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Subject: Re: [OMPI devel] SC13 birds of a feather
From: Isaías A. Comprés Ureña (compresu_at_[hidden])
Date: 2013-12-04 06:25:45


Dear Jeff Squyres,

On 12/03/2013 11:27 PM, Jeff Squyres (jsquyres) wrote:
> I'm sorry; I really wasn't paying attention to my email the week of SC, and then I was on vacation for the Thanksgiving holiday. :-\
>
> More below.
>
> On Nov 20, 2013, at 4:13 PM, Compres <compresu_at_[hidden]> wrote:
>
>> I was at the birds of a feather and wanted to talk to the Open MPI developers, but unfortunately had to leave early. In particular, I would like to discuss about your implementation of the MPI tools interface and possibly contribute to it later on.
> Sorry we missed you.

No problem; I had to be at a booth during times that overlapped with
your session.

> What did you want to discuss? We actually have a full implementation of the MPI_T interface -- meaning that we have all the infrastructure in place for MPI_T control and performance variables.
>
> 1. The MPI_T control variables map directly to OMPI's MCA params, so we automatically expose oodles of cvars through MPI_T. They're all read-only after MPI_INIT, however -- many things are setup during MPI_INIT and it would be quite a Big Deal if they were to change. However, we pretty much *assumed* all cvars shouldn't change after INIT -- we didn't really audit to see if there were actually some cvars that could change after INIT. So there's work that could be done there (i.e., find cvars that could change after INIT, and/or evaluate the amount of work/change it would be to change some read-only cvars to be read-write, etc.).
>
> 2. The MPI_T performance variables are new. There's only a few created right now (e.g., in the Cisco usnic BTL). But the field is pretty wide open here -- the infrastructure is there, but we're really not exposing much information yet. There's lots that can be done here.
>
> What did you have in mind?

I think you made a good guess on what we would like to do here. We are
working on automatic tuning based on both modeling and empirical data.
One of our aims is to accelerate the data collection part (in this case
related to MPI settings), by doing it online without the need of full
application runs or restarts.

Right now we can modify MPI runtime parameters with IBM-MPI or Open
MPI. These require full restarts, since they are set as environment
variables and are not modifiable after MPI_INIT. With your MPIT
implementation, we can do the same programmatically but cannot avoid the
restarts or full runs.

We already did what you describe at the end of 1., but with a (1 year
old) snapshot of MPICH. The idea was to identify which variables could
be made modifiable at runtime, and whether there was any attainable
performance as a result of tuning them. We only explored point to point
and collective communication parameters, and the results are
encouraging. There was no technical reason when picking MPICH for the
first prototype.

With MPICH, we had to examine the code for things that were
configurable. It seems to me that in the case of Open MPI, most of the
work is done and, as you point out, it may just be necessary to identify
which ones can be made modifiable at runtime and at what development cost.

My main intention here is to see if other people are interested and will
benefit from this. Additionally, if the changes (patches) are taken by
the project, we avoid running out of sync (which is what ended up
happening with our MPICH modifications).

- Isaías A. Comprés