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README

This directory contains an experimental, version 2 PMI interface.  Both
the client API and the wire protocol for this version are different than
PMI version 1.  This version is not yet in use in MPICH2, and is 
being developed to prototype the changes proposed.  In particular, this version
adds support for better thread-safe behavior.

Currently, the source files simply define the interfaces.  There is no 
implementation yet.  The files have been added to the repository so that they
can be reviewed by interested parties, particularly 3rd-parties that 
will need to interface to this second-generation interface.

A major issue that PMI verison 2 needs to address is thread-safety and
responsiveness.  In particular, no thread that is waiting on a PMI call can 
block other threads, even PMI_Spawn.  The design sketched out here
addresses these issues as well as providing a relatively simple model
for thread interaction.

To allow multiple threads to share the PMI socket; we use the
following model for mediating atomic access:

enter-atomic
   make use of socket
   create operation with a completion flag "flag"

   waitfor( &flag, atomic-ctx, wait-function, wait-ctx )

   cleanup
exit-atomic

During the "waitfor" operation, other threads may enter this atomic
section.

Here is a sketch of the implementation of "waitfor" that makes use of 
the Posix condition variable to block the threads that are waiting for
a response.

waitfor( ... )
{
  do {
      if (atomic-ctx->wait-function-running) {
          pthread_cond_wait( atomic-ctx->mutex, atomic-ctx->condvar );
      }
      else {
         atomic-ctx->wait-function-running = 1;
         pthread_mutex_unlock( atomic-ctx->mutex );
         wait-function( flag_ptr, wait-ctx );
         pthread_cond_signal( atomic-ctx->condvar )
      }
  } while (!*flag);
  pthread_mutex_lock( atomic-ctx->mutex );
}


Here, the wait-function might look something like this:

int wait-function( int *flag, void *socket_set )
do {
     poll( socket_set ... );
     for each active fd
          process fd (may complete some operations). 
    } while (!*flag);
}

Note that in the single-threaded case, the waitfor routine turns into

waitfor( ... )
{
  do {
     wait-function( flag_ptr, wait-ctx );
      }
  } while (!*flag);
}

and the enter/exit atomic are no-ops.  This makes each of the
implementation of each of the PMI routines essentially independent of
the number of threads.  

For example, the PMI_KVS_Get operation will look more like

int PMI_KVS_Get( ... )
{
    enter-atomic
        create operation, get opid
        enqueue operation
        write ( pmifd, "nnnnnncmd=get;opid=...;name=..." );
        waitfor( &operation->flag, ... );
        dequeue operation
        extract response (e.g., the value and rc)
        free operation
    exit-atomic
}

In principle, once the operation is dequeued, we could exit the atomic
section, but we may want to keep a pool of operations (instead of
malloc'ing it each time), and then we'd need to re-aquire the critical
section in order to free it (another option would be to allow one
operation per thread and allocate it once; that would fit with the
usage model but may not be worth the effort in this case).

The above is sufficient for the needs of PMI.  One additional
optimization is to avoid the mutex by first doing a test:

test-function( test-ctx )
if (!*flag) {
   ... the usual code above, including the trailing mutex lock
}

An easy way to do this with the single wait function is to make the
routine wait only if there is a non-null flag, i.e., 

wait-function( NULL, wait-ctx )
if (!*flag) {
       ....
}

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