= Overview =

First revision of the Locatation Aware Mapping Algorithm (LAMA) RMAPS
component.  This component is used to effect many different types of
regular of process/processor affinity patterns.  Although quite
flexible in the patterns that it provides, it is ''not'' a
fully-arbitrary, rankfile-like solution for process/processor
affinity.  

Inspiried by !BlueGene-like network specifications, LAMA has a core
algorithm that is quite good at specifying regular patterns in
multiple "dimensions" (where "dimensions" are expressed in terms of
different hardware elements: processor hardware threads, cores,
sockets, ...etc.).  The LAMA core algorithm is described here:

  http://www.open-mpi.org/papers/cluster-2011-lama/

= LAMA Usage Levels =

LAMA allows specifying affinity multiple different ways:

 1. None: Speciying no affinity options to mpirun results in exactly
    the same behavior as today: no affinity is used.
 1. Simple: Using the mpirun options "--bind-to <WIDTH>" and "--map-to
    <LEVEL>" to indicate how "wide" each process should be bound
    (i.e., bind to a processor core, or to a processor socket, etc.)
    and how to lay out the processes (i.e., round robin by cores,
    sockets, etc.).  
 1. Expert: Using four new MCA parameters to effect process mapping
    and binding to processors.  These options are a bit complex, and
    are not for the faint at heart, but offer a high degree of
    (regular pattern) flexibility (each of these are described more
    fully below): 
    * rmaps_lama_map: a sequence of characters describing how to lay
      out processes
    * rmaps_lama_bind: a sequence of characters describing the
      resources to bind to each process
    * rmaps_lama_mppr: a sequence of characters describing the maximum
      number of processes to allow per resource (i.e., a specific
      definition of "oversubscription")
    * rmaps_lama_ordering: once all processes are in place, how to
      order the ranks in MPI_COMM_WORLD

We anticipate that most users will utilize the "None" and "Simple"
levels of affinity, and they continue to work just as they do with the
v1.6 series and SVN trunk.  

The Expert level was designed for two purposes:

 1. To provide a precise definition for the "Simple" level (i.e.,
 every
    --bind-to/--map-by option in the "Simple" level has a
 corresponding
    precise specification in the "Expert" level)
 1. As modern computing platforms become more complex, we simply
    cannot predict what application developers will need in terms of
    processor affinity.  LAMA is an attempt to provide a highly
    flexible mechanism that allows applications to utilize a variety
    of complex, unique affinity patterns beyond the common "bind to
    core" and "bind to socket" patterns.

= LAMA Simple Level =

The "Simple" level is pretty much the same as what Open MPI has
offered for years.  It supports the same --bind-to and --map-by
options that Open MPI has supported for a while, but expands their
scope a bit.

Specifically, the following options are available for both --bind-to
and --map-by:

 * slot
 * hwthread
 * core
 * l1cache
 * l2cache
 * l3cache
 * socket
 * numa
 * board
 * node

= LAMA Expert Level =

The "Expert" level requires some explanation.  I'll repeat my
disclaimer here: the LAMA Expert level is not for the meek.  It is
flexible, but complex.  '''Most users won't need the Expert level.'''

LAMA works in three phases: mapping, binding, and ordering.  Each is
described below.

== Expert: Mapping ==

Processes are paired with sets of resources.  For example, each
process may be paired with a single processor core.  Or each process
may be paired with an entire processor socket.  LAMA performs this
mapping, obeying the Max Processes Per Resource ("MPPR", pronounced
"mipper") limits.  More on MPPR, below.

Mapping can be performed across multiple hardware levels:

 * h: Hardware thread
 * c: Processor core
 * s: Processor socket
 * L1: L1 cache
 * L2: L2 cache
 * L3: L3 cache
 * N: NUMA node
 * b: Processor board
 * n: Server node

If the act of mapping is that of pairing MPI processes to the
resources that have been allocated to a job, one can easily imagine
looping through all the resources and assigning processes to them.

But to effect different process process layout patterns across those
resources, one may want to loop over those resources ''in a different
order.''  That is, if the above-mentioned nine hardware resources
(hardware thread, processor core, etc.) can be thought of as an
nine-dimensional space, you can imagine nine nested loops to traverse
all of them.  And you can imagine that changing the order of nesting
would change the traversal pattern.

LAMA accepts a sequence of tokens representing the above-mentioned
nine hardware resources to specify the order of looping when mapping
resources to processes.

For example, consider a "simple" traversal: csL1L2L3Nbnh.  Reading
that sequence of letters from left-to-right, it specifies mapping by
processor core, processor socket, L1 cache, L2 cache, L3 cache, NUMA
node, processor board, server node, and finally hardware thread.

Wait... what?  That string specifies resources from "smallest" to
"largest" -- with the exception of hardware threads.  Why are they
tacked on to the end?

In short, this string of letters means "map by round robin by core" --
(indeed, it exactly corresponds to the Simple level "--map-by core").
Specifically, LAMA traverses the string from left-to-right and maps
processes to all the resources indicated by that token (e.g., "c" for
processor core).  When there are no more resources indicated by that
token, it goes on to the next token.

