[mpiwg-sessions] Next meeting - *Tuesday* next week - midday Eastern US time

[Ralph H Castain]

> On Aug 10, 2018, at 4:09 AM, HOLMES Daniel <d.holmes at epcc.ed.ac.uk> wrote:
> 
> Hi Ralph,
> 
> In general, I agree with all of this. Two points:
> 
> 1) I think your assignment of special case and general case are the wrong way around. A fully connected graph (your normal/general case) is a special case of a general graph (your special case), which may be completely disconnected, partially connected, disjointly connected, or fully connected. Requiring every process to specify every other process (your normal/general case) is therefore restricting their behaviour from the general graph (your special case) to a particular special case. This does not need to be an issue: generalising a special-case API does not break backwards compatibility, in general. On the other hand, I’m not sure how much different either the specification’s wording or the implementation’s code would have to be in order to support the general graph case as well as the fully connected special case. The general case definitely messes with the brain cells a lot more though.

I think we can agree to disagree on the case assignments (as I look at them in terms of usage and not graph theory) - it isn’t really important to the API definition. :-)

If someone is using the async construct method, then we have to know who to invite. Those processes don’t call “construct” - they call “join” to accept/decline the invitation. Thus, they don’t have the ability to extend the invitations. We could bring back the PMIx_Group_invite API to allow that, but it created some nasty race conditions that make it very difficult to track completion. I suppose an alternative is to eliminate the “join” API and just have the invitee call “construct”, but that _really_ complicates the implementation of the “construct” API. Definitely would at least require them to provide an attribute indicating they are responding to an invite, but that doesn’t do anything to resolve the race conditions.

If someone is instead using the collective construct method, then it seems a little confusing if other procs keep extending it. This is basically equivalent to an MPI collective operation - you have to know the participants to efficiently handle it. If we say that all procs don’t need to provide us with the list of participants, then implementation becomes a little more complex and we possibly lose scalability. I can see a mode where we treat a process that specifies the same group identifier but doesn’t include a matching list of all participants as a potential sub-leader (akin to what you describe below) and do the collective over its sub-group, rolling things up as we go. However, that still has scalability impacts that other use-cases may not want to embrace (e.g., I now have to circulate the list of participants to check for completeness) - probably better to just define an attribute the procs would have to provide to request this use-case. I’m okay with allowing that as a special case that comes with the scalability warning - if the user wants to do so, they pay the price.

> 
> 2a) Having a single leader may not be sufficient. I’m thinking intercommunicators, which have naturally have two leaders - one for the local group and one for the remote group. This statement immediately gives us a clue as to how to handle that particular use-case: group construct two separate groups then worry about linking them together later. Perhaps, a third group containing just the two leaders? OK, maybe that works fine as is. Taking a different tack, unless the leader treats no news as good news, all processes must notify the leader that they believe the operation is successful. For a really large group, wouldn’t the n-to-1 ‘notify the leader of local outcome’ gather/reduce operation cause a bottleneck at the leader? You’d want a hierarchy (e.g. a tree or something like it) just like for normal gather/reduce implementations, i.e. different processes would perceive different leaders (who in turn may know they are just middle-management and there is a big boss that their underlings don’t know about). Failure of a leaf would be notified to their parent/leader process, which can fix/swallow the problem or bubble it up to their parent/leader. Failure of a leader causes an election in their sub-tree(s)/children. This gives the possibility of a free choice of the size of the containment domain - preferable to being told it’s all or nothing. A general graph naturally gives a structure but it would be hard, in general, to use that for containment - because each process can specify more than one other process, which is their leader? Thus, each process could nominate a single process (probably one of the entries in their process array, but not necessarily) to be their leader, or a rule could be devised that figures out which process is leader for each process.
> 
> 2b) Nominating/electing a (successor) leader could be a simple as “proceed in index order” (of the locally provided array). PMIx taking over by self-nominating leadership and canceling the group construct at the first sign of trouble seems a bit harsh.

I’m not sure I understand your comment - when did PMIx propose to “take over” the leadership assignment?? All I proposed is that a leader exist (either the proc that started an async construct or some proc designated by the app) and that we notify it about issues instead of broadcasting failure events to everyone. Again, this is for scalability - there are system and app-level costs to broadcasting events. If that process fails, then we would notify everyone that “leader failed” so the app can decide who should take over that role. I’m okay with that broadcast as it represents an “exception” case and shouldn’t happen very often.

