Re: map-par slower than map

[Zelphir Kaltstahl <zelphirkaltstahl@posteo.de>]

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