A piece of background on par-map:

When I introduced par-map et al the only ambition was to have simple
language constructs to invoke parallelism. The use case I had in mind was
course grained parallelism where each piece of work is somewhat
substantial. Back then, a thread was launched for each piece of work,
however there was also a thread pool such that not all of the overhead of
launching new threads always was required.

Since then, par-map has been rewritten (by others) to be based on futures.
(And now the thread pool is localized in the futures implementation---as
"workers".) Looking in the code now, I think it is fair to say that it is
still intended for coarse grained parallelism. There is some heavy lifting
going on with mutexes and condition variables as well as shuffling around
with list pairs.

So, applying par-map on a huge list with small amount of work per item was
never and still isn't the intended use case.

It would of course be interesting if the code could be improved to support
fine grained parallelism. :-)

Best regards,
Mikael

On Wed, Oct 12, 2022 at 11:30 PM Zelphir Kaltstahl <
zelphirkaltstahl@posteo.de> wrote:

> Hi!
>
> On 10/12/22 22:27, Damien Mattei wrote:
> >
> https://github.com/damien-mattei/library-FunctProg/blob/master/guile/logiki%2B.scm#L1674
> >
> > i commited the current version of code here with all files but it is
> > huge.... :-/
> >
> > On Wed, Oct 12, 2022 at 10:20 PM Damien Mattei <damien.mattei@gmail.com>
> > wrote:
> >
> >> Mutex? i do not think code has situation where dead lock could happen,
> it
> >> is a code about minimalising logic expressions, it uses minterms ,
> minterms
> >> set is a set of minterms :like this:
> >>
> >> example:
> >> ((1 1 0) (1 1 1)) will be unified : (1 1 x)
> >> because 0 and 1 are replaced by x
> >> the minterms-set could have thousands of pair (mathematic not lisp)
> >> minterms to unify
> >> if there is more than one x as result there is no need to continue so i
> >> escape with a continuation:
> >>
> >> minterms-set =
> >> {
> >> ((1 0 1 0) (1 1 1 0))
> >> ((1 0 1 0) (1 1 0 1))
> >> ((1 0 1 0) (1 0 1 1))
> >> ((1 0 1 0) (0 1 1 1))
> >> ((0 1 1 0) (1 1 1 0))
> >> ((0 1 1 0) (1 1 0 1))
> >> ((0 1 1 0) (1 0 1 1))
> >> ((0 1 1 0) (0 1 1 1))
> >> ((0 1 0 1) (1 1 1 0))
> >> ((0 1 0 1) (1 1 0 1))
> >> ((0 1 0 1) (1 0 1 1))
> >> ((0 1 0 1) (0 1 1 1))
> >> ((0 0 1 1) (1 1 1 0))
> >> ((0 0 1 1) (1 1 0 1))
> >> ((0 0 1 1) (1 0 1 1))
> >> ((0 0 1 1) (0 1 1 1))
> >> }
> >>
> >> replace { } by () to have the list, other example at another level :
> >>
> >> minterms-set =
> >> {
> >> ((0 x 1 1) (x 1 1 1))
> >> ((0 x 1 1) (1 x 1 1))
> >> ((0 x 1 1) (1 1 x 1))
> >> ((0 x 1 1) (1 1 1 x))
> >> ((x 0 1 1) (x 1 1 1))
> >> ((x 0 1 1) (1 x 1 1))
> >> ((x 0 1 1) (1 1 x 1))
> >> ((x 0 1 1) (1 1 1 x))
> >> ((0 1 x 1) (x 1 1 1))
> >> ((0 1 x 1) (1 x 1 1))
> >> ((0 1 x 1) (1 1 x 1))
> >> ((0 1 x 1) (1 1 1 x))
> >> ((x 1 0 1) (x 1 1 1))
> >> ((x 1 0 1) (1 x 1 1))
> >> ((x 1 0 1) (1 1 x 1))
> >> ((x 1 0 1) (1 1 1 x))
> >> ((0 1 1 x) (x 1 1 1))
> >> ((0 1 1 x) (1 x 1 1))
> >> ((0 1 1 x) (1 1 x 1))
> >> ((0 1 1 x) (1 1 1 x))
> >> ((x 1 1 0) (x 1 1 1))
> >> ((x 1 1 0) (1 x 1 1))
> >> ((x 1 1 0) (1 1 x 1))
> >> ((x 1 1 0) (1 1 1 x))
> >> ((1 0 1 x) (x 1 1 1))
> >> ((1 0 1 x) (1 x 1 1))
> >> ((1 0 1 x) (1 1 x 1))
> >> ((1 0 1 x) (1 1 1 x))
> >> ((1 x 1 0) (x 1 1 1))
> >> ((1 x 1 0) (1 x 1 1))
> >> ((1 x 1 0) (1 1 x 1))
> >> ((1 x 1 0) (1 1 1 x))
> >> }
> >>
> >> here we see some minterms are already unified
> >>
> >>   it is not easy to read even by me because i wrote the code many years
> ago
> >> and is split in many files, but here it is:
> >>
> >> (par-map function-unify-minterms-list minterms-set)
> >>
> >> {function-unify-minterms-list <+ (λ (L) (apply
> >> function-unify-two-minterms-and-tag L))}
> >>
> >> (define (unify-two-minterms mt1 mt2)
> >>    (function-map-with-escaping-by-kontinuation2
> >>   (macro-function-compare-2-bits-with-continuation) mt1 mt2))
> >>
> >> ;; (function-map-with-escaping-by-kontinuation2
> >> (macro-function-compare-2-bits-with-continuation)   '(1 1 0 1 0 1 1 0)
> '(1
> >> 1 0 1 1 1 1 1))
> >>
> >> ;; list1 = (1 1 0 1 0 1 1 0)
> >> ;; more-lists = ((1 1 0 1 1 1 1 1))
> >> ;; lists = ((1 1 0 1 0 1 1 0) (1 1 0 1 1 1 1 1))
> >> ;; clozure = #<procedure:...gos-DrRacket.scm:195:11>
> >>
> >> ;; #f
> >> ;;
> >> ;;  (function-map-with-escaping-by-kontinuation2
> >> (macro-function-compare-2-bits-with-continuation)    '(1 1 0 1 0 1 1 0)
> '(1
> >> 1 0 1 1 1 1 0))