If the proper attributes aren’t provided, then the user is telling us to terminate the construct on first failure. This isn’t PMIx making that decision - the user told us to do it. The notification/leader procedures only take affect if the user asks us to do it. Other use-cases don’t need or want that overhead.

> 
> Conceptually, I think we can take your 1-to-n model, replicate it m times (possibly with different values of n in each replica), then add one more process that is the leader of all the m “leaders”. Rinse and repeat to obtain any tree-based hierarchy (partway towards the general graph case). Now draw in “connections” from each process to all processes in their local array (which might be all other processes in the group, in the fully connected special case) and we have the general graph case. The tree/hierarchy defines the flow for control messages, the arrays give the final connectivity topology.
> 
> Calling this proposed function with the intention of creating a fully connected group (your normal case), requires each process to specify an array of processes (supported by the current API) containing all other processes. The first process in the array is nominated leader - PMIx sends termination notices there. If it fails, then the process at index (last_leader_index +1) is nominated until there are no more processes to nominate, at which point PMIx self-nominates and cancels the operation. If a non-leader fails, its current leader is notified. That process decides what to do. A cancel decision is broadcast to all other processes, an group join invitation is send to the invited process, a "don’t care, carry on” is silent.

I think you are making this way too complex and slow. I have no problem defining attributes to allow someone to specify such behavior, but I wouldn’t agree to make that the default as other use-cases really want a lightweight, fast operation. No issue with declaring there is a default succession strategy, but we would again define that as an attribute so someone can choose to do what I described above (i.e., let the app decide the succession).

> When a non-leader joins the group, its leader is notified.

I assume you mean this as the non-leader accepting an invite and the leader being notified per the previously-defined “join” operation?

> When the leader has responses from all its non-leaders, it declares the group complete, which is broadcast to all non-leader processes.
> 

I’m a tad concerned about this one. If the leader determines the group is complete but fails before it generates the broadcast announcing it to everyone, then how do I explain that to the rest of the group? I’ll have to wrap my head around this - I confess it bothers me. Seems like it opens a door for the group construct to “hang”, or forces us to notify everyone of everything which then impacts scalability.

> Calling this proposed function with the intention of creating a dist_graph (your special case), requires each process to specify an array of processes (supported by the current API) containing only its neighbour processes. The implementation sketched above is the same for this case. A middle-manager plays the leader role to some processes and the non-leader role to exactly one other process (at a time).

As I said above, this can work for the async case but raises some issues for the collective case. I honestly don’t know how to scalably support what you are describing, but can allow it as a special case with a warning (not printed out, but just in the text explaining that option)

> 
> In both use-cases, all processes must register for leader events and non-leader event, because they can be nominated/promoted at any time during the group construct by PMIx depending on what happens.

Agreed - but we don’t necessarily have to send all of those events to everyone. We can make that an option for those willing to pay the price.

