[erlang-questions] Process spawn time with -smp

Rickard Green <>
Mon Apr 9 04:49:04 CEST 2007


Hi!

This test program isn't especially nice to the smp emulator. When a 
process is placed in the run-queue, scheduled in, or scheduled out, a 
lock on the run-queue has to be held. When a child processes in this 
program has been spawned, it is put in the run-queue, later scheduled 
in, and then immediately scheduled out. Since the test will make sure 
that there will be plenty of processes in the run-queue, all scheduler 
threads available will be woken. Since the child processes doesn't do 
anything, the scheduler threads will more or less only fight about the 
run-queue lock.

I ran the test program myself on a couple of our machines, but I didn't 
get as bad figures as you got. The worst I got was a little more than 
twice the time for the smp emulator compared to the non-smp emulator on 
a machine with four cores. When I ran the smp emulator with only one 
scheduler thread on that same machine I got times 5-30% longer than for 
the non-smp emulator. This comparison is more interesting since there 
will not be such a fight about the run-queue lock, as described above, 
when processes actually do something.

BR,
Rickard Green, Erlang/OTP, Ericsson AB.

lang er wrote:
> Hi!
> I run code from Joe's new book on my macbook(2G RAM and  2GHz Intel
> Core Duo CPU) to test how fast erlang create processes.
> 
> I compile Erlang with smp support, and I test with smp flag on and off.
> 
> The following is the results :
> 
> erl -noshell +P 1000000 -s processes max 800000 -s init stop
> Maximum allowed processes:1000000
> Process spawn time=4.16250 (8.02000) microseconds
> 
> erl -smp -noshell +P 1000000 -s processes max 800000 -s init stop
> Maximum allowed processes:1000000
> Process spawn time=19.7000 (28.0012) microseconds
> 
> So  why process spawn is so slow with smp?
> 
> -------------
> test code from joe's book:
> 
> max(N) ->
>    Max = erlang:system_info(process_limit),
>    io:format("Maximum allowed processes:~p~n" ,[Max]),
>    statistics(runtime),
>    statistics(wall_clock),
>    L = for(1, N, fun() -> spawn(fun() -> wait() end) end),
>   {_, Time1} = statistics(runtime),
>   {_, Time2} = statistics(wall_clock),
>   lists:foreach(fun(Pid) -> Pid ! die end, L),
>   U1 = Time1 * 1000 / N,
>   U2 = Time2 * 1000 / N,
>   io:format("Process spawn time=~p (~p) microseconds" ,
>   [U1, U2]).
> 
> wait() ->
>   receive
>    die -> void
>  end.
>  for(N, N, F) -> [F()];
>  for(I, N, F) -> [F()|for(I+1, N, F)].
> 
> 
> 
> BR!
> James
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