[mpiwg-rma] Single RMA synchronization for several window handles

[Rolf Rabenseifner]

Jim and all,

It is not syntactic sugar.
It is a latency optimizing enhancement:

If you are doing neighbor communication with 1-sided 
communication to 6 neighbors - based on 6 windows that
are all defined in MPI_COMM_WORLD, then currently 
you need to call 6 times the synchronization calls
which implies at maximum a 6 times larger latency,
e.g., 6 times MPI_Win_fence instead of one call.

If you are using MPI on shared memory windows,
then the same latency problem exists, but here, 
the trick with dynamic windows is impossible 
because shared memory windows must be allocated 
with MPI_Win_allocate_shared.    

Of course, it is more complicated to define 
MPI_Win_combine on a superset of communicators used
for all the combined windows.

As a first effort,  it would be already helpful
to define it for the same communicator.
To allow enhancements in a future MPI version, 
I still would recommend to have the comm argument 
as part of the argument list.

Rolf

----- Original Message -----
> From: "Jim Dinan" <james.dinan at gmail.com>
> To: "MPI WG Remote Memory Access working group" <mpiwg-rma at lists.mpi-forum.org>
> Sent: Sunday, August 10, 2014 5:48:02 PM
> Subject: Re: [mpiwg-rma] Single RMA synchronization for several window        handles
> 
> 
> 
> Hi Rolf,
> 
> 
> I had initially proposed this in the context of passive target RMA.
>  Active target RMA already notifies the receiver when data has
> arrived, but there is no efficient way to get such a notification in
> passive target RMA.  I think what you are proposing here would be
> syntactic sugar on top of the existing interface --- could I
> implement this by fencing every window to determine that all
> transfers are completed?
> 
> 
> I agree with the comments regarding dynamic windows.  The merged
> window would contain discontiguous buffers; thus, it would lose the
> ability to do offset-based addressing and would need to use absolute
> (BOTTOM-based) addressing.
> 
> 
>  ~Jim.
> 
> 
> 
> On Fri, Aug 8, 2014 at 10:18 AM, Rolf Rabenseifner <
> rabenseifner at hlrs.de > wrote:
> 
> 
> Jim,
> 
> your topic "Reducing Synchronization Overhead Through Bundled
> Communication" may get also help if we would be able to
> combine several window handles to one superset window handle.
> 
> If you have several windows for different buffers, but
> only one synchronization pattern, e.g. MPI_Win_fince
> then currently you must call MPI_Win_fence seperately
> for each window handle.
> 
> I would propose:
> 
> MPI_Win_combine (/*IN*/  int count,
>                  /*IN*/  MPI_Win *win,
>                  /*IN*/  MPI_Comm comm,
>                  /*OUT*/ MPI_Win *win_combined)
> 
> The process group of comm must contain the process groups of all win.
> The resulting window handle win_combined can be used only
> in RMA synchronization calls and other helper routines,
> but not for dynamic window allocation nor for any
> RMA communication routine.
> Collective synchronization routines must be called by all processes
> of comm.
> The semantics of an RMA synchronization call using win_combined
> is defined as if the calls were seperately issued for
> each window handle of the array win. If group handles
> are part of the argument list of the synchronization call
> then the appropriate subset is used for each window handle in win.
> 
> What do you think about this idea for MPI-4.0?
> 
> Best regards
> Rolf
> 
> ----- Original Message -----
> > From: "Jim Dinan" < james.dinan at gmail.com >
> > To: "MPI WG Remote Memory Access working group" <
> > mpiwg-rma at lists.mpi-forum.org >
> > Sent: Thursday, August 7, 2014 4:08:32 PM
> > Subject: [mpiwg-rma] RMA Notification
> > 
> > 
> > 
> > Hi All,
> > 
> > 
> > I have added a new proposal for an RMA notification extension: 
> > https://svn.mpi-forum.org/trac/mpi-forum-web/ticket/439
> > 
> > 
> > I would like to bring this forward for the RMA WG to consider as an
> > MPI-4 extension.
> > 
> > 
> > Cheers,
> >  ~Jim.
> > _______________________________________________
> > mpiwg-rma mailing list
> > mpiwg-rma at lists.mpi-forum.org
> > http://lists.mpi-forum.org/mailman/listinfo.cgi/mpiwg-rma
> 
> --
> Dr. Rolf Rabenseifner . . . . . . . . . .. email rabenseifner at hlrs.de
> High Performance Computing Center (HLRS) . phone ++49(0)711/685-65530
> University of Stuttgart . . . . . . . . .. fax ++49(0)711 / 685-65832
> Head of Dpmt Parallel Computing . . . www.hlrs.de/people/rabenseifner
> Nobelstr. 19, D-70550 Stuttgart, Germany . . . . (Office: Room 1.307)
> _______________________________________________
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> 
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-- 
Dr. Rolf Rabenseifner . . . . . . . . . .. email rabenseifner at hlrs.de
High Performance Computing Center (HLRS) . phone ++49(0)711/685-65530
University of Stuttgart . . . . . . . . .. fax ++49(0)711 / 685-65832
Head of Dpmt Parallel Computing . . . www.hlrs.de/people/rabenseifner
Nobelstr. 19, D-70550 Stuttgart, Germany . . . . (Office: Room 1.307)