(Unfortunately, only two of those 11 implementations are actually
ready for production use, but by the end of the year that number
will probably go up to four or five.) (Update: it's now 5: MRI, JRuby, YARV (the interpreter for Ruby 1.9), Rubinius and IronRuby).
The first implementation doesn't actually have a name, which
makes it quite awkward to refer to it and is really annoying and
confusing. It is most often referred to as "Ruby", which is even
more annoying and confusing than having no name, because it
leads to endless confusion between the features of the Ruby
Programming Language and a particular Ruby Implementation.
It is also sometimes called "MRI" (for "Matz's Ruby
Implementation"), CRuby or MatzRuby.
MRI implements Ruby Threads as Green Threads within its
interpreter. Unfortunately, it doesn't allow those threads
to be scheduled in parallel, they can only run one thread at a
time.
However, any number of C Threads (POSIX Threads etc.) can run
in parallel to the Ruby Thread, so external C Libraries, or MRI
C Extensions that create threads of their own can still run in
parallel.
The second implementation is YARV (short for "Yet
Another Ruby VM"). YARV implements Ruby Threads as POSIX or
Windows NT Threads, however, it uses a Global Interpreter
Lock (GIL) to ensure that only one Ruby Thread can actually be
scheduled at any one time.
Like MRI, C Threads can actually run parallel to Ruby Threads.
In the future, it is possible, that the GIL might get broken
down into more fine-grained locks, thus allowing more and more
code to actually run in parallel, but that's so far away, it is
not even planned yet.
JRuby implements Ruby Threads as Native Threads,
where "Native Threads" in case of the JVM obviously means "JVM
Threads". JRuby imposes no additional locking on them. So,
whether those threads can actually run in parallel depends on
the JVM: some JVMs implement JVM Threads as OS Threads and some
as Green Threads. (The mainstream JVMs from Sun/Oracle use exclusively OS threads since JDK 1.3)
XRuby also implements Ruby Threads as JVM Threads. Update: XRuby is dead.
IronRuby implements Ruby Threads as Native Threads,
where "Native Threads" in case of the CLR obviously means
"CLR Threads". IronRuby imposes no additional locking on them,
so, they should run in parallel, as long as your CLR supports
that.
Ruby.NET also implements Ruby Threads as CLR
Threads. Update: Ruby.NET is dead.
Rubinius implements Ruby Threads as Green Threads
within its Virtual Machine. More precisely: the Rubinius
VM exports a very lightweight, very flexible
concurrency/parallelism/non-local control-flow construct, called
a "Task", and all other concurrency constructs (Threads in
this discussion, but also Continuations, Actors and
other stuff) are implemented in pure Ruby, using Tasks.
Rubinius can not (currently) schedule Threads in parallel,
however, adding that isn't too much of a problem: Rubinius can
already run several VM instances in several POSIX Threads in
parallel, within one Rubinius process. Since Threads are
actually implemented in Ruby, they can, like any other Ruby
object, be serialized and sent to a different VM in a different
POSIX Thread. (That's the same model the BEAM Erlang VM
uses for SMP concurrency. It is already implemented for
Rubinius Actors.)
Update: The information about Rubinius in this answer is about the Shotgun VM, which doesn't exist anymore. The "new" C++ VM does not use green threads scheduled across multiple VMs (i.e. Erlang/BEAM style), it uses a more traditional single VM with multiple native OS threads model, just like the one employed by, say, the CLR, Mono, and pretty much every JVM.
MacRuby started out as a port of YARV on top of the
Objective-C Runtime and CoreFoundation and Cocoa Frameworks. It
has now significantly diverged from YARV, but AFAIK it currently
still shares the same Threading Model with YARV.
Update: MacRuby depends on apples garbage collector which is declared deprecated and will be removed in later versions of MacOSX, MacRuby is undead.
Cardinal is a Ruby Implementation for the Parrot
Virtual Machine. It doesn't implement threads yet, however,
when it does, it will probably implement them as Parrot
Threads. Update: Cardinal seems very inactive/dead.
MagLev is a Ruby Implementation for the GemStone/S
Smalltalk VM. I have no information what threading model
GemStone/S uses, what threading model MagLev uses or even if
threads are even implemented yet (probably not).
HotRuby is not a full Ruby Implementation of its
own. It is an implementation of a YARV bytecode VM in
JavaScript. HotRuby doesn't support threads (yet?) and when it
does, they won't be able to run in parallel, because JavaScript
has no support for true parallelism. There is an ActionScript
version of HotRuby, however, and ActionScript might actually
support parallelism. Update: HotRuby is dead.
Unfortunately, only two of these 11 Ruby Implementations are
actually production-ready: MRI and JRuby.
So, if you want true parallel threads, JRuby is currently your
only choice – not that that's a bad one: JRuby is actually faster
than MRI, and arguably more stable.
Otherwise, the "classical" Ruby solution is to use processes
instead of threads for parallelism. The Ruby Core Library
contains the Process module with the Process.fork
method which makes it dead easy to fork off another Ruby
process. Also, the Ruby Standard Library contains the
Distributed Ruby (dRuby / dRb) library, which allows Ruby
code to be trivially distributed across multiple processes, not
only on the same machine but also across the network.
Best Answer
C++98 does not have any support for threading, neither in the language nor the standard library. You need to use a third party library and you have already listed a number of the main candidates.