Hence, in this case, LAMA will map the first process to the first
core, then it will map the second process to the second core, and so
on.

Once all the cores are exhausted, LAMA effectively ignores all the
other letters until "h" (because all the other resources are made up
of cores; when cores are exhausted, those resources are exhausted,
too).  

If there are still more processes to be mapped, LAMA will then
traverse all the hyperthreads -- meaning that the next process will be
mapped to the second hyperthread on the first core.  And the next
process will be mapped to the second hyperthread on the second core.
And so on.

Keep in mind that the cores involved may span many server nodes; we're
not just talking about the cores (etc.) in a single machine.

As another example, the sequence "sL1L2L3Nbnch" is exactly equivalent
to "--map-by socket" (i.e., LAMA maps the first process to the first
socket, the second process to the second socket, and so on).

The sequence of letter can be combined in many, many different ways to
produce many different regular mapping patterns.

=== Max Processes Per Resource (MPPR) ===

The MPPR is an expression that precisely defines the maximum number of
processes that can be mapped to any single resource.  In effect, it
defines the concept of "oversubscription."  Specifically, traditional
HPC wisdom is that "oversubscription" is when there is more than one
MPI process per processor core.

This conventional defintion is expressed in a MPPR string of "1:c"
(one process per core).  

But what if your MPI processes are multi-threaded, and they need
multiple processes per core?  You'd need a different description of
"oversubscription" in this case.  Perhaps you want to have one MPI
process per socket.  This would be expressed in a MPPR string of
"1:s".

The general form of an individual MPPR specification is an integer
follow by a colon, followed by any of the tokens from mapping can be
used in the MPPR specification.  For example "1:c" is pronounced "one
process per core."

Multiple MPPR specifications can be strung together into a
comma-delimited list, too.  All of these MPPR values and then taken
into account when mapping.  Here's some examples:

 * 1:c -- allow, at most, one process per processor core (i.e., don't
   schedule by hyperthread)
 * 1:s -- allow, at most, one process per processor socket (e.g., 
   that process may be multithreaded, or wants exclusive use of the
   socket's caches)
 * 1:s,2:n -- only allow one process per processor socket, but, at
   most, two processes per server node (e.g., if the two MPI processes
   will consume all the RAM on the server node, even if there are more
   processor cores available)

If mapping all processes to resources would exceed a MPPR limit, this
job is ruled to be oversubscribed.  If --oversubscribe was specified
on the mpirun command line, the job continues.  Otherwise, LAMA will
abort the job.

Additionally, if --oversubscribe is specified, LAMA will endlessly
cycle through the mapping token string untill all processes have been
mapped.

== Expert: Binding == 

Once processes have been paired with resources during the Mapping
stage, they are optionally bound to a (potentially different) set of
resources.  For example, processes may be mapped round robin by
processor socket, but bound to an individual processor core.

To be clear: if binding is not used, then mapping is effectively
reduced to "counting how many processes end up on each server node."
Without binding, there's no enforcement that a process will stay where
LAMA thinks it was placed.  

With binding, however, processes are bound to a set of hardware
threads.  The number of threads to which the process is bound is
sometimes referred to as the "binding width".  For example, if a
process is bound to all the hardware threads in a processor socket,
its "width" is the processor socket.

(note that we specifically do not say that the hardware threads are
sequential, even if they are all within a single resource such as a
processor core or socket.  BIOS ordering of hardware threads can be
wonky; so we only refer to "sets of hardware threads")

Bindings are expressed as an integer and a token from the mapping
string.  For example "1s" means "bind each process to one processor
socket" (there is no ":" in the binding string because the ":" is
pronounced as "per" when reading the MPPR string).

Note that it only makes sense to bind processes to a single resource
specification (unlike the MPPR specification, where multiple limits
can be specified).

== Expert: Ordering ==

Finally, processes are assigned a rank in MPI_COMM_WORLD.  LAMA
currently offers two ordering modes: sequential or natural:

 * Sequential: if you laid out all the hardware resources in a single
   line, and then overlaid all the MPI processes on top of them, they
   are ordered from 0 to (N-1) from left-to-right.
 * Natural: the ordering of ranks follows the mapping ordering.  For
   example, consider a server node with two processor sockets, each
   containing four cores.  The command line "mpirun -np 8 --bind-to
   core --map-by socket --order n a.out" would result in MCW ranks
   that look like this: [0 2 4 6] [1 3 5 7].

= Execution =

At this point, the job is fully mapped, optionally bound, and its
ranks in MPI_COMM_WORLD are ordered.  It now starts its execution.

= Final Notes =

Note that at this point, lama is not the default mapper.  It must be
activiated with "--mca rmaps lama".  We'll continue to do further
testing and comparitive analysis with the current set of ORTE mappers.

Also, note that the LAMA algorithm can handle heterogeneity between
hardware resources (e.g., an MPI job spanning server nodes with
differing numbers of processor sockets).  For lack of a longer
explanation (this commit message already long enough!), LAMA considers
each server node individually during mapping and binding.

See the LAMA paper for more details:
http://www.open-mpi.org/papers/cluster-2011-lama/