=============
  Lua Lanes
=============

With the introduction of Lindas (Jun-2008), Lua Lanes simplifies its API while
simultaneously adding more power and speed.

Pros:
    - regular Lua 5.1/5.2 module
    - completely separate Lua states, one per OS thread
    - message passing, or shared data using Lindas
    - no application level locking, ever
    - scales well, up to 1000's of threads
    - priorities (-2..+2) for launched threads
    - threads are cancellable (with complete cleanup)
    - timeouts on all pending operations
    - thread contents are given as regular Lua functions; syntax checked early on,
      syntax highlighting works
    - standard libraries opened to subthreads can be granually selected
    - fast stack-to-stack copies, via hidden "keeper states". No serialization needed.
    - protects calls to 'require', allowing wide compatibility with existing
      modules (and all, with minor changes)

Cons:
    - requires OS threads
    - currently 1:1 mapping to OS threads (limits scalability and maybe performance)
    - not utilizing network parallelism (all threads on one CPU)

Sample:
<<
  lanes = require "lanes".configure()
  
  local function calculate(a,b,c)
    if not a then 
      error "sample error; propagated to main lane when reading results"
    end
    return a+b+c
  end

  local h1= lanes.gen("base", calculate)(1,2,3)
  local h2= lanes.new("base", calculate)(10,20,30)
  local h3= lanes.new("base", calculate)(100,200,300)

  print( h1[1], h2[1], h3[1] )     -- pends for the results, or propagates error
<<


===========
  luaproc    (by Skyrme, Rodriguez and Ierusalimschy)
===========

http://www.inf.puc-rio.br/~roberto/docs/ry08-05.pdf

The PDF seems to be an authorative voyage into how Lua could handle multithreading,
in a multicore "separate universes" manner (not like what coroutines already do).

Pros:
    - Tackles both multicore and network parallelism
    - M:N relationship to kernel threads (one kernel thread runs multiple luaprocs)
    - Simple (so they say)
    - Lua author (Roberto) included
    - Can be used also without _any_ OS threading support (works like Rings, then)

Cons:
    - Data passing for "strings, number, or booleans" only
      "More complex types must be encoded in some form"
      (serializing data is slower than the stack-to-stack copies used by i.e. Lanes)
      (yet, serializing allows for network parallelism)
    - Message passing is synchronous (only). The sender will wait until the
      receiver has taken the message.


==================
  Lua coroutines    (by Lua authors)
==================

http://www.lua.org/manual/5.1/manual.html#2.11
http://lua-users.org/wiki/CoroutinesTutorial

Lua coroutines is an integral part of Lua 5 itself. It is listed here, since
it should be the _first_ multitasking mechanism to consider. It can also be
used together with Lanes, or the other mechanisms listed below.

Coroutines are very usable in provider/consumer situations, allowing you to
queue Lua functions on an as-needed dataflow basis with each other.
 
Pros:
    - works with plain Lua (no extensions)
    - works on any platform
    - lightweight (no OS level threading or locking involved)

Cons:
    - co-operative, meaning your code will need to decide, who gets to run
    - does not utilize multiple CPUs/cores

Sample:

    ..TBD: sample code doing the "child" "parent" output here (see below)..


=============
  LuaThread     (by Diego Nehab)
=============

http://www.cs.princeton.edu/~diego/professional/luathread/

LuaThread provides thread creation, mutexes, condition variables, and inter-
thread queues to the Lua scripts. It takes a C-kind of approach, where Lua
globals are shared by the threads running, and need therefore to be guarded
against multithreading conflicts. 

Whether this is exactly what you want, or whether a more loosely implemented
multithreading (s.a. Lanes) would be better, is up to you. One can argue that
a loose implementation is easier for the developer, since no application level
lockings need to be considered.

