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). 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 monitor-
ing 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 con-
tents. On the second level, the MPI library is instrumented to
produce a cumulative record of communication, which can be visu-
alized either at runtime or post-mortem. Another strength of
this MPI implementation is the movement of non-blocking communi-
cation 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 communica-
tion 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 robust-
ness. The LAM home page can be found on the World Wide Web at:
It should be consulted for the most current information about
LAM, as well as updates, patches, etc. 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 rou-
tines is used, so recompilation is not necessary. The default
implementation uses LAM's network message-passing subsystem, in-
cluding its buffer daemon. In this ";daemon" mode, LAM's exten-
sive monitoring features are fully available. The main purpose
of daemon based communication is development, but depending on
the application's decomposition granularity and communication re-
quirement, it may also be entirely adequate for production execu-
tion. 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 di-
rectly between application processes. This is the ";client to
client"; mode (C2C). The availability of optimal C2C implementa-
tions 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. MPI process
and message monitoring commands and tools will be much less ef-
fective in C2C mode, usually reporting running processes and emp-
ty message queues. Signal delivery with doom(1) is unaffected.
Applications may fail, legitimately, on some implementations but
not others due to an escape hatch in the MPI Standard called ";re-
source 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. LAM has a property called ";Guaranteed Enve-
lope 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 neces-
sary. 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, be-
cause process-pair protection is provided by TCP/IP itself. Er-
rors are only reported to the receiving process in C2C mode. An
option to mpirun(1) disables GER. 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). 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. ROMIO has some important limita-
tions under LAM; the RELEASE_NOTES file in the LAM distribution
should be consulted before writing MPI programs that use MPI I/O.
LAM includes an implementation of MPI-2 dynamic process creation.
The non-blocking functions are not implemented. Opaque MPI ob-
jects 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 iden-
tifiers from MPI communicators and datatypes. See
MPIL_Comm_id(2) and MPIL_Type_id(2). Additionally, LAM provides
the capability to launch non-MPI programs on remote nodes. This
includes shell scripts, debuggers, etc. As long as an MPI pro-
gram 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. To
avoid being swamped with trace data from a long running applica-
tion, LAM supplies collective operations to turn the tap on and
off. See MPIL_Trace_on(2) and MPIL_Trace_off(2). LAM has an
signal handling package which mirrors but does not interfere with
POSIX signal handling. An MPI extension routine delivers a sig-
nal to a process. See MPIL_Signal(2). introu(1), introc(2), IN-
TROF(2) recon(1), lamboot(1), lamhalt(1), lamnodes(1), wipe(1),
tping(1), lamgrow(1), lamshrink(1) mpicc(1), mpiCC(1), mpif77(1)
mpirun(1), lamclean(1) lamexec(1) mpitask(1), mpimsg(1) lam-
trace(1) lam_rfposix(2), tstdio(3), cubix(3) "LAM Frequently
Asked Questions";
at ";MPI Primer / Developing with LAM", Ohio Supercomputer Center
"MPI: A Message-Passing Interface Standard", Message-Passing In-
terface Forum, version 1.1
at ";MPI-2: Extensions to the Message Passing Interface", Message
Passing Interface Forum, version 2.0
at ";LAM/MPI ND User Guide / Introduction"
at ";MPI: It's Easy to Get Started" "MPI: Everyday Datatypes"
"MPI: Everyday Collective Communication" "Robust MPI Message De-
livery Through Guaranteed Resources";, MPI Developer's Conference,
1995