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Re: map-par slower than map


From: Zelphir Kaltstahl
Subject: Re: map-par slower than map
Date: Thu, 10 Nov 2022 10:52:58 +0000

Hi Damien!

I think Racket futures and Guile futures are a bit different. According to the Racket documentation "The level of parallelism available from those constructs, however, is limited by several factors, and the current implementation is best suited to numerical tasks." (https://docs.racket-lang.org/guide/parallelism.html#%28part._effective-futures%29). So I am not sure, if they will work well for your use-case. I think I remember there having been a discussion on the Guile mailing list, where I asked, whether the Guile futures suffer from the same limitations, but I am not sure, that this question was sufficiently answered. I personally haven't noticed any blocking in my pure mathematical project Euler code.

That said. If you can send me some example code, which does not require me to set up the whole thing of Scheme+, then I can take a look and check on my end, how what when blocks. Or at least send me some snippet, which I can run without setting up lots of things, maybe with 1 simple command, where the entry point to `run-in-parallel` is obvious.

Regards,
Zelphir

On 11/10/22 11:41, Damien Mattei wrote:
note that it is not a Guile problem, the same code give also no speed up with Racket 'future ,i have not already test it but it should block also on 'touch future...

On Thu, Nov 10, 2022 at 11:32 AM Damien Mattei <damien.mattei@gmail.com> wrote:

    Hello Zelphir,

    i finally find a possible cause of no speed up of my code, i find that
    using your code the procedure keep blocked on the first 'touch at line 27
    here:

    
https://notabug.org/ZelphirKaltstahl/guile-project-euler-solutions/src/ebb19b11b465903105924adb6252f1e2ecf63859/lib/parallelism.scm#L27

    if i add a 'display i got this output, see the second part ,i cut it
    waiting the rest of output , it is blockers on the first 'touch until it
    return ,after all the touch are fast as if all the job is done in the
    first 'touch

    unct-unify-minterms-set-1-unit-future : begin
    set1-length = 930
    set2-length = 1270
    before Cartesian product set
    after Cartesian product set
    minterms-set-length = 1181100
    minterms-set-first = ((1 1 1 x x 0 0 0 0 1) (1 1 1 1 x x 0 0 0 1))
    segmts = ((0 . 196850) (196851 . 393701) (393702 . 590552) (590553 .
    787403) (787404 . 984254) (984255 . 1181099))
    before //
    run-in-parallel : making future
    run-in-parallel : making future
    run-in-parallel : making future
    run-in-parallel : making future
    run-in-parallel : making future
    run-in-parallel : making future
    run-in-parallel : touching future
    run-in-parallel : touching future
    run-in-parallel : touching future
    run-in-parallel : touching future
    run-in-parallel : touching future
    run-in-parallel : touching future
    after //
    unified-minterms-vector-1-length = 1181100

    funct-unify-minterms-set-1-unit-future : end
    funct-unify-minterms-set-1-unit-future : begin
    set1-length = 1270
    set2-length = 888
    before Cartesian product set
    after Cartesian product set
    minterms-set-length = 1127760
    minterms-set-first = ((1 1 1 1 x x 0 0 0 1) (1 1 1 1 1 x x 0 0 1))
    segmts = ((0 . 187960) (187961 . 375921) (375922 . 563882) (563883 .
    751843) (751844 . 939804) (939805 . 1127759))
    before //
    run-in-parallel : making future
    run-in-parallel : making future
    run-in-parallel : making future
    run-in-parallel : making future
    run-in-parallel : making future
    run-in-parallel : making future
    run-in-parallel : touching future

    blocking just above

    i find no explanation in Guile doc:

    Scheme Procedure: *touch* /f/

        Return the result of the expression embedded in future f.

        If the result was already computed in parallel, |touch| returns
        instantaneously. Otherwise, it waits for the computation to complete,
        if it already started, or initiates it. In the former case, the
        calling thread may process other futures in the meantime.

    perheaps 'map is not the good way to "launch" futures?

    here is my version of code with display that genrate the output above:

    (define run-in-parallel
      (λ (segments map-proc) ;;reduce-proc reduce-init)
        "Use futures to run a procedure in parallel, if
    multiple cores are available. Take a list of SEGMENTS as
    input, which are ranges of values to work on. MAP-PROC is
    applied to the SEGMENTS using map. When the MAP-PROC calls
    for all segments finished and returned values, the
    REDUCE-PROC is applied to the map result using reduce and
    the REDUCE-INIT argument."
        (let ([futures
      (map (λ (seg)
     (display-nl "run-in-parallel : making future")
     (make-future
      ;; Need to wrap in a thunk, to not
      ;; immediately start evaluating.
      (λ () (map-proc seg))))
    segments)])
          ;;(let ([segment-results (map touch futures)])
          (let ([segment-results (map (lambda (f)
       (display-nl "run-in-parallel : touching future")
       (touch f))
     futures)])
    segment-results
    ;; (reduce reduce-proc
    ;; reduce-init
    ;; segment-results)
    ))))


    Best regards,

    Damien

    On Wed, Oct 12, 2022 at 11:29 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

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