Ubuntu Manpage: lamssi_rpi - overview of LAM's RPI SSI modules

NAME

       lamssi_rpi - overview of LAM's RPI SSI modules

DESCRIPTION

       The  "kind"  for RPI SSI modules is "rpi".  Specifically, the string "rpi" (without the quotes) should be
       used to specify which RPI should be used on the mpirun command line with the -ssi switch.  For example:

       mpirun -ssi rpi tcp C my_mpi_program
           Specifies to use the tcp RPI (and to launch a single copy of the executable "foo" on each node).

       The "rpi" string is also used as a prefix send parameters to specific RPI modules.  For example:

       mpirun -ssi rpi tcp -ssi rpi_tcp_short 131072 C my_mpi_program
           Specifies to use the tcp RPI, and to pass in the value of 131072 (128K) as the short  message  length
           for  TCP messages.  See each RPI section below for a full description of parameters that are accepted
           by each RPI.

       LAM currently supports five different RPI SSI modules: gm, lamd, tcp, sysv, usysv.

SELECTING AN RPI MODULE

       Only one RPI module may be selected per command execution.  The selection of which module  occurs  during
       MPI_INIT,  and  is  used  for  the  duration of the MPI process.  It is erroneous to select different RPI
       modules for different processes.

       The kind for selecting an RPI is "rpi".  For example:

       mpriun -ssi rpi tcp C my_mpi_program
           Selects to use the tcp RPI and run a single copy of the foo exectuable on each node.

AVAILABLE MODULES

As with all SSI modules, it is possible to pass parameters at run time. This section discusses the
built-in LAM RPI modules, as well as the run-time parameters that they accept.

In the discussion below, the parameters are discussed in terms of kind and name. The kind and name may
be specified as command line arguments to the mpirun command with the -ssi switch, or they may be set in
environment variables of the form LAM_MPI_SSI_name=value. Note that using the -ssi command line switch
will take precendence over any environment variables.

If the RPI that is selected is unable to run (e.g., attempting to use the gm RPI when gm support was not
compiled into LAM, or if no gm hardware is available on the nodes), an appropriate error message will be
printed and execution will abort.

crtcp RPI
The crtcp RPI is a checkpoint/restart-able version of the tcp RPI (see below). It is separate from the
tcp RPI because the current implementation imposes a slight performance penalty to enable the ability to
checkpoint and restart MPI jobs. Its tunable parameters are the same as the tcp RPI. This RPI probably
only needs to be used when the ability to checkpoint and restart MPI jobs is required.

See the LAM/MPI User's Guide for more details on the crtcp RPI as well as the checkpoint/restart
capabilities of LAM/MPI. The lamssi_cr(7) manual page also contains additional information.

gm RPI
The gm RPI is used with native Myrinet networks. Please note that the gm RPI exists, but has not yet
been optimized. It gives significantly better performance than TCP over Myrinet networks, but has not
yet been properly tuned and instrumented in LAM.

That being said, there are several tunable parameters in the gm RPI:

rpi_gm_maxport N
If rpi_gm_port is not specified, LAM will attempt to find an open GM port to use for MPI
communications starting with port 1 and ending with the N value speified by the rpi_gm_maxport
parameter. If unspecified, LAM will try all existing GM ports.

rpi_gm_port N
LAM will attempt to use gm port N for MPI communications.

rpi_gm_tinymsglen N
Specifies the maximum message size (in bytes) for "tiny" messages (i.e., messages that are sent
entirely in one gm message). Tiny messages are memcpy'ed into the header before it is sent to the
destination, and memcpy'ed out of the header into the destination buffer on the receiver. Hence, it
is not advisable to make this value too large.

rpi_gm_fast 1
Specifies to use the "fast" protocol for sending short gm messages. Unreliable in the presence of GM
errors or timeouts; this parameter is not advised for MPI applications that essentially do not make
continual progress within MPI.

rpi_gm_cr 1
Enable checkpoint/restart behavior for gm. This can only be enabled if the gm rpi module was
compiled with support for the gm_get() function, which is disabled by default. See the LAM
Installation and User's Guides for more information on this parameter before you use it.

lamd RPI
The lamd RPI uses LAM's "out-of-band" communication mechanism for passing MPI messages. Specifically,
MPI messages are sent from the user process to the local LAM daemon, then to the remote LAM daemon (if
the destination process is on a different node), and then to the destination process.

