Re: Lua on cell phones

[Mike Pall <mikelu-0711@...>]

Roberto Ierusalimschy wrote:
> I do not think I am unbiased enough to present the best arguments.

I guess we need a (completely biased) Lua PR department then? :-)
(no, I'm not volunteering)

Ok, here's my list of arguments why Lua would be a good choice
for a cell phone VM:

- Lua is small: the Lua interpreter easily fits in under 100
  kilobytes including the base libraries. Cell phones are often
  very memory limited -- every kilobyte saved counts.

  It's practically hopeless to try to strip the non-essential
  parts from larger runtimes (Python, Java -- you name it)
  without severely limiting their functionality (J2ME anyone?).
  And you'll still not be able to match Lua's small footprint.
  One really cannot afford a "batteries-included" and
  all-memory-you-can-eat VM on a cell phone.

  Minimal memory use is important even for smartphones with more
  generous resource limits. Their CPU caches are usually small
  and memory access is rather slow compared to desktop CPUs.

- Lua is fast: independent benchmarks show that Lua is often
  amongst the fastest interpreted dynamic languages. The Lua
  interpreter has been extensively tuned for high performance.
  The call-out mechanism to C is one of the fastest, too (compare
  to JNI).

  It has been shown that Lua can be compiled to machine code with
  a JIT compiler (x86 only right now) for even better performance
  at minimal cost (30% larger footprint). Given how underpowered
  many cell phone CPUs are, this means that Lua could be used to
  supplant even more tasks usually delegated only to systems
  programming languages (i.e. C).
  [Disclaimer: I wrote LuaJIT.]

- Extending Lua is easy: Lua has been designed to work together
  with C. The Lua VM offers a small, yet flexible C API.

  Binding to C libraries is the forte of Lua. Often enough a few
  lines suffice. Even beginners can write a binding in minutes
  without studying huge manuals. There are plenty of automatic
  binding generators for larger tasks, too.

  Compared to other language's binding mechanisms, Lua's C API
  effectively provides a shield between the internals of the Lua
  VM and the outer world. One does not have to care about
  internal structures of the Lua VM or even the garbage collector
  (how many refcounting bugs do your Python bindings have?).

- Embedding Lua is easy: the Lua core uses zero static data. All
  C APIs require a single opaque handle which carries along the
  complete state of the VM.

  Memory use is strictly contained. The Lua core does not leak
  memory. Even custom memory allocation functions can be used.
  It's easy to use multiple completely independent Lua
  interpreters in a single process if there should be a need.

  Lua needs only a handful of ANSI C library functions for
  operation. And even these can be further reduced. Lua has
  excellent portability to even the most restricted embedded
  environments.

- Lua has an incremental GC: amongst the options for implementing
  a garbage collector, an incremental GC is most suitable for
  embedded devices. It has low latency, no additional memory cost
  and little implementation complexity.

  A traditional mark-and-sweep GC may cause lengthy pauses which
  are incompatible with semi-real-time requirements (imagine:
  "Please hold the line while we're cleaning up -- beep").
  Generational GC has a much higher memory overhead and its
  heuristic approach to reclamation may hold on to important
  resources much longer than tolerable in such an environment.

  [It has been debated at length whether GC is an appropriate
  mechanism for managing memory in resource constrained devices.
  But manual memory management is tedious and error-prone. The
  benefits of GC, coupled with modern languages by far outweigh
  its disadvantages wrt. average memory use IMHO.]

- Lua has coroutines: multitasking is desirable, even on small
  devices. Too often one encounters annoying limits on cell
  phones, e.g. one cannot open the calendar or lookup a number in
  the phonebook while on the phone with someone. But fully
  preemptive multitasking is either not supported or is
  considered too costly for these environments.
  
  Lua's coroutines provide a fast and memory efficient way for
  non-preemptive multitasking. Lua's coroutines are built-in and
  are independent of the capabilities of the underlying OS. Lua
  even happily runs on devices without an MMU.

- Lua can be sandboxed: it's easy to restrict access to library
  functions on a function-by-function basis. Memory use as well
  as CPU usage can be restricted, too.

  This is very important for a cell phone environment. You really
  want to prevent any random application you've downloaded from
  blocking the GUI forever or silently making expensive phone
  calls on your behalf.

  Removing certain dangerous library functions or (better yet)
  just offering a handful of "safe" functions for use by
  user-written applications is easy. It's even possible to manage
  multiple protection domains with different privilege levels in
  the same Lua VM.

  Other VMs provide either no support for sandboxing (short of
  completely ripping out huge parts of their libraries) or offer
  only complicated and inflexible protection mechanisms. Lua can
  be sandboxed _in_ Lua and with standard Lua calls, providing
  for maximum flexibility at minimum cost.

- Lua is malleable: Lua has just enough syntactic sugar and
  meta-mechanisms to be easily repurposed for domain specific
  languages (DSL).

  It's nice for (readable!) configuration files or (say) a GUI
  layout language. Tasks where you'd otherwise have to design,
  implement and teach extra languages (hand-written XML anyone?).
  With Lua you have to put in the effort only once.

- Lua is simple, but not simplistic: Lua has a simple, yet
  powerful syntax which fits on a single page. The semantics are
  consistent and intuitive.

  Beginners can start to program right away. Teaching Lua is
  easy, excellent programming books are available. No huge and
  expensive IDEs and SDKs are needed, any text editor will do.

  Lua also has advanced features like closures, coroutines,
  meta-mechanism and lots of finely tuned details. All of these
  make it attractive for professional developers, too. But none
  of these features get in the way of beginners.

  The WoW Lua add-ons are a good example of the power of an open
  and collaborative environment. It nicely shows what a huge
  driving factor user contributed (active) content can be for the
  success of a project.
  
  If you want to quickly gain a huge library of gadgets and
  add-ons for your cell phone, then Lua is the way to go. You
  just have to make it easy for them ...

Ok, this list is by no means exhaustive. But it's a start.

--Mike