RE: First timer on cooperative multitasking

["Thijs Schreijer" <thijs@...>]

> > Hi all,
> >
> > I've been using lua for some time as an embedded scripting language.
> The
> > implementation was a pre-emptive multitasking environment. I now have
> the
> > intention to do some lua-only developments and have been looking into
> the
> > cooperative multitasking model.
> >
> > The basics are very clear, but there is one part that I feel
> uncomfortable
> > with; How often and when would I have to yield? Are there any
> guidelines on
> > that?
> 
>   I implemented a cooperative scheduler in C/Lua based around a
> homebrewed
> network API.  Basically, when ever I read from a socket, I would yield,
> and
> possibly with socket writes as well (for instance, I wanted to write
> 8k, but
> only 1k was accepted, yield until I can write to that socket again).  I
> didn't bother with trying to ensure arbitrary code would yield properly
> as I
> was the only one writing the code.
> 
>   The C code was event driven (using epoll() under Linux).  The Lua
> code is
> nice---for instance, an echo service:
> 
> 	-- When a TCP connection is accepted, a Lua coroutine is created
> and
> 	-- a function called "main()" in the script is called.  Any calls
> 	-- through the "socket" object can block, causing another Lua
> 	-- "thread" to run.
> 
> 	function main(socket)
> 	  while true do
> 	    socket:write(socket:read("*a"))
> 	  end
> 	end
> 
>   The C code ... isn't quite so straight forward.
>

I'm looking at lua for the cross platform support mac, win, linux, etc. with
ultimately also embedded linux (wlan routers, nas devices, etc.) So I want
to stick to only cross platform stuff and hence choose luasockets.


> > In general my understanding is that not yielding for too long, will
> make
> > other threads unresponsive, that's obvious. But I intent do use
> luasockets
> > and maybe some RS232 serial stuff, so that's where my concerns are;
> loosing
> > incoming data on those IO channels because another thread doesn't
> yield in
> > time.
> 
>   It's up to the programmer writing the scripts to properly yield---if
> you
> want to enfore this (and don't trust the programmer), you need
> preemption,
> period.

That's clear. No problem, I intend to write the code myself, so that
shouldn't be an issue. Also I want it to use cooperative, as a learning
experience (I have had my share of race conditions, deadlocks and the like).

> 
>   But, like I said, decent places to call yield() behind the scenes
> include
> places that normally block, like network or serial communication.
> 
> > The google mainly returned long debates on preemptive vs cooperative,
> but no
> > info on how to make it work best.
> 
>   Cooperative multitasking is actually very simple---just yield to
> another
> thread.  When to do that, is the not-so-easy part.

And that's exactly my main question... 

> 
>   -spc (If you like, I can provide what I have, although it's mostly
> 	undocumented ... )

Thx Sean for your kind (and elaborate) reply