-> From: dennis@rats-b.nosc.mil (Dennis Cottel)
-> To: mpi-1sided@mcs.anl.gov
-> Subject: MPI handlers need preemption
-> Sender: owner-mpi-core@mcs.anl.gov
-> Precedence: bulk
->
->
->
-> At the April Forum meeting while discussing MPI handlers, I argued that
-> handlers should act like "interrupt routines" because that is not only
-> what users need, but what they expect based on the behavior of past and
-> existing implementations.
->
-> At that point, Joel Clark of Intel spoke up to say that in the current
-> Paragon implementation, NX hrecv handlers don't preempt the running
-> (computing) process. Instead, he said, the thread dealing with handler
-> routines is simply scheduled to run like any other thread and at the
-> same priority as the computing thread.
->
-> Since then, we've done some experiments, which Joel has confirmed, that
-> show that Paragon NX handler code starts running within a few hundred
-> microseconds. The handler does not wait for the computing process to
-> voluntarily give up the CPU, nor does the handler wait for the computing
-> process's time slice to be exhausted.
->
-> In fact, it appears from Joel's measurements that in the newest version
-> of the Paragon OS (R1.4), a separate pthread which has done a blocking
-> receive also runs "immediately". That is, a user could implement
-> interrupt handler functionality using a pthread and normal MPI receive
-> calls.
I have to say I believe this "immediate" handler execution is an artifact
of the unix-like Paragon scheduling and priority ageing and is not
guaranteed by the functional definition.
joel
->
-> This preemptive behavior is a requirement if users are to be able
-> to build layered message passing functionality above MPI. User's
-> can't get the same functionality any other way.
->
-> Since my only experience with message passing handlers has been with NX
-> on the iPSC/860 and Paragon, could someone describe the handler
-> capabilities of message passing libraries on other systems and whether
-> they provide interrupt-like behavior?
->
-> --Dennis
->
-> Dennis Cottel, dennis@nosc.mil, (619) 553-1645
-> NCCOSC, RDT&E Division (NRaD), San Diego, CA
->
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