Notes on comparing effectiveness of random list element selection as found in Erlang's pg2 vs using random:uniform. Also comparing effect under concurrent access of erlang:now() vs os:timestamp().

draw.R

library("lattice")
z <- read.table("all.txt", colClasses=rep("integer", 4))
names(z) <- c("pg_rand", "pg_rand_os", "u_rand", "u_rand_os")
all <- stack(z)
names(all) <- c("item", "func")
tapply(all$item, all$func, table)
histogram(~item|func, data=all, type="count", breaks=0:50)

rand_test.erl

-module(rand_test).
-compile([export_all]).

pg_rand(L) ->
    {_,_,X} = erlang:now(),
    Idx = (X rem length(L)) + 1,
    lists:nth(Idx, L).

pg_rand_os(L) ->
    {_,_,X} = os:timestamp(),
    Idx = (X rem length(L)) + 1,
    lists:nth(Idx, L).

u_rand(L) ->
    random:seed(erlang:now()),
    Idx = random:uniform(length(L)),
    lists:nth(Idx, L).

u_rand_os(L) ->
    random:seed(os:timestamp()),
    Idx = random:uniform(length(L)),
    lists:nth(Idx, L).

test() ->
    N = 10000,
    Workers = 10,
    ListSize = 50,
    List = [ integer_to_list(I) || I <- lists:seq(1, ListSize) ],
    work(N, Workers, List, fun pg_rand/1, "pg_rand.txt"),
    work(N, Workers, List, fun u_rand/1, "u_rand.txt"),
    work(N, Workers, List, fun u_rand_os/1, "u_rand_os.txt"),
    work(N, Workers, List, fun pg_rand_os/1, "pg_rand_os.txt").
    
write_sample(Sample, Name) ->
    {ok, Fd} = file:open(Name, [write]),
    [ io:fwrite(Fd, "~B~n", [S]) || S <- Sample ],
    file:close(Fd).

sample(N, L, F) ->
    sample(N, L, F, []).

sample(0, _L, _F, Acc) ->
    lists:reverse(Acc);
sample(N, L, F, Acc) ->
    sample(N - 1, L, F, [<<"\n">>, F(L)| Acc]).

work(Workers, N, L, F, Name) ->
    Parent = self(),
    Pids = [ spawn(fun() -> worker(N, L, F, Parent) end)
             || _I <- lists:seq(1, Workers) ],
    {ok, Fd} = file:open(Name, [write]),
    gather_workers(Pids, Fd).

worker(N, L, F, Parent) ->
    Ans = sample(N, L, F),
    Parent ! {self(), Ans}.

gather_workers([Pid | Rest], Fd) ->
    receive
        {Pid, Ans} ->
            file:write(Fd, Ans),
            gather_workers(Rest, Fd)
    end;
gather_workers([], Fd) ->
    file:close(Fd).


        

shell.sh

erl -s rand_test test -s init stop
paste pg_rand.txt pg_rand_os.txt u_rand.txt u_rand_os.txt > all.txt