While this adds latency to message passing because of the extra hops that each message must travel, it
allows for true asynchronous message passing. Since the LAM daemon is running in its own execution
space, it can make progress on message passing regardless of the state / status of the user's program.
This can be an overall net savings in performance and execution time for some classes of MPI programs.

It is expected that this RPI will someday become obsolete when LAM becomes multi-threaded and allows
progress to be made on message passing in separate threads rather than in separate processes.

The lamd RPI has no tunable parameters.

tcp RPI
The tcp RPI uses pure TCP for all MPI message passing. TCP sockets are opened between MPI processes and
are used for all MPI traffic.

The tcp RPI has one tunable parameter:

rpi_tcp_short <bytes>
Tells the tcp RPI the smallest size (in bytes) for a message to be considered "long". Short messages
are sent eagerly (even if the receiving side is not expecting them). Long messages use a rendevouz
protocol (i.e., a three-way handshake) such that the message is not actually sent until the receiver
is expecting it. This value defaults to 64k.

sysv RPI
The sysv RPI uses shared memory for communication between MPI processes on the same node, and TCP sockets
for communication between MPI processes on different nodes. System V semaphores are used to lock the
shared memory pools. This RPI is best used when running multiple MPI processes on uniprocessors (or
oversubscribed SMPs) because of the blocking / yielding nature of semaphores.

The sysv RPI has the following tunable parameters:

rpi_tcp_short <bytes>
Since the sysv RPI uses parts of the tcp RPI for off-node communication, this parameter also has
relevance to the sysv RPI. The meaning of this parameter is discussed in the tcp RPI section.

rpi_sysv_short <bytes>
Tells the sysv RPI the smallest size (in bytes) for a message to be considered "long". Short shared
memory messages are sent using a small "postbox" protocol; long messages use a more general shared
memory pool method. This value defaults to 8k.

rpi_sysv_pollyield <bool>
If set to a nonzero number, force the use of a system call to yield the processor. The system call
will be yield(), sched_yield(), or select() (with a 1ms timeout), depending what LAM's configure
script finds at configuration time. This value defaults to 1.

rpi_sysv_shmpoolsize <bytes>
The size of the shared memory pool that is used for long message transfers. It is allocated once on
each node for each MPI parallel job. Specifically, if multiple MPI processes from the same parallel
job are spawned on a single node, this pool will only be allocated once.

The configure script will try to determine a default size for the pool if none is explicitly
specified (you should always check this to see if it is reasonable). Larger values should improve
performance especially when an application passes large messages, but will also increase the system
resources used by each task.

rpi_sysv_shmmaxalloc <bytes>
To prevent a single large message transfer from monopolizing the global pool, allocations from the
pool are actually restricted to a maximum of rpi_sysv_shmmaxalloc bytes each. Even with this
restriction, it is possible for the global pool to temporarily become exhausted. In this case, the
transport will fall back to using the postbox area to transfer the message. Performance will be
degraded, but the application will progress.

The configure script will try to determine a default size for the maximum atomic transfer size if
none is explicitly specified (you should always check this to see if it is reasonable). Larger
values should improve performance especially when an application passes large messages, but will also
increase the system resources used by each task.

usysv RPI
The usysv RPI uses shared memory for communication between MPI processes on the same node, and TCP
sockets for communication between MPI processes on different nodes. Spin locks are used to lock the
shared memory pools. This RPI is best used when the multiple of MPI processes on a single node is less
than or equal to the number of processors because it allows LAM to fully occupy the processor while
waiting for a message and never be swapped out.

The usysv RPI has many of the same tunable parameters as the sysv RPI:

rpi_tcp_short <bytes>
Same meaning as in the sysv RPI.

rpi_usysv_short <bytes>
Same meaning as rpi_sysv_short in the sysv RPI.

rpi_usysv_pollyield <bool>
Same meaning as rpi_sysv_pollyield in the sysv RPI.

rpi_usysv_shmpoolsize <bytes>
Same meaning as rpi_sysv_shmpoolsize in the sysv RPI.

rpi_usysv_shmmaxalloc <bytes>
Same meaning as rpi_sysv_shmmaxalloc in the sysv RPI.

rpi_usysv_readlockpoll <iterations>
Number of iterations to spin before yielding the processor while waiting to read. This value
defaults to 10,000.

rpi_usysv_writelockpoll <iterations>
Number of iterations to spin before yielding the processor while waiting to write. This value
defaults to 10.

NAME

DESCRIPTION

SELECTING AN RPI MODULE

AVAILABLE MODULES

SEE ALSO