LAM features a full implementation of Message-Passing Interface, with
the exception that the MPI_CANCEL function will not properly cancel
messages that have been sent.  Pending receive requests can be
canceled; implementation of canceling send messages was judged to be
too difficult (and not enough LAM users asked for it).
.PP
Compliant applications are source code portable between LAM and any
other implementation of MPI.  In addition to meeting the standard in a
high quality manner, LAM offers extensive monitoring capabilities to
support debugging.  Monitoring happens on two levels.  LAM has the
hooks to allow a snapshot of process and message status to be taken at
any time during an application run.  The status includes all aspects
of synchronization plus datatype map / signature, communicator group
membership and message contents.  On the second level, the MPI library
is instrumented to produce a cumulative record of communication, which
can be visualized either at runtime or post-mortem.
.PP
Another strength of this MPI implementation is the movement of
non-blocking communication requests, including those that result from
buffered sends.  This is the real challenge of implementing MPI;
everything else is mostly a straight forward wrapping of an underlying
communication mechanism.  LAM allows messages to be buffered on the
source end in any state of progression, including partially
transmitted packets.  This capability leads to great portability and
robustness.
.SS Up-to-Date Information
The LAM home page can be found on the World Wide Web at:
.I http://www.lam-mpi.org/ .
It should be consulted for the most current information about LAM, as
well as updates, patches, etc.
.SS Direct MPI Communication
The sophisticated message advancing engine at the heart of the MPI
library uses only a handful of routines to drive the underlying
communication system.  Runtime flags decide which implementation of
these low-level routines is used, so recompilation is not necessary.
The default implementation uses LAM's network message-passing
subsystem, including its buffer daemon.  In this "daemon" mode, LAM's
extensive monitoring features are fully available.  The main purpose
of daemon based communication is development, but depending on the
application's decomposition granularity and communication requirement,
it may also be entirely adequate for production execution.
.PP
The other implementation of the MPI library's low-level communication
intends to use the highest performance underlying mechanism, certainly
bypassing the LAM daemon and connecting directly between application
processes.  This is the "client to client" mode (C2C).
.PP
The availability of optimal C2C implementations will continue to
change as architectures come and go.  At the least, LAM includes a
combination TCP/IP and shared memory implementation of C2C that
bypasses the LAM daemon.
.PP
MPI process and message monitoring commands and tools will be much
less effective in C2C mode, usually reporting running processes and
empty message queues.  Signal delivery with doom(1) is unaffected.
.SS Guaranteed Envelope Resources
Applications may fail, legitimately, on some implementations but not
others due to an escape hatch in the MPI Standard called "resource
limitations".  Most resources are managed locally and it is easy for
an implementation to provide a helpful error code and/or message when
a resource is exhausted.  Buffer space for message envelopes is often
a remote resource (as in LAM) which is difficult to manage.  An
overflow may not be reported (as in some other implementations) to the
process that caused the overflow.  Moreover, interpretation of the MPI
guarantee on message progress may confuse the debugging of an
application that actually died on envelope overflow.
.PP
LAM has a property called "Guaranteed Envelope Resources" (GER) which
serves two purposes.  It is a promise from the implementation to the
application that a minimum amount of envelope buffering will be
available to each process pair.  Secondly, it ensures that the
producer of messages that overflows this resource will be throttled or
cited with an error as necessary.
.PP
A minimum GER is configured when LAM is built.  The MPI library uses a
protocol to ensure GER when running in daemon mode.  The default C2C
mode (TCP/IP) does not use a protocol, because process-pair protection
is provided by TCP/IP itself.  Errors are only reported to the
receiving process in C2C mode.  An option to mpirun(1) disables GER.
.SS Input and Output
The MPI standard does not specify standard I/O functionality.  LAM
does not interfere with the I/O capabilities of the underlying system
but it does make special provisions for remote terminal I/O using the
ANSI/POSIX routines.  See mpirun(1) and tstdio(3).
.PP
LAM now includes the ROMIO distribution for MPI-2 file input and
output.  If ROMIO support is compiled into LAM, the functionality from
Chapter 9 of the MPI-2 standard is provided.
.PP
ROMIO has some important limitations under LAM; the RELEASE_NOTES file
in the LAM distribution should be consulted before writing MPI
programs that use MPI I/O.
.SS Dynamic Processes
LAM includes an implementation of MPI-2 dynamic process creation.
The non-blocking functions are not implemented.
.SH LAM Extensions to MPI
.SS Debugging Aids
Opaque MPI objects are monitored by LAM commands but the user needs a
way to cross-reference the identification given by the commands with
variables within MPI application.
These extensions extract identifiers from MPI communicators and datatypes.
See MPIL_Comm_id(2) and MPIL_Type_id(2).
.PP
Additionally, LAM provides the capability to launch non-MPI programs
on remote nodes.  This includes shell scripts, debuggers, etc.  As
long as an MPI program is eventually launched (as a child, grandchild,
etc.), LAM can handle executing as many intermediate programs as
necessary.  This can greatly help debugging and logging of user
programs.
.SS Trace Generation
To avoid being swamped with trace data from a long running application,
LAM supplies collective operations to turn the tap on and off.
See MPIL_Trace_on(2) and MPIL_Trace_off(2).
.SS Asynchronous Signals
LAM has an signal handling package which mirrors but does not interfere
with POSIX signal handling.
An MPI extension routine delivers a signal to a process.
See MPIL_Signal(2).
.SH SEE ALSO
.SS Overview of Commands and Libraries
introu(1), introc(2), INTROF(2)
.SS Starting / Stopping LAM
recon(1), lamboot(1), lamhalt(1), lamnodes(1), wipe(1),
tping(1), lamgrow(1), lamshrink(1)
.SS Compiling MPI Applications
mpicc(1), mpiCC(1), mpif77(1)
.SS Running MPI Applications
mpirun(1), lamclean(1)
.SS Running Non-MPI Applications
lamexec(1)
.SS Monitoring MPI Applications
mpitask(1), mpimsg(1)
.SS Unloading MPI Trace Data
lamtrace(1)
.SS Enhanced I/O in MPI Programs
lam_rfposix(2), tstdio(3), cubix(3)
.SS Reference Documents
.HP
"LAM Frequently Asked Questions"
.br
at
.I http://www.lam-mpi.org/faq/
.HP
"MPI Primer / Developing with LAM", Ohio Supercomputer Center
.HP
"MPI: A Message-Passing Interface Standard", Message-Passing
Interface Forum, version 1.1
.br
at 
.I http://www.mpi-forum.org/
.HP
"MPI-2: Extensions to the Message Passing Interface", Message Passing
Interface Forum, version 2.0
.br
at
.I http://www.mpi-forum.org/
.SS MPI Quick Tutorials
.HP
"LAM/MPI ND User Guide / Introduction"
.br
at
.I http://www.lam-mpi.org/mpi/tutorials/lam/
.HP
"MPI: It's Easy to Get Started"
.HP
"MPI: Everyday Datatypes"
.HP
"MPI: Everyday Collective Communication"
.SS Guaranteed Envelope Resources
.HP
"Robust MPI Message Delivery Through Guaranteed Resources",
MPI Developer's Conference, 1995
