On Fri, Jun 10, 2011 at 7:37 AM, Ralph Castain <r...@open-mpi.org> wrote:
>
> On Jun 9, 2011, at 6:12 PM, Joshua Hursey wrote:
>
>>
>> On Jun 9, 2011, at 6:47 PM, George Bosilca wrote:
>>
>>> Well, you're way to trusty. ;)
>>
>> It's the midwestern boy in me :)
>
> Still need to shake that corn out of your head... :-)
>
>>
>>>
>>> This only works if all component play the game, and even then there it is
>>> difficult if you want to allow components to deregister themselves in the
>>> middle of the execution. The problem is that a callback will be previous
>>> for some component, and that when you want to remove a callback you have to
>>> inform the "next" component on the callback chain to change its previous.
>>
>> This is a fair point. I think hiding the ordering of callbacks in the errmgr
>> could be dangerous since it takes control from the upper layers, but,
>> conversely, trusting the upper layers to 'do the right thing' with the
>> previous callback is probably too optimistic, esp. for layers that are not
>> designed together.
>>
>> To that I would suggest that you leave the code as is - registering a
>> callback overwrites the existing callback. That will allow me to replace the
>> default OMPI callback when I am able to in MPI_Init, and, if I need to, swap
>> back in the default version at MPI_Finalize.
>>
>> Does that sound like a reasonable way forward on this design point?
>
> It doesn't solve the problem that George alluded to - just because you
> overwrite the callback, it doesn't mean that someone else won't overwrite you
> when their component initializes. Only the last one wins - the rest of you
> lose.
>
> I'm not sure how you guarantee that you win, which is why I'm unclear how
> this callback can really work unless everyone agrees that only one place gets
> it. Put that callback in a base function of a new error handling framework,
> and then let everyone create components within that for handling desired
> error responses?
Yep, that is a problem, but one that we can deal with in the immediate
case. Since OMPI is the only layer registering the callback, when I
replace it in OMPI I will have to make sure that no other place in
OMPI replaces the callback.
If at some point we need more than one callback above ORTE then we may
want to revisit this point. But since we only have one layer on top of
ORTE, it is the responsibility of that layer to be internally
consistent with regard to which callback it wants to be triggered.
If the layers above ORTE want more than one callback I would suggest
that that layer design some mechanism for coordinating these multiple
- possibly conflicting - callbacks (by the way this is policy
management, which can get complex fast as you add more interested
parties). Meaning that if OMPI wanted multiple callbacks to be active
at the same time, then OMPI would create a mechanism for managing
these callbacks, not ORTE. ORTE should just have one callback provided
to the upper layer, and keep it -simple-. If the upper layer wants to
toy around with something more complex it must manage the complexity
instead of artificially pushing it down to the ORTE layer.
-- Josh
>>
>> -- Josh
>>
>>>
>>> george.
>>>
>>> On Jun 9, 2011, at 13:21 , Josh Hursey wrote:
>>>
>>>> So the "Resilient ORTE" patch has a registration in ompi_mpi_init.c:
>>>> -------------
>>>> orte_errmgr.set_fault_callback(&ompi_errhandler_runtime_callback);
>>>> -------------
>>>>
>>>> Which is a callback that just calls abort (which is what we want to do
>>>> by default):
>>>> -------------
>>>> void ompi_errhandler_runtime_callback(orte_process_name_t *proc) {
>>>> ompi_mpi_abort(MPI_COMM_WORLD, 1, false);
>>>> }
>>>> -------------
>>>>
>>>> This is what I want to replace. I do -not- want ompi to abort just
>>>> because a process failed. So I need a way to replace or remove this
>>>> callback, and put in my own callback that 'does the right thing'.
>>>>
>>>> The current patch allows me to overwrite the callback when I call:
>>>> -------------
>>>> orte_errmgr.set_fault_callback(&my_callback);
>>>> -------------
>>>> Which is fine with me.
>>>>
>>>> At the point I do not want my_callback to be active any more (say in
>>>> MPI_Finalize) I would like to replace it with the old callback. To do
>>>> so, with the patch's interface, I would have to know what the previous
>>>> callback was and do:
>>>> -------------
>>>> orte_errmgr.set_fault_callback(&ompi_errhandler_runtime_callback);
>>>> -------------
>>>>
>>>> This comes at a slight maintenance burden since now there will be two
>>>> places in the code that must explicitly reference
>>>> 'ompi_errhandler_runtime_callback' - if it ever changed then both
>>>> sites would have to be updated.