> 
> Cheers,
> Dan.
> —
> Dr Daniel Holmes PhD
> Applications Consultant in HPC Research
> d.holmes at epcc.ed.ac.uk <mailto:d.holmes at epcc.ed.ac.uk>
> Phone: +44 (0) 131 651 3465
> Mobile: +44 (0) 7940 524 088
> Address: Room 3415, JCMB, The King’s Buildings, Edinburgh, EH9 3FD
> —
> The University of Edinburgh is a charitable body, registered in Scotland, with registration number SC005336.
> —
> 
>> On 9 Aug 2018, at 16:42, Ralph H Castain <rhc at open-mpi.org <mailto:rhc at open-mpi.org>> wrote:
>> 
>> 
>> 
>>> On Aug 9, 2018, at 7:53 AM, HOLMES Daniel <d.holmes at epcc.ed.ac.uk <mailto:d.holmes at epcc.ed.ac.uk>> wrote:
>>> 
>>> Hi Ralph,
>>> 
>>> I particularly like the self-fulfilling paradox at the end there: if the group construct fails then I won’t even call group construct, which guarantees that it’ll fail. Of course it’s not really a paradox because there are multiple processes involved but I can see why your head is hurting.
>> 
>> Yes, it is easy to rapidly get trapped in the weeds here.
>> 
>>> 
>>> Taking a further step back, scalable group construction (without a priori knowledge of membership and with the possibility of errors) requires a distributed graph specification of the membership and handling of errors. If each process only specifies its neighbours, and only handles errors from its neighbours, then the event explosion can be contained to a neighbourhood.
>>> 
>>> For example, if all processes specified the group membership as {back_proc, forward_proc} then a failed process will cause a NOTIFY_TERMINATION event in its back_proc and its forward_proc but no others, irrespective of the total size of the intended group. Those two processes must then decide what to do, e.g. link to each other (form a ring with one fewer processes than originally intended) or invite a new process (hopefully the same one ;) which will fix the originally intended ring). A “burn the world” decision from either of these two processes must then be propagated to all other participants (not necessarily around the ring, an OOB method works too). None of the other processes can have left/completed their group construct yet because not all members have given their consent (implies: connect up local neighbourhood, wait for go signal, complete locally).
>>> 
>>> If the user chooses to specify all other processes at all processes, then they are explicitly requesting to be notified about all events for which they have registered from all other processes. This may cause explosions for large groups.
>>> 
>>> Note, the user does not actually have to specify the intended size of the group when constructing it. The size and topology of the (connected portion of the) graph can be determined once the group exists, by doing collective operations using the group. Collective operations can be done on the entire (connected portion of the) graph by each process interacting only with its neighbours. Having discovered information about processes outside of one’s local neighbourhood, direct communication can be done between any pair or sub-group of processes. Thus, a fully connected MPI communicator can (if required) be built up from a MPI_DIST_GRAPH topology specification.
>> 
>> I’m reluctant to customize PMIx Groups for just the graph use-case as other programming models also have interest (and might not fit that case) and it isn’t clear to me that everyone in MPI will want to base themselves on a graph-based group approach . What we could do, though, is provide attributes to support the use model you describe and let the PMIx Group implementation deal with it as a special case.
>> 
>> For the more general case, I think we just need to ensure that there is a clean failure path that ensures the user gets out of the operation (i.e., doesn’t hang or incorrectly think the group exists) when failures occur. We can provide failure notification and recovery methods - we just need to acknowledge that these only really work in the (expected) case where failures are relatively rare events. After all, if lots of processes are failing or refusing to join the proposed group during a construct operation, then you probably need to do some triage on your cluster and/or your application!
>> 
>> If we take that approach, then we can limit notifications to the “leader” and let it decide what to do about it. If the leader fails, then we could just have PMIx automatically terminate group construction, issuing “cancel” events to all other participants.
