On Mon, 01 Nov 2010 10:24:43 -0400, Bruno Medeiros
<brunodomedeiros+s...@com.gmail> wrote:
On 29/10/2010 02:32, Robert Jacques wrote:
[snip]
The programming language Cilk popularized the concept of parallelization
through many small tasks combined with a work stealing runtime. Futures
are essentially the same concept, but because futures were generally
implemented with OS-threads, a thread pool or fibers/coroutines, that
term is generally avoided. Like message passing, tasks are often
implemented in libraries with Intel's threading building blocks probably
being the most famous, though both Microsoft's Task Parallel Library and
Apple's Grand Central are gaining mind-share. David Simcha currently has
a task library in review for inclusion to phobos. Basically, the point
of tasks is to provide parallelization with extremely low overhead (on
average a Cilk spawn is less than 4 function calls). That way, instead
of having a few coarse grain threads which neither scale nor load
balance well, you're encouraged to use tasks everywhere and therefore
reap the benefits of a balanced N-way scalable system.
Hum, I see what you mean know, but tasks only help with the *creation
overhead* of otherwise spawning lots of OS threads, they don't solve the
main problems I mentioned.
First, it may be fine to spawn 100 tasks, but there is still the issue
of deciding how many OS threads the tasks will run in! Obviously, you
won't run them in just one OS thread, otherwise you won't get any
parallelism. Ideally for your program only, your program would have as
many OS threads as there are cores. But here there is still the same
issue of whether its ok for you program to use up all the cores in your
machine. The compiler doesn't know that. Could it be enough to have a
global compiler option to specify that? I don't think so: What if you
want some code of your program to use as much OS-threads as possible,
but not some other code?
Second, and perhaps more importantly, the very same issue occurs in the
scope of your program alone. So, even if you use all OS threads, and
don't care about other programs, spawning 100 tasks for some loop might
take time away from other more important tasks of your program. The
compiler/task-scheduler/whatever would not automatically know what is
acceptable and what is not. (the only exception being if your program
was logically single-threaded)
Controlling the task runtime thread-pool size is trivial. Indeed, you'll
often want to reduce the number of daemon threads by the number of active
program threads. And if you need fine grain control over pool sizes, you
can always create separate pools and assign tasks to them. I think a
reasonable default would be (# of cores - 2) daemons with automatic
decreases/increases with every spawn/termination. But, all those settings
should be controllable at runtime.