Pros:
    - no marshalling overhead, since threads share the same Lua state

Cons:
    - requires a modified Lua core
    - application level locking required

Sample:
<<
  local function flood(output, word)
    while 1 do 
        output:lock()
        io.write(word, ", ")
        output:unlock()
    end
  end

  local output = thread.newmutex()
  thread.newthread(flood, {output, "child"})
  flood(output, "parent")
<<


=============
  Lua Rings     (by Roberto Ierusalimschy & Tomás Guisasola)
=============

http://www.keplerproject.org/rings/

".. library which provides a way to create new Lua states from within Lua. 
It also offers a simple way to communicate between the creator (master) and 
the created (slave) states."

".. master can execute code in any of its slaves but each slave only has
access to its master (or its own slaves)."

Rings offers separate Lua states, but no multithreading. This makes it simple,
but it won't use more than one CPU core. Other differences include:

    - opens all Lua standard libraries for subthreads
      (Lanes opens the needed ones)

    - marshalls numbers, strings, booleans, userdata
      (Lanes also marshalls tables, functions, upvalues, ..)

    - "remotedostring" allows executing code in the master state
      (Lanes does _not_ allow subthreads to trouble/modify master automatically,
      to allow effective sandboxing. The same can be achieved by sending code 
      between the threads, but master needs to explicitly allow this = receive
      a function and execute it)

    - offers "Stable, a very simple API to manage a shared table at the master
      state"
      (Lanes 2008 offers keeper tables)

Pros:
    - "offers Stable, a very simple API to manage a shared table at the master 
    state"

Cons:
    - thread contents defined as strings, not Lua source as such; does not
      give syntax check at file parsing, does not allow syntax highlight

Sample:
<<
  require"rings"
  S = rings.new ()

  data = { 12, 13, 14, }
  print (S:dostring ([[
  aux = {}
  for i, v in ipairs (arg) do
	table.insert (aux, 1, v)
  end
  return unpack (aux)]], unpack (data))) -- true, 14, 13, 12

  S:close ()
<<


==========================
  Helper Threads Toolkit    (by Javier Guerra G.)
==========================

http://luaforge.net/projects/helper-threads/

"Provides a consistent framework to write non-blocking C libraries, with a Lua
interface for starting tasks and managing the Futures, Queues and Threads."

This seems like a companion of the "occasional threading" model (see below);
Lua side is kept clear of multithreading, while C side can be "spawn" off to
do things on the background.

Pros:
    - provides an "update" mechanism, allowing the (C) thread and controlling
      Lua to interact during execution of the thread
    - ...

Cons:
    - thread is "only for C code and it can't touch or access the Lua state",
      in other words there is no Lua-side multithreading concept (by design)


========================
  Occasional threading      (by Russ Cox)
========================

http://lua-users.org/lists/lua-l/2006-11/msg00368.html

".. able to have certain C calls run in parallel with the [Lua] VM, but
otherwise keep the VM single-threaded."

That pretty much says it all.

Pros:
    - simple, yet providing for the "occasional" need to run really multithreaded
    - can be made into a loadable module (says the message)

Cons:
    - only for occasional usage, the programming paradigm is still essentially
      singlethreaded (by definition)
    - not a real project, just a message on the Lua list (but a good one!)
    

=================
  ConcurrentLua 
=================

http://concurrentlua.luaforge.net/index.html

ConcurrentLua is based on the Lua model for concurrency, namely coroutines, and
extends this model by providing message-passing primitives. 

".. implementation of the share-nothing asynchronous message-passing model"

".. process can check its mailbox for new messages at any time, and if there 
are any, they can be read in the order of arrival."

".. processes in the system are implemented with Lua coroutines"

".. still based on the cooperative multithreading model that Lua uses"

Recent, released on 21 June 2008.

Pros:
    - From ground up designed for distributed computing (multiple computers,
    not only CPU cores)
    - Does not require a pre-emptive operating system

Cons:
    - Serialization must degrade raw performance in one-computer scenarios
      (vs. stack-to-stack copying ala Lanes)
    - Depends on LuaSocket and Copas modules.
    - Each thread has a single mailbox tied to it (vs. separating threads and
      connection resources)