> >>
> >> ;; list1 = (1 1 0 1 0 1 1 0)
> >> ;; more-lists = ((1 1 0 1 1 1 1 0))
> >> ;; lists = ((1 1 0 1 0 1 1 0) (1 1 0 1 1 1 1 0))
> >> ;; clozure = #<procedure:...gos-DrRacket.scm:195:11>
> >>
> >> ;; '(1 1 0 1 x 1 1 0)
> >> (define (function-map-with-escaping-by-kontinuation2 clozure list1 .
> >> more-lists)
> >>    (call/cc (lambda (kontinuation)
> >>      (let ((lists (cons list1 more-lists))
> >>    (funct-continu ;; this function have the kontinuation in his
> environment
> >>     (lambda (arg1 . more-args)
> >>       (let ((args (cons arg1 more-args)))
> >> (apply clozure kontinuation args))))) ;; a tester: (apply clozure (cons
> >> conti args))
> >>
> >>           ;; (newline)
> >>           ;; (dv list1)
> >>           ;; (dv more-lists)
> >>           ;; (dv lists)
> >>   ;; (dv clozure)
> >>           ;; (newline)
> >>
> >>        (apply map funct-continu lists)))))
> >>
> >> (define-syntax macro-function-compare-2-bits-with-continuation ;;
> >> continuation version of macro-compare-2-bits
> >>    ;; i need a macro because of external function to the clozure
> >>    (syntax-rules ()
> >>      ((_) (let ((cnt 0)) ;; counter
> >>    (lambda (continuation b1 b2) (if (equal? b1 b2)
> >>   b1
> >>   (begin
> >>     (set! cnt (add1 cnt)) ;; we leave with continuation in case cpt >
> 1, we
> >> can have used a flag too instead of a counter
> >>     (when (> cnt 1) (continuation #f)) ;; escaping with the continuation
> >>     'x))))))) ;; return x in case of (b1,b2) = (O,1) or (1,0)
> >>
> >> what could have caused mutex if in the latter definition above (let
> ((cnt
> >> 0)) ;; counter was defined at top level and shared by all threads!!! yes
> >> there could have be some mutex  but this is not the case, i think even
> all
> >> function are pure so why is it more slow with // than without?
> >> Damien
> >>
> >> On Wed, Oct 12, 2022 at 8:45 PM Maxime Devos <maximedevos@telenet.be>
> >> wrote:
> >>
> >>> On 12-10-2022 19:19, Damien Mattei wrote:
> >>>> Hello,
> >>>> all is in the title, i test on a approximately 30000 element list , i
> >>> got
> >>>> 9s with map and 3min 30s with par-map on exactly the same piece of
> >>> code!?
> >>>   > [...]
> >>>   >
> >>>> translated from Scheme+ to Scheme:
> >>>> (define unified-minterms-set-1 (map function-unify-minterms-list
> >>>> minterms-set)) ;;(par-map function-unify-minterms-list minterms-set))
> >>> The definition of 'function-unify-minterms-list' and 'minterms-set' is
> >>> missing.  Without a test case, we can only speculate what's going on.
> >>> (E.g., maybe it grabs a mutex).
> >>>
> >>> Greetings,
> >>> Maxime.
> I don't want to scare anyone, just maybe warn about parallel map. I once
> tried
> to use Guile's parallel map function for a decision tree implementation
> (
> https://notabug.org/ZelphirKaltstahl/guile-ml/src/cf666801fea91c9fa8fa290764ff6c60b7f3949d/decision-tree.scm),
>
> where each branch while learning the tree would call parallel map again
> for sub
> branches and so on. Somehow it made Guile crash (I don't have the error
> message
> any longer, but I did post about it on the mailing list back then.). I
> never
> figured out, what went wrong. All I had was pure function calls and math
> inside
> the thing that parallel map was supposed to run.
>
> Ultimately I simply tried other parallelism constructs and when I switched
> to
> using futures instead, everything worked fine, no crashes, no errors.
>
> Since that time, I did not use parallel map and instead used futures.
> Recently I
> made a parallelization thing for solving exercises of Project Euler using
> multiple cores, so that some solutions are calculated faster. Maybe this
> can
> help or can be adapted to another use case:
>
>
> https://notabug.org/ZelphirKaltstahl/guile-project-euler-solutions/src/ebb19b11b465903105924adb6252f1e2ecf63859/lib/parallelism.scm#L11-L30
>
> It expects ranges of things, which are called `segments` in the code.
> Usually
> ranges of numbers for Project Euler things. Here is the code to split a
> range
> into segments:
>
>
> https://notabug.org/ZelphirKaltstahl/guile-project-euler-solutions/src/ebb19b11b465903105924adb6252f1e2ecf63859/lib/segment.scm
>
> (Check any solution using it for an example.)
>
> So this might be a bit too specific for general parallel things, but I
> guess one
> could change the way futures are used in `run-in-parallel`, to fit any
> other
> purpose.
>
> Best regards,
> Zelphir
>
> --
> repositories: https://notabug.org/ZelphirKaltstahl
>
>
>