On Fri, 14 Mar 2008, Manoj Vaghela wrote:
> When a derived datatype is created (let say, using Vector), at what point
> does it buffer memory? I mean, after committing a variable, how does it
> store the buffer the memory reserved by that variable?
I think you migt be a bit confused about MPI datatypes. Creating and
committing a datatype doesn't have anything to do with buffering for
actually moving data -- it's only creating a description of the data you
want to move. There is obviously some storage to hold the description of
the data layout, but when that is allocated / released is an internal
detail of the MPI (and is different from MPI to MPI).
> As in Pack/Unpack, a buffer of required size should be allocated, and
> after unpacking data, buffer is deallocated. How does the same thing work in
> Vector? When is the memory freed? How does MPI_Type_free work?
> (MPI_Type_free marks the variable for deallocation).
Again, derived datatypes aren't like pack/unpack -- you are only creating
/ destroying the *description* of the data loayout when you create / free
an MPI type handle. The memory required to buffer the actual data (if
there is any at all) is allocated / released as part of the communication
process. It's possible that there will never be such a buffer, as the
payload will just be compied into network bounce buffers directly --
again, this is an MPI implementation detail, and not something the user
has to worry about.
Hope this helps,
Brian
--
Brian Barrett
LAM/MPI Developer
Make today a LAM/MPI day!
|
