On Thursday 10 June 2010, Han Dong wrote:
> Hi,
>
> There's this function inside a DistArray
>
>
> *reduce<http://dist.codehaus.org/x10/xdoc/2.0.3/x10/array/DistArray.html#re
> duce%28op:%28id$10:T,%20id$11:T%29%7B%7D=>
> *(x10.lang.Fun_0_2<T<http://dist.codehaus.org/x10/xdoc/2.0.3/x10/array/Dis
> tArray.html> ,T
> <http://dist.codehaus.org/x10/xdoc/2.0.3/x10/array/DistArray.html>,T<http:
> //dist.codehaus.org/x10/xdoc/2.0.3/x10/array/DistArray.html>> op,
> T <http://dist.codehaus.org/x10/xdoc/2.0.3/x10/array/DistArray.html> unit)
> *Declaration*: reduce(op:(id$10:T, id$11:T){}=> Tunit:T): T.
> Could anybody explain to me what does it actually do and a sample
> utilization example? Is it similar to the reduce used in map reduce
> applications?
>
> Thanks,
>
> Han Dong
> h...@umbc.edu
> University of Maryland '10
>
My understanding is the following (please anyone correct me if I am wrong).
If T is (say) Double and op:(id$10:T, id$11:T){}=> Tunit:T is Double.+ then
reduce computes the global sum of all elements of DistArray across all places.
If T is (say) Double and op:(id$10:T, id$11:T){}=> Tunit:T is Double.* then
reduce computes the global product of all elements of DistArray across all
places.
The order in which the operands are added or multiplied, respectively, is
chosen by X10 runtime and depends on the number of places and the Distribution
under DistArray. Therefore, in general any operand op:(id$10:T, id$11:T){}=>
Tunit:T needs to be commutative and associative to allow for the freedom of
changing the order of the reduction.
Also, for Double.+ in particular, you have to make sure that the round of
errors don't bite you. Albeit addition is commutative and associative in
general, it no longer holds when applied to IEEE floating point numbers of
finite length. All values positive is fine, a mix of positive and negative
values might be dangerous. Please note that this is not restricted to X10, but
true for every (!) programming language on every possible CPU (past, present
and future) from every vendor.
--
Mit freundlichen Grüßen / Kind regards
Dr. Christoph Pospiech
High Performance & Parallel Computing
Phone: +49-351 86269826
Mobile: +49-171-765 5871
E-Mail: christoph.pospi...@de.ibm.com
-------------------------------------
IBM Deutschland GmbH
Vorsitzender des Aufsichtsrats: Erich Clementi
Geschäftsführung: Martin Jetter (Vorsitzender),
Reinhard Reschke, Christoph Grandpierre,
Klaus Lintelmann, Michael Diemer, Martina Koederitz
Sitz der Gesellschaft: Ehningen / Registergericht: Amtsgericht Stuttgart, HRB
14562 WEEE-Reg.-Nr. DE 99369940
------------------------------------------------------------------------------
ThinkGeek and WIRED's GeekDad team up for the Ultimate
GeekDad Father's Day Giveaway. ONE MASSIVE PRIZE to the
lucky parental unit. See the prize list and enter to win:
http://p.sf.net/sfu/thinkgeek-promo
_______________________________________________
X10-users mailing list
X10-users@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/x10-users
