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Subject: Re: [OMPI devel] RFC: Resilient ORTE
From: Josh Hursey (jjhursey_at_[hidden])
Date: 2011-06-07 10:51:48

I briefly looked over the patch. Excluding the epochs (which we don't
need now, but will soon) it looks similar to what I have setup on my
MPI run-through stabilization branch - so it should support that work
nicely. I'll try to test it this week and send back any other

Good work.


On Tue, Jun 7, 2011 at 10:46 AM, Wesley Bland <wbland_at_[hidden]> wrote:
> This could certainly work alongside another ORCM or any other fault
> detection/prediction/recovery mechanism. Most of the code is just dedicated
> to keeping the epoch up to date and tracking the status of the processes.
> The underlying idea was to provide a way for the application to decide what
> its fault policy would be rather than trying to dictate one in the runtime.
> If any other layer wanted to register a callback function with this code, it
> could do anything it wanted to on top of it.
> Wesley
> On Tuesday, June 7, 2011 at 7:41 AM, Ralph Castain wrote:
> I'm on travel this week, but will look this over when I return. From the
> description, it sounds nearly identical to what we did in ORCM, so I expect
> there won't be many issues. You do get some race conditions that the new
> state machine code should help resolve.
> Only difference I can quickly see is that we chose not to modify the process
> name structure, keeping the "epoch" (we called it "incarnation") as a
> separate value. Since we aren't terribly concerned about backward
> compatibility, I don't consider this a significant issue - but something the
> community should recognize.
> My main concern will be to ensure that the new code contains enough
> flexibility to allow integration with other layers such as ORCM without
> creating potential conflict over "double protection" - i.e., if the layer
> above ORTE wants to provide a certain level of fault protection, then ORTE
> needs to get out of the way.
> On Mon, Jun 6, 2011 at 1:00 PM, George Bosilca <bosilca_at_[hidden]> wrote:
> WHAT: Allow the runtime to handle fail-stop failures for both runtime
> (daemons) or application level processes. This patch extends the
> orte_process_name_t structure with a field to store the process epoch (the
> number of times it died so far), and add an application failure notification
> callback function to be registered in the runtime.
> WHY: Necessary to correctly implement the error handling in the MPI 2.2
> standard. In addition, such a resilient runtime is a cornerstone for any
> level of fault tolerance support we want to provide in the future (such as
> the MPI-3 Run-Through Stabilization or FT-MPI).
> WHERE: Patch attached to this email, based on trunk r24747.
> TIMEOUT: 2 weeks from now, on Monday 20 June.
> ------
> Currently the infrastructure required to enable any kind of fault tolerance
> development in Open MPI (with the exception of the checkpoint/restart) is
> missing. However, before developing any fault tolerant support at the
> application (MPI) level, we need to have a resilient runtime. The changes in
> this patch address this lack of support and would allow anyone to implement
> a fault tolerance protocol at the MPI layer without having to worry about
> the ORTE stabilization.
> This patch will allow the runtime to drop any dead daemons, and re-route all
> communications around the holes in order to __ALWAYS__ deliver a message as
> long as the destination process is alive. The application is informed (via a
> callback) about the loss of the processes with the same jobid. In this patch
> we do not address the MPI_ERROR_RETURN type of failures, we focused on the
> MPI_ERROR_ABORT ones. Moreover, we empowered the application level with the
> decision, instead of taking it down in the runtime.
> Epoch - A counter that tracks the number of times a process has been
> detected to have terminated, either from a failure or an expected
> termination. After the termination is detected, the HNP coordinates all
> other process’s knowledge of the new epoch. Each ORTED will know the epoch
> of the other processes in the job, but it will not actually store anything
> until the epochs change.
> Run-Through Stabilization - When an ORTED (or HNP) detects that another
> process has terminated, it repairs the routing layer and informs the HNP.
> The HNP tells all other processes about the failure so they can also repair
> their routing layers an update their internal bookkeeping. The processes do
> not abort after the termination is detected.
> Callback Function - When the HNP tells all the ORTEDs about the failures,
> they tell the ORTE layers within the applications. The application level
> ORTE layers have a callback function that they use to inform the OMPI layer
> about the error. Currently the OMPI errhandler code fills in this callback
> function so it is informed when there is an error and it aborts (to maintain
> the current default behavior of MPI). This callback function can also be
> used in an ORTE only application to perform application based fault
> tolerance (ABFT) and allow the application to continue.
> Epoch - The orte_process_name_t struct now has a field for epoch. This means
> that whenever sending a message, the most current version of the epoch needs
> to be in this field. This is a simple look up using the function in
> orte/util/nidmap.c: orte_util_lookup_epoch(). In the orte/orted/orted_comm.c
> code, there is a check to make sure that it isn’t trying to send messages to
> a process that has already terminated (don’t send to a process with an epoch
> less than the current epoch). Make sure that if you are sending a message,
> you have the most up to date data here.
> Routing - So far, only the binomial routing layer has been updated to use
> the new resilience features. To modify other routing layers to be able to
> continue running after a process failure, they need to be able to detect
> which processes are not currently running and route around them. The errmgr
> gives the routing layer two chances to do this. First it calls delete_route
> for each process that fails, then it calls update_routing_tree after it has
> appropriately marked each process. Before either of these things happen the
> epoch and process state have already been updates so the routing layer can
> use this data to determine which processes are alive and which are dead. A
> convenience function has been added to orte/util/nidmap.h called
> orte_util_proc_is_running() which allows the ORTEDs to determine the status
> of a process. Keep in mind that a process is not running if it hasn’t
> started up yet so it is wise to check the epoch (to make sure that it isn’t
> ORTE_EPOCH_MIN) as well to make sure that you’re actually detecting an error
> and not just noticing that an ORTED hasn’t finished starting.
> Callback - If you want to implement some sort of fault tolerance on top of
> this code, use the callback function in the errmgr framework. There is a new
> function in the errmgr code called set_fault_callback that takes a function
> pointer. The ompi_init code sets this to a default value just after it calls
> orte_init (to make sure that there is an errmgr to call into). If you later
> set this to a new function, you will get the callback to notify you of
> process failures. Remember that you’ll need to handle any sort of MPI level
> fault tolerance at this point because you’ve taken away the callback for the
> OMPI layer.
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Joshua Hursey
Postdoctoral Research Associate
Oak Ridge National Laboratory