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Re: RFC: Implicit Parallelization Pragma

by Limbic~Region (Chancellor)
on Jan 25, 2005 at 17:43 UTC ( #424960=note: print w/replies, xml ) Need Help??

in reply to RFC: Implicit Parallelization Pragma

You didn't mention it, so I will ask. Does your proposed threadsafe pragma make any guarantees concerning the finishing order? In your example
my @str_lengths = map { length } @strs;
There is no reason not to get the lengths in parallel, but there certainly might be a reason to want/expect the results come out in the same order they went in (FIFO).

Cheers - L~R

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Re^2: RFC: Implicit Parallelization Pragma
by hardburn (Abbot) on Jan 25, 2005 at 17:47 UTC

    Certainly it would have to guarentee ordered results, or it woud be very limited in usefulness.

    "There is no shame in being self-taught, only in not trying to learn in the first place." -- Atrus, Myst: The Book of D'ni.

      Might I suggest you use a different example then. I didn't think you were proposing FIFO, but the example seemed that way to me. Additionally, I think having a second more constrained version that did guarantee order would still be useful. Imagine the following project:
      • Get the length of all words in the list
      • Look each word up in a dictionary
      • Count the number of syllables in a word
      Now for the sake of argument, looking each word up in the dictionary takes twice as long as both getting the length of the word and counting the number of syllables together. If these 3 tasks were done in parallel, it should take only roughly the same time as just looking up each word. On the other hand, if the order isn't guaranteed - the results coming out might not be very useful.

      In other words, the elapsed time becomes close to the length for the longest pole in the tent. Some time is also spent ordering the results as they become available. Of course, I may be way out in left field here.

      Cheers - L~R

        How about the Schwartzian as an example?

        my @sorted = map { $_->[0] } sort { $a->[1] <=> $b->[1] } map {[ $_, substr( $_, 2, 5 ) ]} @list;

        The maps can be parellelized, and the sort can, too, if perl (lowercase) implemented it with a parellel sorting algorithm. The second map (in execution order) must come out in correct order--otherwise, why did you even bother sorting? Intrestingly, the first map does not need to come out in order, though there is a problem of letting perl in on this fact.

        "There is no shame in being self-taught, only in not trying to learn in the first place." -- Atrus, Myst: The Book of D'ni.

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