>> 
>> For flexibility, we can add an attribute that modifies that behavior and add a new event to notify other group participants of the leader’s failure (we know the leader already agreed to join the group!). We can then add an attribute by which a process can declare itself the new leader, thereby causing an event to the rest of the group participants to update their leader assignment (this is implemented today as a broadcast and so scales relatively well). The new leader is the one that will decide what to do about giving up on constructing the group.
>> 
>> Since we cache notifications, we know that any “cancel” event received by a proc prior to registering for it will still be delivered. We then specify in the standard that procs should register for all group-related events prior to engaging in any PMIx Group operations. This ensures that the app knows about the “cancel” before calling construct, and that procs which call the blocking form of construct prior to the event arriving will still have a mechanism for getting out of the operation.
>> 
>> To make the registration easier, PMIx could add an ability to register for a “class” of events - e.g., register for the “group” class of events. This would provide for future compatibility should new group-related events get added. You currently have to specify the events you want to know about.
>> 
>> Make sense?
>> Ralph
>> 
>>> 
>>> Cheers,
>>> Dan.
>>> —
>>> Dr Daniel Holmes PhD
>>> Applications Consultant in HPC Research
>>> d.holmes at epcc.ed.ac.uk <mailto:d.holmes at epcc.ed.ac.uk>
>>> Phone: +44 (0) 131 651 3465
>>> Mobile: +44 (0) 7940 524 088
>>> Address: Room 3415, JCMB, The King’s Buildings, Edinburgh, EH9 3FD
>>> —
>>> The University of Edinburgh is a charitable body, registered in Scotland, with registration number SC005336.
>>> —
>>> 
>>>> On 9 Aug 2018, at 15:00, Ralph H Castain <rhc at open-mpi.org <mailto:rhc at open-mpi.org>> wrote:
>>>> 
>>>> So let’s step back for a moment at look at the failed construct problem in more depth. There are a couple of issues that come to mind.
>>>> 
>>>> First, do we really want to send the NOTIFY_TERMINATION event to all participants, or just to the leader? If the group under construction is large and we see a number of failures, then we could wind up in an event “storm”. If we alert only the leader, then it begs the question: what if the leader is the one who fails? Do we need a mechanism by which someone else can declare themselves to be the “leader”?
>>>> 
>>>> It isn’t too difficult for us to examine the returned results array from an event handler, though I’d want to (a) generalize it a bit so we can (b) limit how much of that we do to avoid making the event notification code explode with special cases. If we go that route, which seems the right thing to me, then.we again have a couple of choices:
>>>> 
>>>> * if the NOTIFY_TERMINATION event is only going to the leader, then we would provide the ability for the leader to declare “burn the world” and send a corresponding event to all participants. It does create a bit of a race condition as a remote participating proc may get the event prior to calling Group_join and thus (a) has no idea what the event is talking about and (b) would have to retain the cancellation notice pending the call to Group_join so it could return an error. Doable - just a tad tricky and difficult to test that race condition
>>>> 
>>>> * if the event goes to all participants, then they could locally decide to abandon the group. If they have already joined, they could leave. If not, then they could simply decline the invite. Again, there are race conditions that could bite us (particularly in multi-threaded apps), but maybe we resolve some of those by imposing requirements on the app.
>>>> 
>>>> Now that was all based on the async construct - but what do we do about a blocking call to PMIx_Group_construct? Only think I can think of would be to provide an attribute in the results array that tells the PMIx library to “kick me out of the current operation” and includes some tag(s) to indicate what operation it is talking about. We actually talked about that at some length during the last in-person PMIx devel meeting and came up with a scheme to support such a request (hasn’t been implemented yet), so this could work. However, it again creates that race condition for procs that receive the TERMINATION event prior to calling “construct” as the operation hasn’t been initiated yet. I guess we could just put the burden on the app to realize that it got a group_construct termination event and should therefore not call “construct” on that group?
