Re: [GSoC'19, libgomp work-stealing] Task parallelism runtime

[김규래 <msca8h@naver.com>]

> On Wed, Jun 5, 2019 at 9:25 PM 김규래 <msca8h@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? 
 
Hi Janne,
I believe you're talking about spawning child tasks within an already running task, which I believe is supported by OpenMP.
In this case, pushing to the local queue could introduce a deadlock if the master thread waits on the spawned tasks. 
A short one though since work-stealing can resolve the deadlock.
A better way to handle this is to round-robin the child tasks to the queues excluding the queue of the thread consuming the current task.
Then waiting on the tasks should never cause a deadlock.
Or did I misunderstand your question?
Maybe there is a specific reason for avoiding avoid round-robin that I missed?
 
Ray Kim