[mpich-devel] small MPI_Win_allocate

[Jeff Hammond]

I always ask for noncontig window allocations.  I use as many RMA info keys
as possible :-)

I am not so worried about running out of VA bits, although 48 bits don't go
as far as they used to, as I am about TLB slots.

However, this problem can be solved above MPI, it just requires me to do it
in every MPI-3 code I write.

Jeff

On Tue, May 17, 2016 at 12:20 PM, Balaji, Pavan <balaji at anl.gov> wrote:

>
> It does allocate from shared memory irrespective of the size.  It's not
> page-aligned unless you ask for noncontig allocations.  With respect to
> wasted VM space, you'll run out of window contexts much before you run out
> of VM space, so I'm not sure this is a worthwhile optimization.
>
>   -- Pavan
>
> > On May 12, 2016, at 4:54 PM, Jeff Hammond <jeff.science at gmail.com>
> wrote:
> >
> > If I call MPI_Win_allocate for e.g. 8 bytes, does it allocate shared
> memory every time, and thus potentially allocate with page alignment, which
> could cause a lot of empty VM space?
> >
> > Perhaps I do not understand POSIX shared memory well enough, but it
> strikes me that it might be a worthwhile optimization to have a memory pool
> for small-size MPI_Win_allocate(_shared) calls.
> >
> > Thanks,
> >
> > Jeff
> >
> > --
> > Jeff Hammond
> > jeff.science at gmail.com
> > http://jeffhammond.github.io/
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-- 
Jeff Hammond
jeff.science at gmail.com
http://jeffhammond.github.io/
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