Kor,
I think you've just found a bug (all my fault...) at a very good time. There
are indeed configuration options that affect how the scheduler steals tasks and
it looks like I've set them to very inappropriate values. Stay tuned for a PR.
On the second point you're probably seeing your hpx_main/main function as
run_helper. Because of some changes in APEX the task names in the OTF file get
the first name of the task. We don't have a good solution for this yet. As a
hack you could try doing this as the first thing in your main function (note:
that's main if you include hpx_main.hpp or hpx_main if you include hpx_init.hpp
or hpx_start.hpp):
hpx::util::annotate_function annotation("my_main");
hpx::this_thread::yield();
Once the task is rescheduled it should have the label "my_main".
Mikael
________________________________
From: hpx-users-boun...@stellar.cct.lsu.edu
<hpx-users-boun...@stellar.cct.lsu.edu> on behalf of Jong, K. de (Kor)
<k.dejo...@uu.nl>
Sent: Wednesday, December 18, 2019 3:20:59 PM
To: hpx-users@stellar.cct.lsu.edu
Subject: Re: [hpx-users] parallel_executor::post dominating APEX trace
Hi Mikael,
Thank you for your detailed and helpful answer! It starts to make sense
to me now. My program almost behaves as I expect it should. I still have
two questions though. Maybe you or someone else can point me to the
right direction?
1. I run my program on a single node and expect that all threads receive
about the same number of same-sized tasks. The APEX trace in Vampir
shows that all threads start busy but after a while gaps appear on some
OS threads during which nothing seems to happen, while other threads are
still performing tasks. I would expect tasks to be more evenly
distributed and/or to be stolen from the task queues of other OS
threads. Is this assumption correct? Can I increase the tendency of the
scheduler to steal tasks to keep OS threads busy?
2. I perform scaling tests, and each time my tasks run in parallel there
is a serial 'run_helper' task that runs on a single OS thread. What is
this and can I somehow keep it out of my timings? Based on a quick look
at the HPX code I concluded that run_helper has to do with initializing
the HPX run time. But even if I run my tasks multiple times (from the
same running process), run_helper spends time before my tasks do.
I focus on a single node now (1-96 OS threads) and am not doing anything
too clever, I think. I don't tweak the bindings and scheduler yet.
Thanks!
Kor
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