David,
Thanks. This makes sense and explains why we were seeing an inactive
thread. It's useful to know that values can be passed from one sessions
to the next via the stack - we hadn't fully appreciated that feature.
This behaviour with threads works well for what we are trying to do but
there is still something we can't explain or see how to work around:
after a loadState, when the running thread retrieves a referenced value,
it no longer appears to pick up the latest value. I've provided an
example that demonstrates this below. Is there a way for the thread to
pick up assignments to the ref variable 'c' after the loadState?
Thanks,
Phil
val c = ref 0;
open Thread;
fun run () : Thread.thread =
let
val cv = ConditionVar.conditionVar ();
val m = Mutex.mutex ();
fun aux () =
let
open Time;
val _ = ConditionVar.waitUntil (cv, m, now () + fromSeconds 1);
in
print (Int.toString (!c));
aux ()
end;
in
Thread.fork (aux, [])
end;
val t = run (); (* prints 0 on standard output every second *)
c := 2; (* now prints 2 *)
PolyML.SaveState.saveState "test1.polydb"; (* save state with c = ref 2 *)
c := 3; (* now prints 3 *)
val t = (PolyML.SaveState.loadState "test1.polydb"; t); (* reload state *)
Thread.isActive t; (* thread still active *)
(* still printing 3, even though saved when c = ref 2 *)
c := 1; (* no effect, still prints 3 *)
David Matthews wrote:
Philip Clayton wrote:
Does anyone know what should happen to threads (created with
Thread.Thread.fork) when PolyML.SaveState.loadState is performed? It
appears that they are no longer reported as active but, despite this,
carry on running. Is it up to the user to kill these before doing a
loadState?
Phil,
Threads continue running after loadState. However there is an issue
with thread identifiers and I think this is what is causing the
confusion.
Saved states are primarily intended to allow the state to be restored
in a different session. Certain values, such as thread identifiers
and stream identifiers, only have meaning within a particular session
and aren't persistent. When they are saved and loaded, even within
the same session, the link with the original object is broken. It is
possible to "carry over" such a value within a session by passing it
on the stack.
Consider:
val y = Thread.Mutex.mutex();
Thread.Mutex.lock y;
val v = Thread.Thread.fork(fn () => Thread.Mutex.lock y, []);
(* v now refers to a blocked but running thread. *)
PolyML.SaveState.saveState "save";
val w = (PolyML.SaveState.loadState "save"; v);
After reloading Poly/ML reports:
> Thread.Thread.isActive v;
val it = false : bool
> Thread.Thread.isActive w;
val it = true : bool
There is a similar situation with streams. The reason it works this
way is that there is no guarantee that the saved state loaded by
loadState actually has been loaded in the same session it was saved
in. To solve this would require (expensive) globally unique identifiers.
David
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