[racket] place: terrible performance of place-channel-get?

This does seem extremely slow. A place-message send must copy the
vector to send it as a message, but the copy shouldn't take so long.
I'll investigate further.

Meanwhile, an option in this case might be to created a "shared
flvector", which can be passed directly (i.e., without copying) to
another place. I've enclosed a variant of your example to illustrate.

At Mon, 10 Nov 2014 11:58:21 +0200, Alexey Cherkaev wrote:
> Hi,
> 
> I am looking at parallelising some numerical computation with Racket. I’ve 
> tried future/touch first. However, the data for computation is passed as 
> vectors and in my experiments with future/touch it would always find 
> "synchronisation task” upon which all multicore-threads collapse into one core 
> serialised computation.
> 
> Now, I decided to try place. My idea is to make it similar to Common Lisp’s 
> LPARALLEL: create workers <= number of cores and distribute tasks into those 
> workers. The problem I have encountered, however, is that place-channel-get 
> seems to take forever to compute. Here is an example of some simulated 
> computation on a vector using two places and trying to run them in parallel:
> 
> #lang racket
> 
> (require racket/place)
> 
> (provide test-places1 test-places2 long-computation v1 v2 random-vector)
> 
> ;;; Utilities: 
> (define (random-list n)
>   (let loop ((i n) (r '()))
>     (if (zero? i)
>         r
>         (loop (sub1 i) (cons (random) r)))))
> 
> (define (random-vector n)
>   (let ((l (random-list n)))
>     (list->vector l)))
>   
> (define (vector-reduce f init v)
>   (let ((n (vector-length v)))
>     (let loop ((i 0) (r init))
>       (if (= i n)
>           r
>           (loop (add1 i) (f r (vector-ref v i)))))))
> 
> ;;; This is  computation to be run in each place:
> (define (long-computation v)
>   (let ((n (vector-length v))
>         (v1 (vector-copy v)))  ; v is immutable, if want to mutate, must copy it
>     (let loop ((i 0))
>       (if (= i n)
>           (begin
>             (sleep 2)         ; make it work for a bit longer
>             (vector-reduce + 0.0 v1)) ; to make result printable
>           (begin
>             (vector-set! v1 i (* (exp (- (vector-ref v1 i)))
>                                  (sin (* pi (vector-ref v1 i)))))   ;flonum 
> computation
>             (loop (add1 i)))))))
>       
> ;;; two vectors to be sent to long-computation
> (define v1 (random-vector 100000))
> (define v2 (random-vector 100000))
> 
> ;;; Test using one place:
> (define (test-places1)
>   (define p1
>     (place ch1
>            (define v (place-channel-get ch1))
>            (define w (long-computation v))
>            (place-channel-put ch1 w)))  
>   (place-channel-put p1 v1)
>   (time (place-channel-get p1)))
> 
> ;;; Test using 2 places:
> (define (test-places2)
>   (define p1
>     (place ch1
>            (define v (place-channel-get ch1))
>            (define w (long-computation v))
>            (place-channel-put ch1 w)))
>   (define p2
>     (place ch2
>            (define v (place-channel-get ch2))
>            (define w (long-computation v))
>            (place-channel-put ch2 w)))
>   (place-channel-put p1 v1)
>   (place-channel-put p2 v2)
>   (sleep 2) ; hypothetically, after this results shoud be ready immidiately!
>   (time (list (place-channel-get p1) (place-channel-get p2))))
> 
> Exectution from racket on MacBook Pro with Intel Core 2 Duo:
> 
> -> (time (long-computation v1))
> cpu time: 42 real time: 2043 gc time: 0
> 39523.12275516648
> -> (test-places1)
> cpu time: 7593 real time: 7475 gc time: 7001
> 39523.12275516648
> -> (test-places2)
> cpu time: 16591 real time: 12492 gc time: 15485
> '(39523.12275516648 39505.415738171105)
> 
> So, the time of execution of (long-computation v1) and the time of getting the 
> result out of the channel in (test-places1) should be more or less the same, 
> but it is not. Furthermore, (test-places2) takes almost twice as (test-places1) 
> (note, I put (time …) around just getting the value, so it does not include the 
> time of creating the place).
> 
> Am I doing something wrong?
> 
> Cheers, Alexey
> 
> 
> ____________________
>   Racket Users list:
>   http://lists.racket-lang.org/users
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