ConcurrentMLImplementation
==========================

Here are some notes on MLton's implementation of <:ConcurrentML:>.

Concurrent ML was originally implemented for SML/NJ.  It was ported to
MLton in the summer of 2004.  The main difference between the
implementations is that SML/NJ uses continuations to implement CML
threads, while MLton uses its underlying <:MLtonThread:thread>
package.  Presently, MLton's threads are a little more heavyweight
than SML/NJ's continuations, but it's pretty clear that there is some
fat there that could be trimmed.

The implementation of CML in SML/NJ is built upon the first-class
continuations of the `SMLofNJ.Cont` module.
[source,sml]
----
type 'a cont
val callcc: ('a cont -> 'a) -> 'a
val isolate: ('a -> unit) -> 'a cont
val throw: 'a cont -> 'a -> 'b
----

The implementation of CML in MLton is built upon the first-class
threads of the <:MLtonThread:> module.
[source,sml]
----
type 'a t
val new: ('a -> unit) -> 'a t
val prepare: 'a t * 'a -> Runnable.t
val switch: ('a t -> Runnable.t) -> 'a
----

The port is relatively straightforward, because CML always throws to a
continuation at most once.  Hence, an "abstract" implementation of
CML could be built upon first-class one-shot continuations, which map
equally well to SML/NJ's continuations and MLton's threads.

The "essence" of the port is to transform:
----
callcc (fn k => ... throw k' v')
----
{empty}to
----
switch (fn t => ... prepare (t', v'))
----
which suffices for the vast majority of the CML implementation.

There was only one complicated transformation: blocking multiple base
events.  In SML/NJ CML, the representation of base events is given by:
[source,sml]
----
datatype 'a event_status
  = ENABLED of {prio: int, doFn: unit -> 'a}
  | BLOCKED of {
        transId: trans_id ref,
        cleanUp: unit -> unit,
        next: unit -> unit
      } -> 'a
type 'a base_evt = unit -> 'a event_status
----

When synchronizing on a set of base events, which are all blocked, we
must invoke each `BLOCKED` function with the same `transId` and
`cleanUp` (the `transId` is (checked and) set to `CANCEL` by the
`cleanUp` function, which is invoked by the first enabled event; this
"fizzles" every other event in the synchronization group that later
becomes enabled).  However, each `BLOCKED` function is implemented by
a callcc, so that when the event is enabled, it throws back to the
point of synchronization.  Hence, the next function (which doesn't
return) is invoked by the `BLOCKED` function to escape the callcc and
continue in the thread performing the synchronization.  In SML/NJ this
is implemented as follows:
[source,sml]
----
fun ext ([], blockFns) = callcc (fn k => let
      val throw = throw k
      val (transId, setFlg) = mkFlg()
      fun log [] = S.atomicDispatch ()
        | log (blockFn:: r) =
            throw (blockFn {
                transId = transId,
                cleanUp = setFlg,
                next = fn () => log r
              })
      in
        log blockFns; error "[log]"
      end)
----
(Note that `S.atomicDispatch` invokes the continuation of the next
continuation on the ready queue.)  This doesn't map well to the MLton
thread model.  Although it follows the
----
callcc (fn k => ... throw k v)
----
model, the fact that `blockFn` will also attempt to do
----
callcc (fn k' => ... next ())
----
means that the naive transformation will result in nested `switch`-es.

We need to think a little more about what this code is trying to do.
Essentially, each `blockFn` wants to capture this continuation, hold
on to it until the event is enabled, and continue with next; when the
event is enabled, before invoking the continuation and returning to
the synchronization point, the `cleanUp` and other event specific
operations are performed.

To accomplish the same effect in the MLton thread implementation, we
have the following:
[source,sml]
----
datatype 'a status =
   ENABLED of {prio: int, doitFn: unit -> 'a}
 | BLOCKED of {transId: trans_id,
               cleanUp: unit -> unit,
               next: unit -> rdy_thread} -> 'a

type 'a base = unit -> 'a status

fun ext ([], blockFns): 'a =
   S.atomicSwitch
   (fn (t: 'a S.thread) =>
    let
       val (transId, cleanUp) = TransID.mkFlg ()
       fun log blockFns: S.rdy_thread =
          case blockFns of
             [] => (S.atomicEnd (); S.next ())
           | blockFn::blockFns =>
                let
                   val () = S.atomicBegin ()
                in
                   (S.prep o S.new)
                   (fn _ => fn () =>
                    let
                       val () = S.atomicBegin ()
                       val x = blockFn {transId = transId,
                                        cleanUp = cleanUp,
                                        next = fn () => log blockFns}
                    in S.switch(fn _ => S.prepVal (t, x))
                    end)
                end
    in
       log blockFns
    end)
----

To avoid the nested `switch`-es, I run the `blockFn` in it's own
thread, whose only purpose is to return to the synchronization point.
This corresponds to the `throw (blockFn {...})` in the SML/NJ
implementation.  I'm worried that this implementation might be a
little expensive, starting a new thread for each blocked event (when
there are only multiple blocked events in a synchronization group).
But, I don't see another way of implementing this behavior in the
MLton thread model.

