[argobots-discuss] Attribution of ULT ids

[Iwasaki, Shintaro]

Hi, Matthieu

Thank you for your question.
> Is the ABT_unit_id associated to threads and tasks a number that necessarily increases as new threads/tasks are created?

No.  Even in the current implementation, a ULT ID is determined when it is inquired about (e.g., `by ABT_thread_get_id()`), not when a ULT is created.  This "inquiry" timing depends on configuration options. In the future, this ID can be replaced by a "pointer". I do not recommend you rely on the ID to know when ULTs/tasks are created.

> I would like to implement a custom pool that prioritizes running older ULTs over newer ones when possible.

I do not know how strictly you want to control scheduling, but the following might be easy to implement and beneficial.

sched_run():
  while (1) {
    /* Check old_thread_pool first. */
    if (unit = ABT_pool_pop(old_thread_pool)) {
      /* If "unit" yields, it is pushed to old_thread_pool.
       * If you want, you can push it to very_old_thread_pool. */
      ABT_xstream_run_unit(unit, old_thread_pool);
      continue;
    }
    /* Check new_thread_pool if old_thread_pool is empty */
    if (unit = ABT_pool_pop(new_thread_pool))
      /* If "unit" yields, it is pushed to old_thread_pool.
       * Note that Argobots 1.1+ does not need ABT_unit_set_associated_pool()
       * https://github.com/pmodels/argobots/pull/317 */
      ABT_xstream_run_unit(unit, old_thread_pool);
    }
  }

The scheduling can cause a deadlock.  Please check the solution in the discussion below.
Related discussion: https://lists.argobots.org/pipermail/discuss/2020-August/000085.html

Thanks,
Shintaro

________________________________
From: Dorier, Matthieu via discuss <discuss at lists.argobots.org>
Sent: Sunday, June 20, 2021 10:15 AM
To: discuss at lists.argobots.org <discuss at lists.argobots.org>
Cc: Dorier, Matthieu <mdorier at anl.gov>
Subject: [argobots-discuss] Attribution of ULT ids


Hi,



Is the ABT_unit_id associated to threads and tasks a number that necessarily increases as new threads/tasks are created?



I’m asking because I would like to implement a custom pool that prioritizes running older ULTs over newer ones when possible, and one way of knowing which of multiple ULTs is older could be to look at the ABT_unit_id and prioritize ULTs with the smallest id.



Thanks,



Matthieu
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