Re: Coroutines & scheduler query

[Adrian Sietsma <adrian_groups@...>]

David Given wrote:
> I have a project for which I'm going to want a large number of coroutines, all 
> communicating via a scheduler. Unfortunately, while I'm pretty much up to 
> speed with the rest of Lua, this isn't an area I've had much experience with.
When are these coroutines created ? all at once, or in response to some event ?

> I remember reading on the list that using a scheduler in this way is not 
> compatible with using coroutines to invert control flow, for example when 
> using a producer/consumer system. This seems to be because the scheduler 
> would have to resume a particular thread with coroutine.resume(), which means 
> that when that thread yields, expecting to go back to its caller, it'll 
> instead end up at the scheduler.
Yes, but the scheduler then resumes the "next" coroutine to proceed.

> Are there any nifty ways round this?
The following pseudocode is a simplified version of what i'm using for a 
http proxy server.
(all the coroutine construction/scheduling is wrapped in the real thing)

-- note : threads is a collection of coroutines to be executed
active_thrd = nil

function startup()
  local thrd = coroutine.create(function() print("func1",active_thrd) end)
  -- schedule for immediate action (push onto threads collection)
  threads.push(thrd,0)

  thrd = coroutine.create(function() print("func2",active_thrd) end)
  threads.push(thrd,0)
  print("yielding")
  threads.push(active_thrd,0) -- re queue myself (vital !)
  coroutine.yield()
  print("i'm back")
  -- do more stuff
return true -- done with this thread


-- schedule the warmup
threads.push(coroutine.create(startup),0)

-- main scheduler
while (true)
   local thrd, args = threads.pop()
   if not thrd then break end
   active_thrd = thrd
   local status, ret, err = coroutine.resume(thrd, unpack(args))
   if not status then -- lua error or assert
      break
   end
end


note that this is co-operative multitasking- a thread will run until it yields.

(i'm converting to lua 5.1 - coroutine.running() removes the need to pass 
around the current thread)

> In addition, is there any fundamental difference between coroutine.resume() 
> and coroutine.yield(), apart from the fact that yield knows which coroutine 
> to switch to automatically? 
coroutine.yield() returns control to the caller, which may or may not be a 
coroutine.

Can I use resume() to keep switching between two
> different coroutines without eating huge quantities of stack space?
I have run the proxy server for long periods without memory leakage or stack 
growth.

Adrian