Note that another way of thinking about what is going on is to
consider each `blockFn` as prepending a different set of actions to
the thread `t`.  It might be possible to give a
`MLton.Thread.unsafePrepend`.
[source,sml]
----
fun unsafePrepend (T r: 'a t, f: 'b -> 'a): 'b t =
   let
      val t =
         case !r of
            Dead => raise Fail "prepend to a Dead thread"
          | New g => New (g o f)
          | Paused (g, t) => Paused (fn h => g (f o h), t)
   in (* r := Dead; *)
      T (ref t)
   end
----
I have commented out the `r := Dead`, which would allow multiple
prepends to the same thread (i.e., not destroying the original thread
in the process).  Of course, only one of the threads could be run: if
the original thread were in the `Paused` state, then multiple threads
would share the underlying runtime/primitive thread.  Now, this
matches the "one-shot" nature of CML continuations/threads, but I'm
not comfortable with extending `MLton.Thread` with such an unsafe
operation.

Other than this complication with blocking multiple base events, the
port was quite routine.  (As a very pleasant surprise, the CML
implementation in SML/NJ doesn't use any SML/NJ-isms.)  There is a
slight difference in the way in which critical sections are handled in
SML/NJ and MLton; since `MLton.Thread.switch` _always_ leaves a
critical section, it is sometimes necessary to add additional
`atomicBegin`-s/`atomicEnd`-s to ensure that we remain in a critical
section after a thread switch.

While looking at virtually every file in the core CML implementation,
I took the liberty of simplifying things where it seemed possible; in
terms of style, the implementation is about half-way between Reppy's
original and MLton's.

Some changes of note:

* `util/` contains all pertinent data-structures: (functional and
imperative) queues, (functional) priority queues.  Hence, it should be
easier to switch in more efficient or real-time implementations.

* `core-cml/scheduler.sml`: in both implementations, this is where
most of the interesting action takes place.  I've made the connection
between `MLton.Thread.t`-s and `ThreadId.thread_id`-s more abstract
than it is in the SML/NJ implementation, and encapsulated all of the
`MLton.Thread` operations in this module.

* eliminated all of the "by hand" inlining


== Future Extensions ==

The CML documentation says the following:
____

----
CML.joinEvt: thread_id -> unit event
----

* `joinEvt tid`
+
creates an event value for synchronizing on the termination of the
thread with the ID tid.  There are three ways that a thread may
terminate: the function that was passed to spawn (or spawnc) may
return; it may call the exit function, or it may have an uncaught
exception.  Note that `joinEvt` does not distinguish between these
cases; it also does not become enabled if the named thread deadlocks
(even if it is garbage collected).
____

I believe that the `MLton.Finalizable` might be able to relax that
last restriction.  Upon the creation of a `'a Scheduler.thread`, we
could attach a finalizer to the underlying `'a MLton.Thread.t` that
enables the `joinEvt` (in the associated `ThreadID.thread_id`) when
the `'a MLton.Thread.t` becomes unreachable.

I don't know why CML doesn't have
----
CML.kill: thread_id -> unit
----
which has a fairly simple implementation -- setting a kill flag in the
`thread_id` and adjusting the scheduler to discard any killed threads
that it takes off the ready queue.  The fairness of the scheduler
ensures that a killed thread will eventually be discarded.  The
semantics are little murky for blocked threads that are killed,
though.  For example, consider a thread blocked on `SyncVar.mTake mv`
and a thread blocked on `SyncVar.mGet mv`.  If the first thread is
killed while blocked, and a third thread does `SyncVar.mPut (mv, x)`,
then we might expect that we'll enable the second thread, and never
the first.  But, when only the ready queue is able to discard killed
threads, then the `SyncVar.mPut` could enable the first thread
(putting it on the ready queue, from which it will be discarded) and
leave the second thread blocked.  We could solve this by adjusting the
`TransID.trans_id types` and the "cleaner" functions to look for both
canceled transactions and transactions on killed threads.

John Reppy says that <!Cite(MarlowEtAl01)> and <!Cite(FlattFindler04)>
explain why `CML.kill` would be a bad idea.

Between `CML.timeOutEvt` and `CML.kill`, one could give an efficient
solution to the recent `comp.lang.ml` post about terminating a
function that doesn't complete in a given time.
[source,sml]
----
  fun timeOut (f: unit -> 'a, t: Time.time): 'a option =
    let
       val iv = SyncVar.iVar ()
       val tid = CML.spawn (fn () => SyncVar.iPut (iv, f ()))
    in
       CML.select
       [CML.wrap (CML.timeOutEvt t, fn () => (CML.kill tid; NONE)),
        CML.wrap (SyncVar.iGetEvt iv, fn x => SOME x)]
    end
----


== Space Safety ==

There are some CML related posts on the MLton mailing list:

* http://www.mlton.org/pipermail/mlton/2004-May/

that discuss concerns that SML/NJ's implementation is not space
efficient, because multi-shot continuations can be held indefinitely
on event queues.  MLton is better off because of the one-shot nature
-- when an event enables a thread, all other copies of the thread
waiting in other event queues get turned into dead threads (of zero
size).
