Re: Give me advice on GSoC OpenMP

[Jakub Jelinek <jakub at redhat dot com>]

On Sun, Apr 03, 2011 at 07:27:12PM +0900, Sho Nakatani wrote:
> Then, I'll compare the trees created by gcc and icc, and point out
> that the implementation of OpenMP Task uses Lazy Task Creation while
> gcc does not.

Depends on what you mean by lazy task creation, gcc schedules
tasks lazily if they aren't if (0), some data structure if created
for them when encountering #pragma omp task directive, but I guess
any implementation will do something like that.

What your testcase shows is not whether tasks are created lazily or not, but
how good/poor #pragma omp taskwait implementation is.  And, for your testcase
libgomp/task.c (GOMP_taskwait) definitely could be improved.  Currently it only
tries to schedule in children that will be awaited by the current tasks and if
there are no such children, goes to sleep, waiting for them to complete.
Scheduling in random unrelated tasks is problematic, because the unrelated
task might take too long to complete and delay the taskwait for way too long
(note, gcc doesn't have untied tasks, all tasks are tied once they are scheduled
onto some particular tasks - setcontext/swapcontext is quite fragile thing to do).
But it is true it could very well schedule tasks that are taskwaited by tasks
taskwaited by current task, and transitively further.  Plus, be able to temporarily
awake such a sleeping thread if there are tasks it can transitively taskwait
for, as if those don't complete, the current taskwait won't return.

	Jakub