On Mar 4, 2010, at 7:36 AM, Richard Treumann wrote:
A call to MPI_Init allows the MPI library to return any level of
thread support it chooses.
This is correct, insofar as the MPI implementation can always choose
any level of thread support.
This MPI 1.1 call does not let the application say what it wants and
does not let the implementation reply with what it can guarantee.
Well, sort of. MPI-2.2, sec 12.4.3, page 385, lines 24-25:
------8<------
24| A call to MPI_INIT has the same effect as a call to
MPI_INIT_THREAD with a required
25| = MPI_THREAD_SINGLE.
------8<------
So even though there is no explicit request and response for thread
level support, it is implicitly asking for MPI_THREAD_SINGLE. Since
all implementations must be able to support at least SINGLE (0 threads
running doesn't really make sense), SINGLE will be provided at a
minimum. Callers to plain-old "MPI_Init" should not expect any higher
level of thread support if they wish to maintain portability.
[...snip...]
Consider a made up example:
Imagine some system supports Mutex lock/unlock but with terrible
performance. As a work around, it offers a non-standard substitute
for malloc called st_malloc (single thread malloc) that does not do
locking.
[...snip...]
Dick's example is a great illustration of why FUNNELED might be
necessary. The moral of the story is "don't lie to the MPI
implementation" :)
-Dave
