Thread Subcommittee Minutes from Berlin

N2012=06-0082, 2006-04-18, Lawrence Crowl

In wide-ranging discussions, the thread subcommittee asked and answered a number of questions about how to introduce threads into C++.

Structure

Are threads hierarchical?	No, use OpenMP.
Are threads flat peer-to-peer?	Not completely, some management tasks become difficult.
Is there a master thread?	Yes, it executes main.
Are threads compatible with the system?	Yes. Anything else will be perceived as less than helpful.

Creation and Termination

Shall threads be explicit?	Yes.
Are threads coarse-grained?	Yes.
Is thread creation dynamic?	Yes.
If so, is it program controlled?	Yes.
Is work explicitly asigned to threads?	Yes.
Can one call exit only from the master thread?	Yes, calling exit from a non-master thread induces an abort.
What happens when exit is called while threads are detached?	The result is implementation-defined.
Is there a join operation?	Yes.
Are exceptions propogated through the join?	Yes, otherwise they become unmanagable. This propogation requires syntax or library ABI magic.
Should there be a join any or join all?	Not in the base layer, leave it to higher layers.
How are thread objects allocated?	Dynamically, on the heap. Thread objects on the stack are problematic if an exception deletes the object, better perhaps is dynamic allocation of the exception object.
May threads be detached?	Yes, it means there will be no join operation and the thread object may delete itself on return from the thread function.

Dynamic Libraries

Does the master construct all globals variables?	Maybe. The issue is a dlopen from a non-master thread and the subsequent initialization of globals.
Shall we require dlopen only from a master thread?	No, this inhibits MT-hot libraries.
Shall the implementation serialize all dlopens?	Leave this implementation-defined.
Shall we prohibit calling dlopen while holding a lock?	No, this approach violates encapsulation in the library.
Are recursive dlopens defined on different libries?	Yes.
Must non-recursive dlopens be serialized by user code?	Maybe.

Communication and Synchronization

Is communication via message?	No, use MPI.
Is communication via shared memory?	Yes.
Is synchronization via locks?	Yes.
If so, events or condition variables?	Condition variables. They are much easier to use.
Can threads synchronize between processes?	Make possible, but not defined.
Can threads synchronize with system threads?	Yes.
Can threads obtain handles on system threads?	Yes, for scheduling.
Are static-duration variables synchronizable?	Yes.
Are static-duration variables explicitly synchronized, e.g. __synchronized keyword?	Maybe. The issue is that it might be better if they were all implicitly synchronized.
Are locks in a library?	Yes, there is a substantial variety.

Scheduling

Is progress guaranteed?	No.
Are scheudling hints available?	Maybe. Revisit this issue.
Can one call terminate/abort anywhere?	Yes, and terminate immediately.
Can a thread cancel itself?	Maybe.
Can a thread cancel other threads synchronously?	Maybe.
Can a thread cancel other threads asynchronously?	No, this does terrible damage to code generation.
Can a thread suspend itself?	No.
Can a thread suspend others?	No.
Can a thread yield itself?	Maybe.
Can a thread yield other threads?	No.

Thread-Local Storage

Is there a __thread keyword?	Yes.
May function-local thread-local variables be dynamically initialized?	Yes.
May other thread-local variables be dynamically initialized?	Implementation defined.