On Jul 18, 2013, at 9:53 AM, Ralph Castain <r...@open-mpi.org> wrote:
> On Jul 18, 2013, at 7:05 AM, David Goodell (dgoodell) <dgood...@cisco.com> > wrote: > >> On Jul 18, 2013, at 8:06 AM, Ralph Castain <r...@open-mpi.org> wrote: >> >>> That's a good point, and a bad behavior. IIRC, it results from the MPI >>> Forum's adoption of the MPI-T requirement that stipulates we must allow >>> access to all control and performance variables at startup so they can be >>> externally seen/manipulated. >> >> Minor nit: MPI_T does not require this. However, it does recommend that you >> offer users access to as many variables as possible as early as reasonably >> possible for the convenience and control of the user. >> >> If an implementation chooses to offer 5% of the possible control/performance >> variables to the user just before MPI_Finalize, that's still a valid MPI_T >> implementation. But it may not be a very useful one... > > The problem here is one of use vs startup performance. George is quite > correct with his concerns - this behavior would have been a serious problem > for RoadRunner, for example, where we had a small IO channel feeding a lot of > nodes. It will definitely become an issue at exascale where IO bandwidth and > memory will be at a premium. My point was not that the performance concerns were unfounded. Rather, I wanted to point out that the "load everything" behavior is not a hard requirement from the MPI standard, so we have room for different implementation choices/tradeoffs. -Dave
