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From: Ralph H Castain (rhc_at_[hidden])
Date: 2007-07-26 09:53:14

On 7/26/07 7:33 AM, "Rolf.Vandevaart_at_[hidden]" <Rolf.Vandevaart_at_[hidden]>

> Aurelien Bouteiller wrote:
>> Hi Ralph and everyone,
>> I just want to make sure the proposed usecases does not break one of the
>> current open MPI feature I require. For FT purposes, I need to get some
>> specific hosts (lets say with a better MTBF). Those hosts are not part
>> of the MPI_COMM_WORLD but are used to deploy FT services (like event
>> loggers, checkpoint servers, etc). To enable collaboration between
>> computing nodes and those FT services, I use the usual MPI2 Dynamics
>> with MPI_Accept/Connect. This means that those different instances of
>> mpirun needs to share the same orte registry, so that they can establish
>> the MPI2 connect/accept trough the registered MPI_ports.
>> This background in place, my first concern is how the deployment maps to
>> the allocated resources. The nodes used to deploy FT services are
>> "special". In typical usecase, I get machines with better MTFB, faster
>> or larger disks by requesting special properties to the resources
>> allocation manager. I don't want those to be mixed with regular nodes in
>> the resulting hostfile: these scarce resources should hold only FT
>> services, no computing processes. As I understand things, I don't see
>> any way to avoid mpirun to deploy application processes on my "special"
>> nodes if they are part of the same launch/allocation in your "filtering"
>> usecase. Currently I proceed to two different mpirun with a single orte
>> seed holding the registry. This way I get two different hostfiles, one
>> for computing nodes, one for FT services. I just want to make sure
>> everybody understood this requirement so that this feature does not
>> disappear in the brainstorming :]
> With the use of resource managers, --host, and --hostfile this should
> all be possible.

I'll try to keep this in mind as we implement the change....will have to see
if this ability really can be supported in the revision. I'll certainly let
you know if I run into a conflict.

>> Next requirement is the ability to add during runtime some nodes to the
>> initial pool. Because node may fail (but it is the same with comm_spawn
>> basically) , I might need some (lot of) spare nodes to replace failed
>> ones. As I do not want to request for twice as many nodes as I need
>> (after all, things could just go fine, why should I waste that many
>> computing resources for idle spares ?), I definitely want to be able to
>> allocate some new nodes to the pool of the already running machines. As
>> far as I understand, this is impossible to achieve with the usecase2 and
>> quite difficult in usecase1. In my opinion, having the ability to spawn
>> on nodes which are not part of the initial hostfile is a key feature
>> (and not only for FT purposes).
> I am looking for more detail into the above issue. What
> resource manager are you using?
> Ideally, we would prefer not to support this. Any nodes
> that you run on, or hope to run on, would be designated
> at the start. For example:
> mpirun -np 1 --host a,b,c,d,e,f,g
> This would cause the one process of the mpi job to start on host a.
> Then, the mpi job has available to it the other hosts should it decide
> later to start a job on them. However no ORTE daemons would
> be started on those nodes until calls to MPI_Comm_spawn
> occur. So, the MPI job would not be consuming any resources
> until called upon to.

This has actually been the subject of multiple threads on the user list and
is considered a critical capability by some users and vendors. I believe
there is little problem in allowing those systems that can support it to
dynamically add nodes to ORTE via some API into the resource manager. At the
moment, none of the RMs support it, but LSF will (and TM at least may)
shortly do so, and some of their customers are depending upon it.

The problem is that job startup could be delayed for significant time if all
hosts must be preallocated. Admittedly, this raises all kinds of issues
about how long the job could be stalled waiting for the new hosts. However,
as the other somewhat exhaustive threads have discussed, there are computing
models that can live with this uncertainty, and RMs that will provide async
callbacks to allow the rest of the app to continue working while waiting.

Just my $0.00002 - again, this goes back to...are there use-cases and
customers to which Open MPI is simply going to say "we won't support that"?

> Rolf
>> I know there have been some extra discussions on this subject.
>> Unfortunately it looks like I am not part of the list where it happened.
>> I hope those concerns have not been already discussed.
>> Aurelien
>> Ralph H Castain wrote:
>>> Yo all
>>> As you know, I am working on revamping the hostfile functionality to make it
>>> work better with managed environments (at the moment, the two are
>>> exclusive). The issue that we need to review is how we want the interaction
>>> to work, both for the initial launch and for comm_spawn.
>>> In talking with Jeff, we boiled it down to two options that I have
>>> flow-charted (see attached):
>>> Option 1: in this mode, we read any allocated nodes provided by a resource
>>> manager (e.g., SLURM). These nodes establish a base pool of nodes that can
>>> be used by both the initial launch and any dynamic comm_spawn requests. The
>>> hostfile and any -host info is then used to select nodes from within that
>>> pool for use with the specific launch. The initial launch would use the
>>> -hostfile or -host command line option to provide that info - comm_spawn
>>> would use the MPI_Info fields to provide similar info.
>>> This mode has the advantage of allowing a user to obtain a large allocation,
>>> and then designate hosts within the pool for use by an initial application,
>>> and separately designate (via another hostfile or -host spec) another set of
>>> those hosts from the pool to support a comm_spawn'd child job.
>>> If no resource managed nodes are found, then the hostfile and -host options
>>> would provide the list of hosts to be used. Again, comm_spawn'd jobs would
>>> be able to specify their own hostfile and -host nodes.
>>> The negative to this option is complexity - in the absence of a managed
>>> allocation, I either have to deal with hostfile/dash-host allocations in the
>>> RAS and then again in RMAPS, or I have "allocation-like" functionality
>>> happening in RMAPS.
>>> Option 2: in this mode, we read any allocated nodes provided by a resource
>>> manager, and then filter those using the command line hostfile and -host
>>> options to establish our base pool. Any spawn commands (both the initial one
>>> and comm_spawn'd child jobs) would utilize this filtered pool of nodes.
>>> Thus, comm_spawn is restricted to using hosts from that initial pool.
>>> We could possibly extend this option by only using the hostfile in our
>>> initial filter. In other words, let the hostfile downselect the resource
>>> manager's allocation for the initial launch. Any -host options on the
>>> command line would only apply to the hosts used to launch the initial
>>> application. Any comm_spawn would use the hostfile-filtered pool of hosts.
>>> The advantage here is simplicity. The disadvantage lies in flexibility for
>>> supporting dynamic operations.
>>> The major difference between these options really only impacts the initial
>>> pool of hosts to be used for launches, both the initial one and any
>>> subsequent comm_spawns. Barring any commentary, I will implement option 1 as
>>> this provides the maximum flexibility.
>>> Any thoughts? Other options we should consider?
>>> Thanks
>>> Ralph
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