Why using pmap is not giving me the desired speedup ?
Tushar Deshpande
<>
Tue Nov 17 07:45:46 CET 2009
Hi,
I'm a newcomer to the Erlang world. I'm trying
to learn parallel programming with Erlang.
I tried to use Joe Armstrong's 'pmap' implementation
to write a parallel quicksort.
I imagined that I would get some observable
speedup with parallel quicksort.
I executed parallel quicksort on my dual-core
notebook. I varied the erlang schedulers from
1 to 32.
I ran quicksort on several lists with lengths
1, 10, 100, 1000, 10000 and 100000.
Contrary to my expectations, parallel version
took more time to run. [The pmap gave
significant speedup for example on page
371 (ptests.erl) in the Joe Armstrong's book]
What could be the reason ?
Notice, that the pqsort is non-tail-recursive
function. Does this have anything to do
with my observations ?
Wouls someone please help me ?
Here's my quicksort implementation for
easy reference.
-module(qsort).
-compile(export_all).
-import(lib_parallel, [ pmap/2]).
pqsort([]) ->
[];
pqsort([Pivot|T]) ->
L1 = [X || X <- T, X < Pivot],
L2 = [X || X <- T, X >= Pivot],
[Left,Right] = pmap(fun pqsort/1, [L1,L2]),
Left ++ [Pivot] ++ Right.
qsort([]) ->
[];
qsort([Pivot|T]) ->
L1 = [X || X <- T, X < Pivot],
L2 = [X || X <- T, X >= Pivot],
Left = qsort(L1),
Right = qsort(L2),
Left ++ [Pivot] ++ Right.
--
Warm Regards,
Tushar Deshpande
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