[mpiwg-rma] Questions about multiple overlapped windows

[Tatsuya Abe]

Hi Jeff,

Thank you so much for the reply.

>A good example is using WIN_ALLOCATE_SHARED to create a shared memory
>window, then passing the resulting buffer into WIN_CREATE.  This
>enables a window to support both intranode+interprocess load-store and
>internode RMA.

Doesn't MPI_Win_allocate_shared support internode RMA, i.e., when I
create a window by using MPI_Win_allocate_shared, cannot I call
MPI_Put/MPI_Get for the window?  I haven't found any description about
it in MPI 3.0 yet.

I am a little confused.  Could you let me ask you more details?

1. Do you mean that another window must be created by MPI_Win_create,
   in order to use MPI_Put/MPI_Get for a window created by
   MPI_Win_allocate_shared?  Is this why multiple windows are allowed
   to be overlapped?

2. Do you mean that the multiple overlapped windows (from
   MPI_Win_allocate_shared and MPI_Win_create) become unnecessary in
   this exapmple if your ticket #397 passes?

Tatsuya



Jeff Hammond <jeff.science at gmail.com> wrote:
>On Sun, Dec 7, 2014 at 4:57 PM, Tatsuya Abe <abetatsuya at gmail.com> wrote:
>> Dear all,
>>
>> I am interested in memory consistency models, and studying program
>> verification with memory consistency models.  I started to read MPI
>> 3.0 document (and Dr. Torsten Hoefler's tutorial slides).
>> http://www.eurompi2014.org/tutorials/hoefler-advanced-mpi-eurompi14.pdf
>>
>> I am wondering if you could give replies to my questions.
>> I have two questions about multiple overlapped windows:
>>
>> 1. (Assume that I create two overlapped windows.)  What happens if I
>>    do MPI_Put through one window and MPI_Get through the other window?
>
>It depends entirely on the synchronization epochs you use and whether
>the memory model provided is UNIFIED or SEPARATE.
>
>> 2. I am wondering what are multiple overlapped windows for.
>
>To work around obnoxious shortcomings of MPI RMA.
>
>> 1. I can find no description about the first question in MPI 3.0
>>    document.
>>
>>    I found Torsten's slides (credit by RMA Working Group, MPI Forum),
>>    which explicitly showed relations between operations (Load, Store,
>>    Get, Put, and Acc) on pp.82--84.  But, it seems an explanation
>>    when exactly one window is created,
>>
>>    For example, page 83 of the slide seems to claim that Put and
>>    Get are allowed when they are not NOVL (non-overlapped)
>>    through one window in separate semantics.
>>
>>    One of my questions is ``are Put and Get allowed when they are
>>    not NOVL (non-overlapped) through *two* *overlapped* windows
>>    in separate semantics?''  I would like to get such table about
>>    multiple windows.
>
>Within the unified model, one can reason about overlapping windows by
>recognizing that the private windows of the overlapping windows are
>all the same and thus when one synchronizes the public and private
>windows of each window with WIN_SYNC, then all of the public windows
>are the same as well.
>
>> 2. I can find no example about multiple overlapped windows except
>>    Figure 11.1 in MPI 3.0 document. Also, I suspect that Put and Get
>>    are allowed when they are not NOVL (non-overlapped) through two
>>    overlapped windows in separate semantics.
>>
>>    I cannot come up with any useful example code.  Why are multiple
>>    *and* overlapped windows allowed to be active?
>
>A good example is using WIN_ALLOCATE_SHARED to create a shared memory
>window, then passing the resulting buffer into WIN_CREATE.  This
>enables a window to support both intranode+interprocess load-store and
>internode RMA.  It is useful if one wants to reproduce the behavior of
>e.g. ARMCI in implementing Global Arrays.
>
>The aforementioned motivation will disappear if my ticket
>https://svn.mpi-forum.org/trac/mpi-forum-web/ticket/397 is passed and
>part of MPI-4.
>
>Another reason is to allow ordered and unordered semantics.  The
>option to control this happens during window construction and cannot
>be changed.  If one wants one semantic (unordered) for performance and
>another (ordered) for correctness, then one needs two windows, one for
>each.
>
>Jeff
>
>-- 
>Jeff Hammond
>jeff.science at gmail.com
>http://jeffhammond.github.io/