W dniu 2010-09-17 17:49, Mattias Gaertner pisze:
On Fri, 17 Sep 2010 14:09:59 +0200
Dariusz Mazur<[email protected]> wrote:
[...]
Whats about CILK http://en.wikipedia.org/wiki/Cilk
|_function_ fib (n: integer):integer;
var
x,y : integer;
begin
if (n< 2) then exit(n)
else begin
x :=_spawn_ fib (n-1);
y := fib (n-2);
_sync_;
exit(x+y);
end;
end;
|
Have you noticed that most examples for multi threading are either
- easy to understand, but completely impractical (the above is an
order of magnitude slower than single threaded)
I don;t agee. First with CILK approach we deal with task, not threads.
Thus we need good task passing to workers.
But seems, that this approach will be faster because on each SPAWN
invoke we add only task (and this is very simple and short function, as
add to FIFO queue)
And each worker (worker count is similar to cores) find task to do. Even
when task are very different (shorter and longer) gueue pass task to
worker with balancing . Why You think this will be magnitude slower I
don't know.
- OR practical, but hard to understand
?
Note: afaik CILK is the same as parallel loops:
No. On loop we divide iterations on start loops. When each iteration has
different time of computing, we lost concurrence, because one thread
finish, but rest work. Second: we not initialize thread on SPAWN (with
loops threads are initializing on start loop), Workers wait for task on
idle. Push and pop task from queue can be very fast. Of course this
depend on implementation (for example each worker should deal with own
queue, but should be possibility to draw task form other queue, this
approach is faster than single MPMC FIFO queue)
It allows to make some
special cases need less typing, can increase readability and can
decrease overhead. As always with multithreading: wrongly used it can
make code run much slower.
Multithreading is not only for faster computing.
--
Darek
--
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