the culprit.

Anyway, here's a fast synchronous version of the original code:

start(Filename) -> register(testLogger,spawn(testLogger,listener,[Filename])).

listener(Filename) ->
    case file:open(Filename,[append, raw]) of
        {ok,Fd} -> listener(Fd,0);
        {error,Reason} -> error_logger:error_msg(Reason)
    end.

listener(Fd,N) ->
    receive
        {data, From, Data} ->
            file:write(Fd,Data),
            From ! ack,
            listener(Fd,N+1);
        {count,From} -> From ! {count,N}, listener(Fd,N);
        {stop,From} -> From ! {stop,success}
    end.

write(_Data,0) -> {ok,done};
write(Data,N) ->
    testLogger ! {data, self(), Data},
    receive
        _ -> ok
    end,
    write(Data,N-1).



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