Ralph H Castain wrote:
No Problem. Hope you had a good vacation :) And sorry for my super
delayed response. I have been pondering this a bit.
Just returned from vacation...sorry for delayed response
In the past, I have expressed three concerns about the RSL.
My bottom line recommendation: I have no philosophical issue with the RSL
concept. However, I recommend holding off until the next version of ORTE is
completed and then re-evaluating to see how valuable the RSL might be, as
that next version will include memory footprint reduction and framework
consolidation that may yield much of the RSL's value without the extra work.
1. What problem are we really trying to solve?
If the RSL is intended to solve the Cray support problem (where the Cray OS
really just wants to see OMPI, not ORTE), then it may have some value. The
issue to date has revolved around the difficulty of maintaining the Cray
port in the face of changes to ORTE - as new frameworks are added, special
components for Cray also need to be created to provide a "do-nothing"
capability. In addition, the Cray is memory constrained, and the ORTE
library occupies considerable space while providing very little
This is definitely a motivation, but not the only one.
The degree of value provide by the RSL will therefore depend somewhat on the
efficacy of the changes in development within ORTE. Those changes will,
among other things, significantly consolidate and reduce the number of
frameworks, and reduce the memory footprint. The expectation is that the
result will require only a single CNOS component in one framework. It isn't
clear, therefore, that the RSL will provide a significant value in that
But won't there still be a lot of orte code linked in that will never be
Also, a RSL would simplify ORTE in that there would be no need to do
anything special for CNOs in it.
If the RSL is intended to aid in ORTE development, as hinted at in the RFC,
then I believe that is questionable. Developing ORTE in a tmp branch has
proven reasonably effective as changes to the MPI layer are largely
invisible to ORTE. Creating another layer to the system that would also have
to be maintained seems like a non-productive way of addressing any problems
in that area.
Whether or not it would help in orte development remains to be seen. I
just say that it might. Although I would argue that developing in tmp
branches has caused a lot of problems with merging, etc.
If the RSL is intended as a means of "freezing" the MPI-RTE interface, then
I believe we could better attain that objective by simply defining a set of
requirements for the RTE. As I'll note below, freezing the interface at an
API level could negatively impact other Open MPI objectives.
It is intended to easily allow the development and use of other runtime
systems, so simply defining requirements is not enough.
2. Who is going to maintain old RTE versions, and why?
It isn't clear to me why anyone would want to do this - are we seriously
proposing that we maintain support for the ORTE layer that shipped with Open
MPI 1.0?? Can someone explain why we would want to do that?
I highly doubt anyone would, and see no reason to include support for
older runtime versions. Again, the purpose is to be able to run
different runtimes. The ability to run different versions of the same
runtime is just a side-effect.
Again, I am not particularly concerned with supporting older versions of
orte, but rather supporting different runtime systems.
3. Are we constraining ourselves from further improvements in startup
This is my biggest area of concern. The RSL has been proposed as an
API-level definition. However, the MPI-RTE interaction really is defined in
terms of a flow-of-control - although each point of interaction is
instantiated as an API, the fact is that what happens at that point is not
independent of all prior interactions.
As an example of my concern, consider what we are currently doing with ORTE.
The latest change in requirements involves the need to significantly improve
startup time, reduce memory footprint, and reduce ORTE complexity. What we
are doing to meet that requirement is to review the delineation of
responsibilities between the MPI and RTE layers. The current delineation
evolved over time, with many of the decisions made at a very early point in
the program. For example, we instituted RTE-level stage gates in the MPI
layer because, at the time they were needed, the MPI developers didn't want
to deal with them on their side (e.g., ensuring that failure of one proc
wouldn't hang the system). Given today's level of maturity in the MPI layer,
we are now planning on moving the stage gates to the MPI layer, implemented
as an "all-to-all" - this will remove several thousand lines of code from
ORTE and make it easier for the MPI layer to operate on non-ORTE
Similar efforts are underway to reduce ORTE involvement in the modex
operation and other parts of the MPI application lifecycle. We are able to
do these things because we are now moving towards a tight integration of
ORTE and OMPI layers - i.e., ORTE can be simplified because we can take
advantage of our knowledge of what is happening on the MPI side of the
In order to accomplish this, however, we need to change the
points-of-contact between the MPI and RTE layers, and redefine what happens
at those points. If we require via the RSL that we" those points and what
happens at those points, then making these changes will either prove
impossible or at least will require considerable RSL code. On the other
hand, if we revise the RSL to support the new ORTE/OMPI functionality, then
we will have to write considerable code to make old versions of ORTE work
with the new system.
