Participants: Jö, Steffen, Christian, Markus, Ansgar, Robert

Programming Model

Which Programming model shall we use? Contestants are threads and tasks.

In case of tasks:

• They should be optional
• We need that the thread pool for tasks can be limited, so the user can (temporarily) set a number of threads aside for external libraries which use threads.
• We need something akin to std::async() (TBB provides something)
• Everyone in the WG agrees

Plenary vote: Tasks: 8, other: 0, abstain: 2

API

How much do we bind ourselves to a particular tasking API?

Contestants are TBB and OpenMP, but nobody likes OpenMP, so really only TBB.

• Licence: TBB recently switched to Apache 2, is that compatible with our license? It is if we finally switch to LGPL3, in the current state compatibility is unclear.
• We could introduce an abstraction layer. But that only makes sense if we have at least a rough idea regarding the possible variability of the interface. We do not know about any other possible interface. And wrapping will in some cases be quite expensive (support wise).
• Even if we standardize on TBB, OpenMP should still be usable since both use the same thread pool.

Proposal of the WG:

• We standardise on TBB without abstraction/wrappers. TBB remains optional: if not available there will be not automatic threading in the core modules. Infrastructure that only makes sense in connection with TBB may be missing then.
• We do not allow OpenMP in core modules. Users may use OpenMP in their own modules.
• We generally disallow threads not managed by the thread pool. In special cases you may start threads outside (e.g. for communicating in the background), but you need to ensure that there are no adverse effects, e.g. performance loss due to oversubscription.
• When there is equivalent functionality in TBB and the C++ standard, prefer the standard functionality.

Plenary Vote:

• Standardise in TBB in the core as proposed: Yes: 8, No: 0, abstain 3
• Prefer equivalent functionality from std::: Yes: 9, No: 0, abstain: 4

Integration Plan

• EXADUNE will make a merge request for the baseic TBB infrastructure (e.g. build system)
• EXADUNE will then successively migrate TBB-dependent features into the core

Open Questions

• Is there a minimum TBB version we should require
  (Steffen: 4.3 provides task observer, which are required for pinning.)
  Well probably introduce restrictions as they turn out to be required.
• Initializing of TBB? By default it uses all threads, which is bad if you start your program with mpirun -n $ncpu ....