Sam Lang wrote:
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.
There doesn't need to be any locking- the main server thread only
executes one state function or one transition at a time. The counter
also doesn't need to be visible- it could be hidden inside the job call,
which could lock or not lock as it sees fit.
The parent also couldn't be the one checking the elements in an array
like that - it would have to be done from within the job code somewhere
(which I think you described in your previous email). That means that
somewhere in the job code (or request scheduler, etc.) something will
have to do the following on every job_testcontext() call:
for each active sm
for each child within that sm
check state
Which could get expensive depending on how extensively we use the
child/parallel sm model.
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.
I would prefer that too :) Is this going to work Walt? It would be
nice if the state machine processing code handled transparently
triggering different termination functions depending on whether it was a
top level sm or not without the state functions themselves knowing any
better.
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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