I really wanted to translate some of the ideas in Coroutines for cooperative multitasking to this page and use the interp limit features to create a control structure that would allow a master interpreter to run several slaves while interrupting them when resources were exhausted (time or # of commands). Then the slave procs could be resumed where they left off using the coroutine command.
Conceptually the interp limit command callback would yield and some other code could be run. The master could run its own code or call one of its slave coprocs, waiting for the limit command to yield again.
I didn't make it very far. The fact that coroutines need to loop until done (or forever) and the limit mechanism requires its command to return after adjusting the interp limit seems incompatible.
Is this at all possible with our current coroutine and interp limit features?
OK -- Here is a more clearly laid out attempt at code. It fails with:
$ ./preemptive.tcl
Call one
one 1
one 2
one 3
Preempt one
cannot yield: C stack busy
#!/home/john/bin/tclsh8.6a2
#
namespace path ::tcl::unsupported
set Tasks {}
proc Task { task args } {
interp create I$task
interp eval I$task proc p$task {} $args
coroutine $task RunTask $task
lappend ::Tasks $task
}
proc Drop { task } {
set here [lsearch $::Tasks $task]
set ::Tasks [lreplace $::Tasks $here $here]
interp delete $task
}
proc RunTask { task } {
yield
interp eval I$task p$task
}
proc Preempt { task commands } {
catch {
puts "Preempt $task"
set ::Limit($task) [expr $::Limit($task) + $commands]
interp limit I$task command -value $::Limit($task)
yield
puts "Return to $task"
} reply; puts $reply
}
Task one {} {
while { 1 } {
incr x
puts "one $x"
}
}
Task two {} {
while { 1 } {
incr x
puts "two $x"
}
}
proc PreemptiveRoundRobin { commands } {
foreach task $::Tasks {
set ::Limit($task) [expr [interp eval I$task info cmdcount] + $commands]
interp limit I$task command -command [list Preempt $task $commands]
interp limit I$task command -value $::Limit($task)
}
while { 1 } {
foreach task $::Tasks {
puts "Call $task"
$task
}
}
}
# Run the two tasks 6 Tcl commands at a time
#
PreemptiveRoundRobin 6
ferrieux So you want Green Threads in Tcl too: I bow to your courage ;-) Now as to the implementation, what about using single-step execution traces instead of interp limit ? Of course it will be slower... but exec traces, contrarily to interp limit, are not linked to an interp's end-of-life (assuming this is the problem; is it ?).
JBR We already have cooperative green threads, via coroutines. Now I want a master thread to select and limit the execution of its slaves instead of just dispatching them.
ferrieux I know :) Matter of terminology here: by "green" I meant "userland", which includes both preemptiver and cooperative... But you didn't answer the suggestion about traces.
MS notes that forcing an interp to yield seems quite feasible to me (without having thought too hard about it). I lack the mental bandwidth at the moment to really think things through but: given well designed syntax and semantics (TIP quality), I volunteer to do the basic implementation. Ball's on your court :P
NEM can't answer the question as to whether this is feasible now with interp limit -- perhaps DKF can answer definitively? It's an interesting idea. The fact that it uses a single interpreter per "thread" would make it seem rather similar to Tcl's native threading model. So the advantage would presumably be that these threads are lighter-weight and so you can (a) run more of them (if you have an extremely concurrent application), and/or (b) there is less context-switching overhead. Is that the idea? (I'm not entirely sure about (b) -- is a coroutine switch cheaper than an OS level context switch?)
JBR I have posted some better code above. It "should" work. It seems that the interp eval calls, or the limit callbacks push another layer on the NRE stack?