Advice to implementors. In JNI-based wrapper implementations it may be
necessary to impose some non-interference rules for concurrent read
and write operations on arrays. When an array is passed to an
MPI method such as a send or receive operation, the wrapper code will
probably extract a pointer to the contents of the array using a JNI
Get...ArrayElements routine. If the garbage collector does
not support ``pinning'' (temporarily disabling run-time relocation of
data for specific arrays--see [3] for more
discussion), the pointer returned by this Get function may be to
a temporary copy of the elements. The copy will be written back to the
true Java array when a subsequent call to Release...ArrayElements is made. If two operations involving the
same array are active concurrently, this copy-back may result in
failure to save modifications made by one or more of the concurrent
calls.
Such an implementation may have to enforce a safety rule such as: when several MPI send or receive (etc) operations are active
concurrently, if any one of those operations writes to a particular
array, none of the other operations must read or write any
portion of that array.
If the garbage collector supports pinning, this problem does not arise.(End of advice to implementors.)
