lists passed by reference

roman_pearce — Sat, 03 Jul 2010 05:57:42 Z

Lists and sets in Maple are always passed by reference, and there shouldn't be a lock to read them. They are immutable. You may find that garbage collection and other factors limit the parallel speedup of Maple code. In general, simpler is better, and if there are performance problems then it's probably something that needs to be improved in the kernel.

should be ok

acer — Sat, 03 Jul 2010 10:10:11 Z

You asked,
"if I hold the list as a private module variable and export a module procedure to return the weights, is Maple going to make a copy of the list every time a task invokes my module's "GetWeights" procedure?"

I believe that, if I understand your question properly, the answer is no. Maple only has one unique copy of any given list at any given time. If you assign into a list (or use subsop to try and cheat) then Maple creates a new list because (as Roman mentioned) lists are actually immutable. They may just appear to be mutable, by a Maple kernel sleight-of-hand.

Consider the following example, and ask why the higher level list L is not changed. The answer is not due to Maple passing a full copy of L into procedure p. It is because the later act of altering an entry of p's local T (with initial same ID as p's formal parameter LL) creates a new list. Note that the initial address of local T is the same as that of L, prior to assignment into an entry of T. The copying of L occurred when T[3] was altered, and not when L was passed into `p`. Moral: if you don't try and alter LL (or T), you should be OK as far as non-copying goes for performance. So avoiding the rtable locks, by converting the weights to lists (after all weights are initially computed, of course) looks ok.

> L:=[1,3,5,7]:
> addressof(L);
                                  181177616
> p:=proc(LL)
>   local T;
>   T:=LL;
>   printf("initial address of T: %a\n",addressof(T));
>   T[3]:=z;
>   printf("subsequent address of T: %a\n",addressof(T));
>   T;
> end proc:

> p(L);
initial address of T: 181177616
subsequent address of T: 193667904
                                [1, 3, z, 7]

> L;
addressof(L);
                                [1, 3, 5, 7]
                                  181177616

Joke: If Maple didn't behave like this then its list performance would be trash, and we'd all be passing addressof(L) into a procs which initially set up list locals with pointto().

acer

I'm sold

jimmyinhmb — Sun, 04 Jul 2010 08:19:26 Z

Thanks for the prompt and thorough explanations, gents. Now that FIFA is resting for a couple of days, I'll get back to work.

- Jimmy

MaplePrimes - answers and comments on Question, Array or list for sharing large immutable data between tasks?

lists passed by reference

should be ok

I'm sold