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Subject: Re: [OMPI devel] RFC: make predefined handles extern to pointers
From: Terry Dontje (Terry.Dontje_at_[hidden])
Date: 2009-01-26 11:45:00


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-bounces_at_[hidden] <devel-bounces_at_[hidden]>
>> To: Open MPI Developers <devel_at_[hidden]>
>> 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.Dontje_at_[hidden]> 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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>>>> devel_at_[hidden]
>>>> http://www.open-mpi.org/mailman/listinfo.cgi/devel
>>>>
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>>
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