>>>> 
>>>> My head is beginning to hurt and I’ve probably confused folks anyway, so best to stop here and wait for input.
>>>> Ralph
>>>> 
>>>> 
>>>>> On Aug 9, 2018, at 2:26 AM, HOLMES Daniel <d.holmes at epcc.ed.ac.uk <mailto:d.holmes at epcc.ed.ac.uk>> wrote:
>>>>> 
>>>>> Hi Ralph,
>>>>> 
>>>>> Thanks for the updates, I’ll take a look in a moment (after coffee).
>>>>> 
>>>>> Technically, a cancel API in PMIX *could* be used with a blocking group construction by calling the cancel inside an event callback. For example, an app could keep a count of how many times PMIX_GROUP_NOTIFY_TERMINATION was called for this group construction and invite a new process for the first N times but cancel the operation in response to any subsequent event(s).
>>>>> 
>>>>> MPI is trying to get rid of its cancel API - we deprecated cancel for point-to-point send because it is fundamentally broken. However, cancel for point-to-point receive is still valid and useful. The problem with cancel is always the race between the “all is OK, go ahead” and the “whoa, stop that” signals. With a receive in MPI, the choice of which will succeed (receive or cancel) is always a local decision and can therefore be made atomic and consistent. The choice between send and cancel is a distributed decision, which cannot be atomic, and always suffers from a race. One way to avoid this race in a PMIX group cancel would be specify that it is only valid from within an event callback, e.g. by exposing it as an in-out/by-ref parameter in the callback itself (not as a separate function call). PMIX could then examine this parameter (set by the user, during the event callback) when the callback returns. It’s a binary/boolean choice between “I handled it, carry on” and “I panicked, burn the world”.
>>>>> 
>>>>> Is it useful? Well all systems have finite resources to use as replacements, so eventually this operation must fail. This type of cancel allows the application to choose when to give up based on how many things happened rather than just how many seconds elapsed.
>>>>> 
>>>>> Cheers,
>>>>> Dan.
>>>>> —
>>>>> Dr Daniel Holmes PhD
>>>>> Applications Consultant in HPC Research
>>>>> d.holmes at epcc.ed.ac.uk <mailto:d.holmes at epcc.ed.ac.uk>
>>>>> Phone: +44 (0) 131 651 3465
>>>>> Mobile: +44 (0) 7940 524 088
>>>>> Address: Room 3415, JCMB, The King’s Buildings, Edinburgh, EH9 3FD
>>>>> —
>>>>> The University of Edinburgh is a charitable body, registered in Scotland, with registration number SC005336.
>>>>> —
>>>>> 
>>>>>> On 9 Aug 2018, at 03:54, Ralph H Castain <rhc at open-mpi.org <mailto:rhc at open-mpi.org>> wrote:
>>>>>> 
>>>>>> I have updated the web page to reflect the comments. Let me know what you think and about the “cancel” API.
>>>>>> 
>>>>>>> On Aug 8, 2018, at 5:40 PM, Ralph H Castain <rhc at open-mpi.org <mailto:rhc at open-mpi.org>> wrote:
>>>>>>> 
>>>>>>> 
>>>>>>> 
>>>>>>>> On Aug 8, 2018, at 4:14 PM, HOLMES Daniel <d.holmes at epcc.ed.ac.uk <mailto:d.holmes at epcc.ed.ac.uk>> wrote:
>>>>>>>> 
>>>>>>>> Hi Ralph,
>>>>>>>> 
>>>>>>>> I was nodding whilst reading,
>>>>>>> 
>>>>>>> Great!
>>>>>>> 
>>>>>>>> until I got to PMIX_GROUP_LEAVE_REQUEST (Destruct procedure, PMIx_Groups).
>>>>>>>> 
>>>>>>>> There are situations where a process will leave without the luxury of requesting and being patient first, e.g. faults (handled as termination, I know, bear with me a moment). If this event was instead PMIX_GROUP_LEFT, then processes would be written to be able to cope with sudden exits in other processes. They have to be written like that anyway because of PMIX_GROUP_NOTIFY_TERMINATION. This event simply distinguishes "the process called PMIx_GROUP_LEAVE" from "the RM figured out a process stopped executing (normally or abnormally)”. Is such a distinction useful?
>>>>>>> 
>>>>>>> It’s a good point. My thought was to provide a “clean” way of dynamically leaving a group as opposed to just pulling out. On the other hand, we do need apps to be prepared for unexpected termination - so it isn’t clear that there is any real benefit. I have no issue with making this change.
>>>>>>> 
>>>>>>>> 
>>>>>>>> In terms of outstanding/in-progress collective operations, just state that calling PMIX_GROUP_LEAVE is not allowed unless no such operations are in flight.
>>>>>>> 
