[Pvfs2-developers] terminating state machines

[Sam Lang]

On Jul 26, 2006, at 3:41 PM, Walter B. Ligon III wrote:

> Yeah, the idea is that the SM code would call the job function.  
> Depending on the state actions to do it seems like asking for  
> trouble, all the details that have to be kept up with.
>
> Actually, there are already job structs used by the SM code, now  
> I've had to add a context id to the smcb and there will be job  
> calls.  I think you are right though, the amount of dependency is  
> pretty small.
>
> As for the job funcs I think I'd need one new one to post the  
> parent job, establishing a counter.  The child job would look up  
> the counter, decrement, and if zero, call job_null to relaunch the  
> parent, or just
> replicate what job_null does, whatever seem the easiest.
>

I would rather see the parent get relaunched by the normal job test  
code by putting itself in the job completion queue once its  
finished.  This could happen in a job_sm_test call like I suggested  
in my previous email.  Also, instead of a counter that a test  
function would check, and the child state machines would have to  
decrement, I'd prefer the parent job keep an array of child state  
machines (it does this anyway, no?) and check each element in the  
array for completion of the state machine.  That way the children  
aren't competing to lock the same state to notify of completion, the  
parent just checks each one.

> The implicit call is the child's call when it terminates.  The  
> parent's call could be implicit too, or done by the state action.

Doesn't this require child state machines to only function in the  
child state machine context?  I'd prefer to just have generic state  
machines that can be used as a child state machine or as a top-level  
state machine.

>
> As of this moment we really haven't taken any pains to keep the SM  
> independent from the job system, in fact you have to have the job  
> system to drive things, so in some sense its not really an issue.

I vote for making the interfaces as separate as possible.  If someone  
else wants to use the state machine code somewhere else, it would be  
nice to allow them to take it as-is (mpich2 guys were talking about  
using it, but I think they ended up doing something else).  Also,  
independent layers make testing and debugging easier in my view.

In the current code, the sm_p is passed through to the job descriptor  
as a void*, and we just cast back to a sm_p in the while loop that  
does the job_testcontext and then drives the state machines again.   
The use of job_status does bring in the job code into the state  
machine code,  but it seems like mostly only the error_code field is  
used within the state actions, and the rest of that structure could  
be independent of the state machine code.

-sam

>
> Any more commends?  (Sam I hope this address some of yours)
>
> Walt
>
> Phil Carns wrote:
>> Walter B. Ligon III wrote:
>>>
>>> OK, guys, I have another issue I want input on.  When child SMs  
>>> terminate they have to notify their parent.  The parent has to  
>>> wait for all the children to terminate.  So I've been thinking to  
>>> use the job subsystem for this: the parent would post a job to  
>>> wait for N children,
>>> and each child would post a job, the last one releasing the parent.
>>>
>>> Now I see two ways to implement this - one is to implement this  
>>> directly in the state machine code.  The parent simply stops  
>>> running (because it does not schedule a job yet returns  
>>> DEFERRED).  Each child decrements a counter, and when it hits 0  
>>> the parent is restarted.  This is a little ugly because the  
>>> waiting parent is not being held on any list or queue (up to now  
>>> all waiting SMs are in the job subsystem), also the last  
>>> terminating child becomes the parent as it starts executing the  
>>> parent code.  Things can get weird when one SM starts children  
>>> that start children, and so on.
>>>
>>> Now the other way to implement this is with the job subsystem as  
>>> I suggested above.  Much cleaner except for one thing:  up to now  
>>> the state machine subsystem has had no dependency at all on the  
>>> job subsystem.  If we do it this way, this function only works  
>>> with the job system intact.  I'd prefer not to do this, but it  
>>> does seem the cleanest, most logical means.
>> I like the job approach.  I guess this is an extra dependency  
>> because the sms would be calling these particular job functions  
>> implicitly, rather than relying on the state functions to handle  
>> those posts and releases?  We definitely haven't done that before,  
>> but at least in this case the job function that the sm  
>> infrastructure would be depending on is the simplest one in the  
>> arsenal :)  It shouldn't be hard for someone to reimplement that  
>> particular functionality if they wanted to use the state machine  
>> mechanism in another project.
>> If you weren't planning on these job calls to be implicit, then  
>> I'm not sure where the extra dependency is- we already use jobs to  
>> trigger all of the other "normal" transitions.
>> This reminded me of a question, though- is there going to be a  
>> standard mechanism for the children to report each of their  
>> independent error codes to the parent sm?  Or do the children need  
>> to just keep a reference to the parent sm structure and manually  
>> fill in an array or something?
>> I guess I have a broader question of how data that the children  
>> generate (like a handle value or an attr structure) gets  
>> transferred to the parent.  Does the parent copy this stuff from  
>> the child after the child finishes, or does the child copy it to  
>> the parent before it exits?    I think we talked about this before  
>> at some point but I forgot what the plan is.  It would be nice if  
>> we made the developer define macros or something to dictate what  
>> the input parameters need to be filled in when invoking a child  
>> and what output parameters can be retrieved when it finishes.   
>> Otherwise it starts getting tricky to remember what fields need to  
>> be set in the sm structure before kicking something off.
>> -Phil
>> -Phil
>
> -- 
> Dr. Walter B. Ligon III
> Associate Professor
> ECE Department
> Clemson University
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