Re: Threads and asyncs

[Lynn Winebarger <owinebar@free-expression.org>]

On Wednesday 04 September 2002 19:20, Tom Lord wrote:
>        > so that A really contains only frames generated by eval.
> 
> 
> Why do you need more than one such frame (on a C stack) per Scheme
> thread (without calls out to "non-primitive foreign functions")?
> 
> With only one C frame, call/cc can be _really_ fast and I think you
> can go in this direction without trashing eval's performance.

      The problem with this is that it requires really fast garbage collection.
Guile's commitment to conservative collection can present a serious
problem in this regard.   WIth this proposal, continuations that
do not have a reference after the first execution (i.e. no call/cc)
will be "garbage collected" simply by popping the stack.  New
continuations get allocated only and exactly when needed.
   Indeed, I think it can the proposal can be completely characterized
as merely an allocation strategy for heap-allocated stack frames,
and nothing more.  Now that I think about it, I believe it will do strictly 
less work than applying general GC to individual activation records.
(that is, whatever general GC the interpreter is using - I don't make
the claim for this method plus Guile's GC versus unfettered frame
allocation + some other GC; rather this method plus GC versus
unfettered frame allocation plus that same GC).

> But, loosely speaking -- the idea of mixing special stacks for
> classical C with stacks just for Scheme seems to me to be a useful
> one, and, if the interfaces in Guile are suitably cleaned up, a way to
> evolve forward.
> 
> It's a drag that the work on Guile debugger support has thwarted
> simply dropping in the faster eval from recent SCMs -- but you could
> leapfrog over that with a stackless eval.

       Is all the thread hair in SCM as well?

> 	 > and current C programs can be grandfathered
> 
> One language implementor I met made a big fuss over amdhal's law --
> which has the implication for Guile that cleaning up various apis
> (internal and external) should be a big priority.

     Ok, I give up, how do the two relate exactly?

> I sort of agree, except that there's a trap there: which is to build
> up abstractions for abstraction sake in areas where, really, there are
> right and wrong answers, with the right answers requiring less than
> fully general abstractions (and, in fact, being somewhat incompatible
> with fully general abstractions).  

     It's hard to determine the correct answer without knowing
what the question is.

>      > Did that make sense?  Anyone see a hole in it?
> 
> IM*H*O: nothing major.  Keep going in that direction.   But you're
> nearing the frontier between my experience and my speculation, so I'll
> shut up now.

     It's beyond my experience as well, but I think it is correct.  After getting
my MCSI's syntax-case working, I might use it to experiment with this idea.

Lynn
* Is "metacircular scheme interpreter" a common enough phrase (in the scheme
community) to be recognizable as an acronym?


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