[drmaa-wg] DRMAA test suite

[Daniel Templeton]

Peter,

I know we discussed this at length already, but I remember the 
discussion being about synchronize().  Sorry to beat dead horses, but 
this one seems like a real handicap to me.  Basically, you're saying 
that wait(), a very important element of the API, isn't useful in an MT 
environment.  I've never seen a DRMAA app, other than the most trivial 
example code, that doesn't call wait().  The logical conclusion, then, 
is that DRMAA isn't useful in an MT environment.

I think you misunderstood my example.  Say I want to build an event 
mechanism around jobs finishing.  The most sensible way to do that is to 
dedicate a thread to doing wait() calls.  That thread would then put the 
JobInfo objects in some common data cache and send out an event 
notifying the other threads that a new JobInfo object is available.  (Or 
it could send the job exit info in the event; same difference.)  The 
proposed semantics for wait() effectively prevents this use case.  I 
would basically have to have my wait thread set a short time limit on 
the wait() call so that it can keep the wait() context current, 
effectively turning it into polling instead of a blocking procedure.

As for being more difficult to implement, I completely disagree.  In the 
SGE implementation, wait(ANY) just waits for a job finish event from the 
qmaster.  To implement the proposed semantics, I'd have to take a 
snapshot of the current job list and compare incoming events to that list.

For synchronize(), keeping the call-time context makes perfect sense.  I 
can see where one could strictly argue that if it's good for 
synchronize(), it's good for wait(), but we need to be a little 
pragmatic.  Again, I ask the question, "what problem does it solve?"  
The behavior of the proposed semantics can be duplicated in several ways 
using the current semantics, but I don't see how to reasonably get the 
current behavior from the proposed semantics.  Also, keep in mind that 
you're changing the behavior of a core routine in a way that affects 
some basic use cases.  Not good.

The up-side is that we're talking about the IDL spec, and not the DRMAA 
1.0 spec, so we have room to make changes still.  And, in case it wasn't 
clear from my tirade, the SGE DRMAA implementations do not limit the 
context of wait(), so the proposed semantics will be a change (for the 
worse) for SGE users.

Daniel

Peter Troeger wrote:
>> Looking through the IDL spec, it says that drmaa_wait(ANY) will only
>> work on jobs submitted up to the time of the drmaa_wait() call.  I don't
>> like that.  For drmaa_synchronize(ALL), it makes sense, because
>> otherwise the call would block indefinitely in an active system.  With
>> drmaa_wait(), however, that change prevents a very useful use case.  Say
>> I want to write a thread that waits for jobs to end and places their
>> finish information in a data structure for other threads to read.  With
>> that caveat applied, if I submit one very long-running job before
>> drmaa_wait() gets called, the hundreds of really short jobs that I
>> submit after the drmaa_wait() call have to wait for the long-running job
>> to end so that the next call to drmaa_wait() can see them.  That's bad,
>> and I don't see where it makes anything better.  What problem does
>> limiting drmaa_wait() to previously submitted jobs solve?
>>     
>
> We had so much discussion around the drmaa_wait semantics, I am not sure
> what the exact reason was. For me, it seems like the same argumentation
> as with drmaa_synchronize. The drmaa_wait() call relies on some current
> state of all the jobs in the session. I know that I submitted 3 jobs so
> far, and now I want to wait for all of them. If we allow other threads
> to extend the session while drmaa_wait() is running, you need to clarify
> the point of synchronization within the running drmaa_wait() call. It's
> harder to implement.
>
> In your particular example, my expectation would be that the second
> thread also calls drmaa_wait() in parallel. In this case, our modified
> text from the latest DRMAA doc can be applied:
>
> -- snip
>
> In a multithreaded environment, only the active thread gets the
> status of the finished or failed job in that case, while the rest of the
> threads continue waiting. If there are no more running or completed jobs
> the routine SHOULD return DRMAA_ERRNO_INVALID_JOB error.
>
> -- snip
>
> We can summarize that drmaa_wait(SESSION_ANY) is always a bad idea when
> multiple threads submitting jobs. In order to get a consistent picture,
> it seems to be appropriate to define the function call as
> "synchronization point", where the session state "at this time" acts as
> input to the method.
>
>
> Peter.
>
>
>
>