On Wed, Jun 5, 2019 at 9:25 PM 김규래 <msc...@naver.com> wrote: > > Hi, thanks for the detailed explanation. > I think I now get the picture. > Judging from my current understanding, the task-parallelism currently works > as follows: > 1. Tasks are placed in a global shared queue. > 2. Workers consume the tasks by bashing the queue in a while loop, just as > self-scheduling (dynamic scheduling)/ > > Then the improvements including work-stealing must be done by: > 1. Each worker holds a dedicated task queue reducing the resource contention. > 2. The tasks are distributed in a round-robin fashion
For nested task submission (does OpenMP support that?) you probably want to submit to the local queue rather than round-robin, no? > 3. work-stealing will resolve the load imbalance. > > If the above statements are correct, I guess the task priority should be > given some special treatment? > > Ray Kim > > -----Original Message----- > From: "Jakub Jelinek"<ja...@redhat.com> > To: "김규래"<msc...@naver.com>; > Cc: <gcc@gcc.gnu.org>; > Sent: 2019-06-04 (화) 03:21:01 (GMT+09:00) > Subject: Re: [GSoC'19, libgomp work-stealing] Task parallelism runtime > > On Tue, Jun 04, 2019 at 03:01:13AM +0900, 김규래 wrote: > > Hi, > > I've been studying the libgomp task parallelism system. > > I have a few questions. > > First, Tracing the events shows that only the main thread calls GOMP_task. > > No, any thread can call GOMP_task, in particular the thread that encountered > the #pragma omp task construct. > > The GOMP_task function then decides based on the clauses of the construct > (passed in various ways through the arguments of that function) whether it > will be included (executed by the encountering thread), or queued for > later execution. In the latter case, it will be scheduled during a barrier > (implicit or explicit), see gomp_barrier_handle_tasks called from the > bar.[ch] code, or at other spots, e.g. during taskwait construct > (GOMP_taskwait) or at the end of taskgroup (GOMP_taskgroup_end). > > > How do the other worker threads enter the libgomp runtime? > > If you never encounter a parallel, teams or target construct, then there is > just one thread that does everything (well, the library is written such that > if you by hand pthread_create, each such thread acts as a separate initial > thread from OpenMP POV). > Threads are created e.g. during parallel construct (GOMP_parallel), where > for non-nested parallelism as the standard requires it reuses existing > threads if possible or spawns new ones, see mainly team.c (gomp_team_start) > for the function that spawns new threads or awakes the ones waiting for > work, or gomp_thread_start in the same file for the function actually run by > the libgomp library created threads. > > > I can't find the entry point of the worker threads from the event tracing > > and the assembly dump. > > Second, How is the task priority set? > > By the user, through priority clause, passed to GOMP_task and then taken > into account when handling tasks in the various queues. > > Jakub -- Janne Blomqvist
