Hi Matthew, Thanks for your help. I really can't even pretend to understand the make-coroutine code but this is what I'm seeing:
Original make-coroutine, without stream-ref pre-call => out of memory (1GB) when running (prime-pi 500000) Original make-coroutine, with stream-ref pre-call => runs (prime-pi 500000) in about .4 sec Alternate make-coroutine, with or without stream-ref pre-call => runs (prime-pi 500000) in about 28 sec All runs from "clean" state. -joe On Tue, Feb 21, 2012 at 10:25 AM, Matthew Flatt <[email protected]> wrote: > I don't know the answer, but here's a piece of the puzzle that might be > relevant. > > Try this alternate `make-coroutine': > > (define (make-coroutine fn) > (let ([cont #f]) > (λ () > (call-with-continuation-prompt > (lambda () > (if cont > (cont) > (fn (λ () (let/cc k > (set! cont k) > (abort-current-continuation > (default-continuation-prompt-tag) > void)))))))))) > > The difference between this one and the one on the web page is that it > uses a prompt to avoid capturing the continuation of the initial call > to a coroutine (i.e., it uses a delimited continuation for the > coroutine). For me, this variant makes `(prime-pi 500000)' work without > ``(stream-ref prime-stream (inexact->exact (* 0.1 n)))'. > > My guess is that avoiding capture of continuations converts O(N^2) > space usage into O(N) space for some N-deep forcing of streams. Using > `(stream-ref prime-stream (inexact->exact (* 0.1 n)))', meanwhile, > might similarly avoid an N-deep nesting at the point where O(N^2) > behavior would kick in. > > I haven't studied the code enough to be certain that my explanation is > right, but when I see surprising space consumption with laziness and > undelimited continuations, my first guess is that undelimited > continuations are holding onto too much. > > At Mon, 20 Feb 2012 17:19:02 -0800, Joe Gilray wrote: > > Hi, > > > > I created an infinite stream of primes (using this interesting article: > > http://matthias.benkard.de/journal/116) > > > > I tried to create a prime-pi function: > > > > (define (prime-pi n) > > (let loop-count ([candidate-stream prime-stream]) > > (if (> (stream-first candidate-stream) n) > > 0 > > (+ (loop-count (stream-rest candidate-stream)) 1)))) > > > > (prime-pi 500000) grinds and runs out of memory > > > > But this little cludge makes it run just fine: > > > > (define (prime-pi n) > > (stream-ref prime-stream (inexact->exact (* 0.1 n))) ; cludge added > to > > get the stream to pre-create enough primes > > (let loop-count ([candidate-stream prime-stream]) > > (if (> (stream-first candidate-stream) n) > > 0 > > (+ (loop-count (stream-rest candidate-stream)) 1)))) > > > > What am I not understanding? > > > > Thanks, > > -Joe > > ____________________ > > Racket Users list: > > http://lists.racket-lang.org/users >
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