[mpiwg-rma] {MAX,MIN}LOC in RMA

Jim Dinan james.dinan at gmail.com
Wed Apr 9 06:39:45 CDT 2014

FYI, we added a test case for MPI_Accumulate with MIN/MAX_LOC to the MPICH
test suite:

Also, the mutex idea I had didn't pan out because of the LOC operators
always chose the least loc when the values are equal.  Can't think of a way
to make this work..


On Thu, Mar 13, 2014 at 3:25 PM, Jim Dinan <james.dinan at gmail.com> wrote:

> MAX/MIN_LOC operations have an (albeit odd) atomic compare and replace
> semantic.  They operate on a pair type that has <value, location>
> components.  The user provides the data for both value and location.  The
> min/max comparison (i.e. less-than/greater-than) is done on the value.  If
> the comparison is true, then the remote <value, location> is atomically
> replaced with the supplied data.
> These operations can be useful for one-sided reductions.  I think PetSC
> uses them for this.  In MPI-2 (there are, of course better ways in MPI-3)
> you can also use these operations to implement a spinlock.  The value is a
> boolean indicating if the mutex is taken and the location is the rank that
> hold the mutex.  The mutex initially contains the unlocked value <0,
> MPI_PROC_NULL>.  You do a MAX_LOC accumulate of <1, me> to try to claim the
> mutex then do a get to see if you succeeded (exclusive epochs because of
> mixing accumulates and gets and the same op restriction).  To unlock, you
> write the unlocked value back to the mutex.
> I have a test case that implements this, that's been queued up for a while
> waiting to go out to MPICH.  Should be able to push it out soon.
>  ~Jim.
> On Thu, Mar 13, 2014 at 3:05 PM, Jeff Hammond <jeff.science at gmail.com>wrote:
>> What is the expected behavior of {MAX,MIN}LOC in RMA?  I have a hard
>> time seeing what the use would be.  Does anyone want to use MAXLOC to
>> find the MAX of two numbers and write in the window the rank (chosen
>> from the set of the initiator and the target) of the pair member that
>> is larger?  It's not like one can use this to implement a one-sided
>> reduction since any time the value in the window is larger, the
>> information about the original LOC will be lost.
>> Can we remove these operations from the set of accumulate operations?
>> Thanks,
>> Jeff
>> --
>> Jeff Hammond
>> jeff.science at gmail.com
>> _______________________________________________
>> mpiwg-rma mailing list
>> mpiwg-rma at lists.mpi-forum.org
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