>>>>
>>>>
>>>> If you use the 'sigaction-like' interface then upon registration I
>>>> would get the previous handler back (which would point to
>>>> 'ompi_errhandler_runtime_callback), and I can store it for later:
>>>> -------------
>>>> orte_errmgr.set_fault_callback(&my_callback, prev_callback);
>>>> -------------
>>>>
>>>> And when it comes time to deregister my callback all I need to do is
>>>> replace it with the previous callback - which I have a reference to,
>>>> but do not need the explicit name of (passing NULL as the second
>>>> argument tells the registration function that I don't care about the
>>>> current callback):
>>>> -------------
>>>> orte_errmgr.set_fault_callback(&prev_callback, NULL);
>>>> -------------
>>>>
>>>>
>>>> So the API in the patch is fine, and I can work with it. I just
>>>> suggested that it might be slightly better to return the previous
>>>> callback (as is done in other standard interfaces - e.g., sigaction)
>>>> in case we wanted to do something with it later.
>>>>
>>>>
>>>> What seems to be proposed now is making the errmgr keep a list of all
>>>> registered callbacks and call them in some order. This seems odd, and
>>>> definitely more complex. Maybe it was just not well explained.
>>>>
>>>> Maybe that is just the "computer scientist" in me :)
>>>>
>>>> -- Josh
>>>>
>>>>
>>>> On Thu, Jun 9, 2011 at 1:05 PM, Ralph Castain <r...@open-mpi.org> wrote:
>>>>> You mean you want the abort API to point somewhere else, without using a
>>>>> new
>>>>> component?
>>>>> Perhaps a telecon would help resolve this quicker? I'm available tomorrow
>>>>> or
>>>>> anytime next week, if that helps.
>>>>>
>>>>> On Thu, Jun 9, 2011 at 11:02 AM, Josh Hursey <jjhur...@open-mpi.org>
>>>>> wrote:
>>>>>>
>>>>>> As long as there is the ability to remove and replace a callback I'm
>>>>>> fine. I personally think that forcing the errmgr to track ordering of
>>>>>> callback registration makes it a more complex solution, but as long as
>>>>>> it works.
>>>>>>
>>>>>> In particular I need to replace the default 'abort' errmgr call in
>>>>>> OMPI with something else. If both are called, then this does not help
>>>>>> me at all - since the abort behavior will be activated either before
>>>>>> or after my callback. So can you explain how I would do that with the
>>>>>> current or the proposed interface?
>>>>>>
>>>>>> -- Josh
>>>>>>
>>>>>> On Thu, Jun 9, 2011 at 12:54 PM, Ralph Castain <r...@open-mpi.org> wrote:
>>>>>>> I agree - let's not get overly complex unless we can clearly articulate
>>>>>>> a
>>>>>>> requirement to do so.
>>>>>>>
>>>>>>> On Thu, Jun 9, 2011 at 10:45 AM, George Bosilca <bosi...@eecs.utk.edu>
>>>>>>> wrote:
>>>>>>>>
>>>>>>>> This will require exactly opposite registration and de-registration
>>>>>>>> order,
>>>>>>>> or no de-registration at all (aka no way to unload a component). Or
>>>>>>>> some
>>>>>>>> even more complex code to deal with internally.
>>>>>>>>
>>>>>>>> If the error manager handle the callbacks it can use the registration
>>>>>>>> ordering (which will be what the the approach can do), and can enforce
>>>>>>>> that
>>>>>>>> all callbacks will be called. I would rather prefer this approach.
>>>>>>>>
>>>>>>>> george.
>>>>>>>>
>>>>>>>> On Jun 9, 2011, at 08:36 , Josh Hursey wrote:
>>>>>>>>
>>>>>>>>> I would prefer returning the previous callback instead of relying on
>>>>>>>>> the errmgr to get the ordering right. Additionally, when I want to
>>>>>>>>> unregister (or replace) a call back it is easy to do that with a
>>>>>>>>> single interface, than introducing a new one to remove a particular
>>>>>>>>> callback.
>>>>>>>>> Register:
>>>>>>>>> ompi_errmgr.set_fault_callback(my_callback, prev_callback);
>>>>>>>>> Deregister:
>>>>>>>>> ompi_errmgr.set_fault_callback(prev_callback, old_callback);
>>>>>>>>> or to eliminate all callbacks (if you needed that for somme reason):
>>>>>>>>> ompi_errmgr.set_fault_callback(NULL, old_callback);
>>>>>>>>
>>>>>>>>
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>>>>>>>
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>>>>>>
>>>>>>
>>>>>>
>>>>>> --
>>>>>> Joshua Hursey
>>>>>> Postdoctoral Research Associate
>>>>>> Oak Ridge National Laboratory
>>>>>> http://users.nccs.gov/~jjhursey
>>>>>>
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>>>>>
>>>>>
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>>>>
>>>>
>>>>
>>>> --
>>>> Joshua Hursey
>>>> Postdoctoral Research Associate
>>>> Oak Ridge National Laboratory
>>>> http://users.nccs.gov/~jjhursey
>>>>
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>>>
>>>
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>
--
Joshua Hursey
Postdoctoral Research Associate
Oak Ridge National Laboratory
http://users.nccs.gov/~jjhursey
