Re: Lua and preemptive scheduling for coroutines

----- Ursprüngliche Message -----
> Von: Ross Bencina <rossb-lists@audiomulch.com>
> An: Lua mailing list <lua-l@lists.lua.org>
> CC:
> Gesendet: 4:58 Freitag, 29.März 2013
> Betreff: Re: Lua and preemptive scheduling for coroutines

>
> On 29/03/2013 2:35 PM, Tim Hill wrote:
>>  You're not going to be able to pull that one off. The fundamental
>>  structure of coroutines is different from threads, and nothing you do
>>  can really change that (aside from a VERY significant reworking of
>>  the Lua code base).
>
> I don't know about that, all he's talking about is conditionally
> injecting yeild() into the code at arbitrary points (such as the end of each
> statement)
>
> This could be done manually with a syntactic substitution. The question is
> whether it can be done using a debug hook or similar mechanism.

That's a correct guess and interpretation. I've done something similar for Java using a byte-code instrumentation during class-loading. Indeed, I was injecting a conditional yield() after each n instructions. Alternatively, it could be done at the beginning of each basic block in the control-flow graph of a function. I know that Groovy language, which is also Java based, does something very similar.

But I have no idea how something like byte-code rewriting can be done in Lua. And I have a feeling that debug hooks are a more light-weight approach. They don't need to really change the code by rewriting it. They just intercept execution after each instruction, line, call, return, etc, which seems to be a cleaner way.

Another important point: In case of instruction counting or time slices, I don't need to suspend exactly when the even happens, i.e. I don't need to suspend e.g. inside a C stack (or Lua stack) frames and so on. It is OK to suspend a bit later, once it is safe, i.e. when the execution is easy to suspend.

-Leo