Also, from what I know of these changes (and perhaps I don't understand
them), the proposed changes would fit into the current RSL design.
Hence, my concern is that we not let RSL implementation prevent us from
moving forward with ORTE. The current work is required to meet scaling
demands, and hopefully will resolve much of the Cray issue. I see no value
in creating RSL just to support old versions of ORTE, nor for supporting
ORTE development. It would be nice if we could re-evaluate this after the
next ORTE version becomes solidified to see how the cost/benefit analysis
has changed, and whether the RSL remains a desirable option.
On 8/16/07 7:47 PM, "Tim Prins" <tpr...@cs.indiana.edu> wrote:
WHAT: Solicitation of feedback on the possibility of adding a runtime
services layer to Open MPI to abstract out the runtime.
WHY: To solidify the interface between OMPI and the runtime environment,
and to allow the use of different runtime systems, including different
versions of ORTE.
WHERE: Addition of a new framework to OMPI, and changes to many of the
files in OMPI to funnel all runtime request through this framework. Few
changes should be required in OPAL and ORTE.
WHEN: Development has started in tmp/rsl, but is still in its infancy. We hope
to have a working system in the next month.
I am working on creating an interface between OMPI and the runtime system.
This would make a RSL framework in OMPI which all runtime services would be
accessed from. Attached is a graphic depicting this.
This change would be invasive to the OMPI layer. Few (if any) changes
will be required of the ORTE and OPAL layers.
At this point I am soliciting feedback as to whether people are
supportive or not of this change both in general and for v1.3.
The current model used in Open MPI assumes that one runtime system is
the best for all environments. However, in many environments it may be
beneficial to have specialized runtime systems. With our current system this
is not easy to do.
With this in mind, the idea of creating a 'runtime services layer' was
hatched. This would take the form of a framework within OMPI, through which
all runtime functionality would be accessed. This would allow new or
different runtime systems to be used with Open MPI. Additionally, with such a
system it would be possible to have multiple versions of open rte coexisting,
which may facilitate development and testing. Finally, this would solidify the
interface between OMPI and the runtime system, as well as provide
documentation and side effects of each interface function.
However, such a change would be fairly invasive to the OMPI layer, and
needs a buy-in from everyone for it to be possible.
Here is a summary of the changes required for the RSL (at least how it is
1. Add a framework to ompi for the rsl, and a component to support orte.
2. Change ompi so that it uses the new interface. This involves:
a. Moving runtime specific code into the orte rsl component.
b. Changing the process names in ompi to an opaque object.
c. change all references to orte in ompi to be to the rsl.
3. Change the configuration code so that open-rte is only linked where needed.
Of course, all this would happen on a tmp branch.
The design of the rsl is not solidified. I have been playing in a tmp branch
(located at https://svn.open-mpi.org/svn/ompi/tmp/rsl) which everyone is
welcome to look at and comment on, but be advised that things here are
subject to change (I don't think it even compiles right now). There are
some fairly large open questions on this, including:
1. How to handle mpirun (that is, when a user types 'mpirun', do they
always get ORTE, or do they sometimes get a system specific runtime). Most
likely mpirun will always use ORTE, and alternative launching programs would
be used for other runtimes.
2. Whether there will be any performance implications. My guess is not,
but am not quite sure of this yet.
Again, I am interested in people's comments on whether they think adding
such abstraction is good or not, and whether it is reasonable to do such a
thing for v1.3.
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