>>>>>>> I think putting a requirement that no collective op can be in progress is unenforceable, especially if you take the position that leaving is the same as unexpected termination - i.e., programs need to be written in a way that can adapt to terminations or departures. We can provide an event indicating that departure occurred and user apps need to register for it and decide for themselves how to respond if in a user collective. The PMIx server can adjust any ongoing PMIx collective (e.g., PMIx_Fence) without user intervention. We currently error-out from such operations, but we can provide an attribute to indicate the operation should “self-heal” and proceed to completion.
>>>>>>> 
>>>>>>> 
>>>>>>>> The potential race between a process calling PMIX_GROUP_LEAVE and other process(es) in the group starting a collective operation should not happen in a well-defined program. Also, if PMIX_GROUP_NOTIFY_TERMINATION can state "collective operations will be adjusted appropriately" then why can’t PMIX_GROUP_LEAVE say that too?
>>>>>>> 
>>>>>>> No problem - it can certainly do so.
>>>>>>> 
>>>>>>>> 
>>>>>>>> —
>>>>>>>> 
>>>>>>>> For PMIX_GROUP_JOIN, can the leader process give up creation of the group and somehow tell PMIX to stop trying?
>>>>>>> 
>>>>>>> Sure - it can do so by setting the PMIX_TIMEOUT attribute. We could provide a “cancel” API as well, but that would require that you used the non-blocking form of PMIx_Group_construct as otherwise there would be no way to call it. Would a “cancel” API be of benefit?
>>>>>>> 
>>>>>>>> If so, then processes that accepted a join request should be informed that the group is never going to be constructed, i.e. they should stop waiting for the callback/return of the blocking function. Thus, "once the group has been completely constructed” could be tempered with “or the group construction fails”.
>>>>>>> 
>>>>>>> Agreed - will update.
>>>>>>> 
>>>>>>>> 
>>>>>>>> Cheers,
>>>>>>>> Dan.
>>>>>>>> —
>>>>>>>> Dr Daniel Holmes PhD
>>>>>>>> Applications Consultant in HPC Research
>>>>>>>> d.holmes at epcc.ed.ac.uk <mailto:d.holmes at epcc.ed.ac.uk>
>>>>>>>> Phone: +44 (0) 131 651 3465
>>>>>>>> Mobile: +44 (0) 7940 524 088
>>>>>>>> Address: Room 3415, JCMB, The King’s Buildings, Edinburgh, EH9 3FD
>>>>>>>> —
>>>>>>>> The University of Edinburgh is a charitable body, registered in Scotland, with registration number SC005336.
>>>>>>>> —
>>>>>>>> 
>>>>>>>>> On 8 Aug 2018, at 23:04, Ralph H Castain <rhc at open-mpi.org <mailto:rhc at open-mpi.org>> wrote:
>>>>>>>>> 
>>>>>>>>> Hi folks
>>>>>>>>> 
>>>>>>>>> I have updated the PMIx Group web page to capture the discussion of the prior meeting plus some subsequent thoughts:
>>>>>>>>> 
>>>>>>>>> https://pmix.org/pmix-standard/pmix-groups/ <https://pmix.org/pmix-standard/pmix-groups/>
>>>>>>>>> 
>>>>>>>>> I’ll try to put some initial implementation behind it before the meeting, so please feel free to chime up with any thoughts.
>>>>>>>>> Ralph
>>>>>>>>> 
>>>>>>>>> 
>>>>>>>>>> On Aug 6, 2018, at 11:38 AM, Ralph H Castain <rhc at open-mpi.org <mailto:rhc at open-mpi.org>> wrote:
>>>>>>>>>> 
>>>>>>>>>> Looks like I can free some time up this week for groups - will try to update later this week 
>>>>>>>>>> 
>>>>>>>>>> Sent from my iPhone
>>>>>>>>>> 
>>>>>>>>>> On Aug 6, 2018, at 11:05 AM, HOLMES Daniel <d.holmes at epcc.ed.ac.uk <mailto:d.holmes at epcc.ed.ac.uk>> wrote:to
>>>>>>>>>> 
>>>>>>>>>>> Hi all,
>>>>>>>>>>> 
>>>>>>>>>>> The next meeting for the Sessions WG will be *Tuesday 14th Aug 2018* at 12pm Eastern US time.
>>>>>>>>>>> 
>>>>>>>>>>> Note the change of day and time. This is a one-off change due to vacation time.
>>>>>>>>>>> 
>>>>>>>>>>> The connection details for the call will be sent out on this list nearer the time.
>>>>>>>>>>> 
>>>>>>>>>>> Cheers,
>>>>>>>>>>> Dan.
>>>>>>>>>>> —
>>>>>>>>>>> Dr Daniel Holmes PhD
>>>>>>>>>>> Applications Consultant in HPC Research
>>>>>>>>>>> d.holmes at epcc.ed.ac.uk <mailto:d.holmes at epcc.ed.ac.uk>
>>>>>>>>>>> Phone: +44 (0) 131 651 3465
>>>>>>>>>>> Mobile: +44 (0) 7940 524 088
>>>>>>>>>>> Address: Room 3415, JCMB, The King’s Buildings, Edinburgh, EH9 3FD
>>>>>>>>>>> —
>>>>>>>>>>> The University of Edinburgh is a charitable body, registered in Scotland, with registration number SC005336.
>>>>>>>>>>> —
>>>>>>>>>>> 
>>>>>>>>>>> The University of Edinburgh is a charitable body, registered in
>>>>>>>>>>> Scotland, with registration number SC005336.
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