A few more details:
- only the MPI objects corresponding to MPI predefined handle
constants (e.g., MPI_COMM_WORLD) have padding; user-defined
communicators (etc.) do not have padding
- the HG for this work changed; it's now here:
http://www.open-mpi.org/hg/hgwebdir.cgi/tdd/pdc-pad/
- so you can see exactly what we're talking about, the bulk of Terry's
proof of concept diff for MPI_COMM_WORLD, MPI_COMM_SELF, and
MPI_COMM_NULL is here:
http://www.open-mpi.org/hg/hgwebdir.cgi/tdd/pdc-pad/rev/
3b2650929b98
On Jan 26, 2009, at 11:45 AM, Terry Dontje wrote:
Another update for this RFC. It turns out that using pointers
instead of structures as initializers would prevent someone from
initializing a global to one of the predefined handles. So instead,
we decided to go the route of padding the structures to provide us
with the ability to not overrun the bss section.
I would like to discuss any objections to this solution on
tomorrow's OMPI concall.
thanks,
--td
Terry Dontje wrote:
Just wanted to give an update. On a workspace with just the
predefined communicators converted to opaque pointers I've ran
netpipe and hpcc performance tests and compared the results before
and after the changes. The differences in performance with 10
sample run was undetectable.
I've also tested using comm_world that I can have an a.out compile
and link with a non-debug version of the library and then run the
a.out successfully with a debug version of the library. At a
simple level this proves that the change actually does what we
believe it should.
I will be completing the rest of handles in the next couple days.
Upon completion I will rerun the same tests above and test running
hpcc with a debug and non-debug version of the library without
recompiling.
I believe I am on track to putting this back to the trunk by the
end of next week. So if anyone has any issues with this please
speak up.
thanks,
--td
Graham, Richard L. wrote:
No specific test, just an idea how this might impact an app. I am
guessing it won't even be noticable.
Rich
----- Original Message -----
From: devel-boun...@open-mpi.org <devel-boun...@open-mpi.org>
To: Open MPI Developers <de...@open-mpi.org>
Sent: Thu Dec 18 07:13:08 2008
Subject: Re: [OMPI devel] RFC: make predefined handles extern to
pointers
Richard Graham wrote:
Terry,
Is there any way you can quantify the cost ? This seems
reasonable, but
would be nice to get an idea what the performance cost is (and
not within a
tight loop where everything stays in cache).
Rich
Ok, I guess that would eliminate any of the simple perf tests like
IMB, netperf, and such. So do you have something else in mind,
maybe HPCC?
--td
On 12/16/08 10:41 AM, "Terry D. Dontje" <terry.don...@sun.com>
wrote:
WHAT: To make predefined handles extern to pointers instead of an
address of an extern to a structure.
WHY: To make OMPI more backwards compatible in regards to
changes to
structures that define predefined handles.
WHERE: In the trunk. ompi/include/mpi.h.in and places in ompi
that
directly use the predefined handles.
WHEN: 01/24/2009
TIMEOUT: 01/10/2009
____________________
The point of this change is to improve the odds that an MPI
application
does not have to recompile when changes are made to the OMPI
library.
In this case specifically the predefined handles that use the
structures
for communicators, groups, ops, datatypes, error handlers, win,
file,
and info.
An example of the changes for the communicator predefined
handles can be
found in the hg tmp workspace at
ssh://www.open-mpi.org/~tdd/hg/predefcompat.
Note, the one downfall that Jeff and I could think of by doing
this is
you potentially add one level of indirection but I believe that
will be
a small overhead and if you use one of the predefined handles
repetitively (like in a loop) that the address will probably be
stored
in a register once and no additional over should be seen due to
this change.
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Jeff Squyres
Cisco Systems
