Erlang parallel building blocks


Copyright (c) 2011, Witold Baryluk
All rights reserved.
Licensed under BSD license, see LICENSE for details.



Somtimes we need to start multiple parallal tasks and then wait until
all of them completes.

This is acomplished using epbb module, with new_group/0, spawn/2,
spawn_list/2, go/1, async_go/1, barrier/0,1 and sync/1 functions:

Example:

P = self(),
Group = epbb2:new_group(),
Group1 = epbb2:spawn_list([
                 fun(U) -> io:format("A~n"), epbb2:barrier(U), io:format("K~n") end,
                 fun(U) -> io:format("B~n"), epbb2:barrier(U), io:format("L~n") end,
                 fun(U) -> io:format("C~n"), epbb2:barrier(U), io:format("M~n") end
            ], Group),
Group2 = epbb2:spawn(fun(U) -> io:format("D~n"), epbb2:barrier(U), io:format("N~n") end, Group1),
{Group3, goed} = epbb2:go(Group2),
{_, synced} = epbb2:sync(Group3),
io:format("Z~n").


It will display A, B, C, D before K, L, M, N which will be before Z.
Actuall A, B, C, D can be permuted. Similary K, L, M, N. But ordering
beetween groups will be conserved.

For example on my machine it output gives:

C
B
A
D
N
K
L
M
Z
ok



There are two identical APIs which can be used. epbb and epbb2, they
just differ in implementation.

epbb is based on direct handling of processes in sync function in
calling processes.

epbb2 starts new controling processes in new_group, and all other calls
are actually just sending messages to this controling process. It is
more flexible, more robust and should be faster, if you have multiple
groups, or want to perform something in the calling processes without
worring about usage of barriers in spawned processes.


Instead of fun/1, one can pass fun/0 to spawn/spawn_list functions, but
then using of epbb2:barrier/1 is impossible (or rather not recommended,
because it is possible to use epbb2:barrier/0, for same functionality,
Pid/Ref of parent/controlling process, will be just implicitly stored in
process' dictionary under key '$epbb2_parent').


Spawned tasks, can perform currently actions:
   - epbb2:barrier() - must be called by all spawned processes
               to make progress through it
   - epbb2:iamdone() - called automatically when processes ends


TODO:
   - reduce
   - broadcast_to_other
   - send_to_parent
   - receive_from_parent

Two last functions involves parent to communicate with all processes,
this means that all spawned processes should follow same code path.