[erlang-questions] Load testing in parallel

Serge Aleynikov saleyn@REDACTED
Mon Aug 13 13:56:59 CEST 2007

You have several problems with this code:

1. Since you don't provide implementation of generate_lists/3, I assume
    it returns a flat list.  In this case in the start function you call:

       spawn(?MODULE, run_process,  [Num, Op, Head, Pid]),

    if here Head is not a list, whereas run_process/4 expects a list
    as the third argument, lists:map/2 function would
    crashes the process silently.  You can fix it by changing that line
       spawn(?MODULE, run_process,  [Num, Op, [Head], Pid]),

2. Also you use the following call to obtain time:

    {_, Wallclock_Time_Since_Last_Call} = statistics(wall_clock).

    Wallclock_Time_Since_Last_Call is time in milliseconds, so unless
    evaluated function takes more than a millisecond you'd get a 0.

    Moreover, unfortunately Wallclock_Time_Since_Last_Call is a *globally
    shared* counter, so any process that calls statistics would cause a
    reset of this value.  So in a concurrent system where many processes
    use this function you'll likely always get a zero.

    use T1 = now(), ..., T2 = now(), ... timer:now_diff(T2, T1)
    to measure time.


Ahmed Ali wrote:
> Hi all,
> I've been trying to load test a function in my code. The way I'm doing it is
> to generate lists of the data I want to process, start a process for each
> list and calculate runtime for each process. The function that I implemented
> will call a  WebService operation in a different host for each data.
> I have the code below for this test. what I do is basically run
> load_test(10, 1000) to generate 10 lists, each with 1000 data in it.
> The problem is that WebService call is done successfully but I don't get any
> output for the statistics. When I run the code sequentially (i.e. instead of
> spawn the call to run_process, I call run_process directly) I get the output
> with no issues. Is there something that I missed in the code below? I
> appreciate your help.
> Best regards,
> Ahmed Al-Issaei
> run_process(Num, Fun, List, _Pid) ->
>     statistics(wall_clock),
>     io:format("load testing process~n"),
>     lists:map(Fun, List),
>     {_, Time2} = statistics(wall_clock),
>     U2 = Time2 / 1000,
>     io:format("Num (~s) done: total time = ~p~n",[Num, U2]).
> start(_N, _Op, [], _Pid) ->
>     true;
> start(Num, Op, Final, Pid) ->
>     [Head | Rest] = Final,
>     spawn(?MODULE, run_process,  [Num, Op, Head, Pid]),
>     start(Num-1, Op, Rest, Pid).
> load_test(Threads, Size) ->
>     List = generate_lists(Threads, Size, []),
>     Op = fun(X) -> call_ws_create(X) end,
>     start(Threads, Op, List, self()).
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