From: Jakub Jelinek <ja...@redhat.com> Date: Tue, 5 Apr 2011 08:33:23 +0200
> On Tue, Apr 05, 2011 at 11:05:01AM +0900, Sho Nakatani wrote: >> - When task A encounters "#pragma omp task" derective, worker creates a >> task >> and immediately execute it. Worker pushes A to the head of deque. > > Immediately starting a freshly created task on #pragma omp task is nice for > cache > locality, ... Yes. Also, Lazy Task Creation has another good aspect. Look at the pictures below. Both shows the call-tree of fibonacci(8). One is compiled by gcc and the other is by icc. (GCC) https://github.com/laysakura/GCC-OpenMP-Speedup/raw/e5671e7f4175c3ac17c1543c93edf25dda2ae6ac/test/calltree/openmp-fibonacci-calltree-gcc.png (ICC) https://github.com/laysakura/GCC-OpenMP-Speedup/raw/e5671e7f4175c3ac17c1543c93edf25dda2ae6ac/test/calltree/openmp-fibonacci-calltree-icc.png Each node with the same color represents the task which is processed on the same thread (or core). These pictures show that the program compiled by icc, which might use Lazy Task Creation, executes nodes in subtree on the same core, while that compiled by gcc doesn't. Of course, fib(8) is doesn't clearly show if Lazy Task Creation really has good effect for the programs that need sophisticated parallelization. But other researches like https://iwomp.zih.tu-dresden.de/downloads/runtime-olivier.pdf also point out it. My teacher and senior associates know good about Lazy Task Creation and I also learned it by some Japanese paper. My teacher recommended me a paper related to Lazy Task Creation (in English). Please google the query and take a look at it if you have a time. "Lazy Task Creation: A Technique for Increasing the Granularity of Parallel Programs" > Immediately starting a freshly created task on #pragma omp task is nice for > cache > locality, except it doesn't work at all for tied tasks which you can't move > to other > threads. For tied tasks (and GCC currently has all tasks tied) it serializes > everything. Then, how about implementing `untied task' as a GSoC project? If it goes successful, I'll tackle better `tied task' also. What do you think about this idea? I'd like to write my proposal tomorrow :-) -- Sho Nakatani
