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RedHat 9 (Linux i386) - man page for mpi (redhat section 7)

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,

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