Hi Joseph
OK, that makes sense. Thank you for your help!
Thanks again
Claire
On 27/04/2020, 11:28, "Joseph Schuchart" <schuch...@hlrs.de> wrote:
Hi Claire,
You cannot use MPI_Get (or any other RMA communication routine) on a
window for which no access epoch has been started. MPI_Win_fence starts
an active target access epoch, MPI_Win_lock[_all] start a passive target
access epoch. Window locks are synchronizing in the sense that they
provide a means for mutual exclusion if an exclusive lock is involved (a
process holding a shared window lock allows for other processes to
acquire shared locks but prevents them from taking an exclusive lock,
and vice versa).
One common strategy is to call MPI_Win_lock_all on all processes to let
all processes acquire a shared lock, which they hold until the end of
the application run. Communication is then done using a combination of
MPI_Get/MPI_Put/accumulate functions and flushes. As said earlier, you
likely will need to take care of synchronization among the processes if
they also modify data in the window.
Cheers
Joseph
On 4/27/20 12:14 PM, Claire Cashmore wrote:
> Hi Joseph
>
> Thank you for your reply. From what I had been reading I thought they
were both called "synchronization calls" just that one was passive (lock) and
one was active (fence), sorry if I've got confused!
> So I'm asking do need either MPI_Win_fence or MPI_Win_unlock/lock in
order to use one-sided calls, and is it not possible to use one-sided
communication without them? So just a stand alone MPI_Get, without the other
calls before and after? It seems not from what you are saying, but I just
wanted to confirm.
>
> Thanks again
>
> Claire
>
> On 27/04/2020, 07:50, "Joseph Schuchart via users"
<users@lists.open-mpi.org> wrote:
>
> Claire,
>
> > Is it possible to use the one-sided communication without
combining
> it with synchronization calls?
>
> What exactly do you mean by "synchronization calls"? MPI_Win_fence is
> indeed synchronizing (basically flush+barrier) but MPI_Win_lock (and
the
> passive target synchronization interface at large) is not. It does
incur
> some overhead because the lock has to be taken somehow at some point.
> However, it does not require a matching call at the target to
complete.
>
> You can lock a window using a (shared or exclusive) lock, initiate
RMA
> operations, flush them to wait for their completion, and initiate the
> next set of RMA operations to flush later. None of these calls are
> synchronizing. You will have to perform your own synchronization at
some
> point though to make sure processes read consistent data.
>
> HTH!
> Joseph
>
>
> On 4/24/20 5:34 PM, Claire Cashmore via users wrote:
> > Hello
> >
> > I was wondering if someone could help me with a question.
> >
> > When using RMA is there a requirement to use some type of
> > synchronization? When using one-sided communication such as
MPI_Get the
> > code will only run when I combine it with MPI_Win_fence or
> > MPI_Win_lock/unlock. I do not want to use MPI_Win_fence as I’m
using the
> > one-sided communication to allow some communication when processes
are
> > not synchronised, so this defeats the point. I could use
> > MPI_Win_lock/unlock, however someone I’ve spoken to has said that I
> > should be able to use RMA without any synchronization calls, if so
then
> > I would prefer to do this to reduce any overheads using
MPI_Win_lock
> > every time I use the one-sided communication may produce.
> >
> > Is it possible to use the one-sided communication without
combining it
> > with synchronization calls?
> >
> > (It doesn’t seem to matter what version of openmpi I use).
> >
> > Thank you
> >
> > Claire
> >
>
