remove useless XBT_DEBUG

[simgrid.git] / doc / doxygen / options.doc

/*! \page options Configure SimGrid

A number of options can be given at runtime to change the default
SimGrid behavior. For a complete list of all configuration options
accepted by the SimGrid version used in your simulator, simply pass
the --help configuration flag to your program. If some of the options
are not documented on this page, this is a bug that you should please
report so that we can fix it. Note that some of the options presented
here may not be available in your simulators, depending on the
@ref install_src_config "compile-time options" that you used.

\tableofcontents

\section options_using Passing configuration options to the simulators

There is several way to pass configuration options to the simulators.
The most common way is to use the \c --cfg command line argument. For
example, to set the item \c Item to the value \c Value, simply
type the following: \verbatim
my_simulator --cfg=Item:Value (other arguments)
\endverbatim

Several \c `--cfg` command line arguments can naturally be used. If you
need to include spaces in the argument, don't forget to quote the
argument. You can even escape the included quotes (write \' for ' if
you have your argument between ').

Another solution is to use the \c \<config\> tag in the platform file. The
only restriction is that this tag must occure before the first
platform element (be it \c \<AS\>, \c \<cluster\>, \c \<peer\> or whatever).
The \c \<config\> tag takes an \c id attribute, but it is currently
ignored so you don't really need to pass it. The important par is that
within that tag, you can pass one or several \c \<prop\> tags to specify
the configuration to use. For example, setting \c Item to \c Value
can be done by adding the following to the beginning of your platform
file:
\verbatim
<config>
  <prop id="Item" value="Value"/>
</config>
\endverbatim

A last solution is to pass your configuration directly using the C
interface. If you happen to use the MSG interface, this is very easy
with the MSG_config() function. If you do not use MSG, that's a bit
more complex, as you have to mess with the internal configuration set
directly as follows. Check the \ref XBT_config "relevant page" for
details on all the functions you can use in this context, \c
_sg_cfg_set being the only configuration set currently used in
SimGrid.

@code
#include <xbt/config.h>

extern xbt_cfg_t _sg_cfg_set;

int main(int argc, char *argv[]) {
     SD_init(&argc, argv);

     /* Prefer MSG_config() if you use MSG!! */
     xbt_cfg_set_parse(_sg_cfg_set,"Item:Value");

     // Rest of your code
}
@endcode

\section options_index Index of all existing configuration options

\note
  The full list can be retrieved by passing "--help" and
     "--help-cfg" to an executable that uses SimGrid.

- \c clean-atexit: \ref options_generic_clean_atexit

- \c contexts/factory: \ref options_virt_factory
- \c contexts/guard-size: \ref options_virt_guard_size
- \c contexts/nthreads: \ref options_virt_parallel
- \c contexts/parallel_threshold: \ref options_virt_parallel
- \c contexts/stack-size: \ref options_virt_stacksize
- \c contexts/synchro: \ref options_virt_parallel

- \c cpu/maxmin-selective-update: \ref options_model_optim
- \c cpu/model: \ref options_model_select
- \c cpu/optim: \ref options_model_optim

- \c exception/cutpath: \ref options_exception_cutpath

- \c host/model: \ref options_model_select

- \c maxmin/precision: \ref options_model_precision
- \c maxmin/concurrency-limit: \ref options_concurrency_limit

- \c msg/debug-multiple-use: \ref options_msg_debug_multiple_use

- \c model-check: \ref options_modelchecking
- \c model-check/checkpoint: \ref options_modelchecking_steps
- \c model-check/communications-determinism: \ref options_modelchecking_comm_determinism
- \c model-check/dot-output: \ref options_modelchecking_dot_output
- \c model-check/hash: \ref options_modelchecking_hash
- \c model-check/property: \ref options_modelchecking_liveness
- \c model-check/max-depth: \ref options_modelchecking_max_depth
- \c model-check/record: \ref options_modelchecking_recordreplay
- \c model-check/reduction: \ref options_modelchecking_reduction
- \c model-check/replay: \ref options_modelchecking_recordreplay
- \c model-check/send-determinism: \ref options_modelchecking_comm_determinism
- \c model-check/sparse-checkpoint: \ref options_modelchecking_sparse_checkpoint
- \c model-check/termination: \ref options_modelchecking_termination
- \c model-check/timeout: \ref options_modelchecking_timeout
- \c model-check/visited: \ref options_modelchecking_visited

- \c network/bandwidth-factor: \ref options_model_network_coefs
- \c network/crosstraffic: \ref options_model_network_crosstraffic
- \c network/latency-factor: \ref options_model_network_coefs
- \c network/maxmin-selective-update: \ref options_model_optim
- \c network/model: \ref options_model_select
- \c network/optim: \ref options_model_optim
- \c network/sender_gap: \ref options_model_network_sendergap
- \c network/TCP-gamma: \ref options_model_network_gamma
- \c network/weight-S: \ref options_model_network_coefs

- \c ns3/TcpModel: \ref options_pls
- \c path: \ref options_generic_path
- \c plugin: \ref options_generic_plugin

- \c storage/max_file_descriptors: \ref option_model_storage_maxfd

- \c surf/precision: \ref options_model_precision

- \c <b>For collective operations of SMPI, please refer to Section \ref options_index_smpi_coll</b>
- \c smpi/async-small-thresh: \ref options_model_network_asyncsend
- \c smpi/bw-factor: \ref options_model_smpi_bw_factor
- \c smpi/coll-selector: \ref options_model_smpi_collectives
- \c smpi/comp-adjustment-file: \ref options_model_smpi_adj_file
- \c smpi/cpu-threshold: \ref options_smpi_bench
- \c smpi/display-timing: \ref options_smpi_timing
- \c smpi/grow-injected-times: \ref options_model_smpi_test
- \c smpi/host-speed: \ref options_smpi_bench
- \c smpi/IB-penalty-factors: \ref options_model_network_coefs
- \c smpi/iprobe: \ref options_model_smpi_iprobe
- \c smpi/iprobe-cpu-usage: \ref options_model_smpi_iprobe_cpu_usage
- \c smpi/init: \ref options_model_smpi_init
- \c smpi/keep-temps: \ref options_smpi_temps
- \c smpi/lat-factor: \ref options_model_smpi_lat_factor
- \c smpi/ois: \ref options_model_smpi_ois
- \c smpi/or: \ref options_model_smpi_or
- \c smpi/os: \ref options_model_smpi_os
- \c smpi/papi-events: \ref options_smpi_papi_events
- \c smpi/privatization: \ref options_smpi_privatization
- \c smpi/send-is-detached-thresh: \ref options_model_smpi_detached
- \c smpi/shared-malloc: \ref options_model_smpi_shared_malloc
- \c smpi/shared-malloc-hugepage: \ref options_model_smpi_shared_malloc
- \c smpi/simulate-computation: \ref options_smpi_bench
- \c smpi/test: \ref options_model_smpi_test
- \c smpi/wtime: \ref options_model_smpi_wtime

- \c <b>Tracing configuration options can be found in Section \ref tracing_tracing_options</b>.

- \c storage/model: \ref options_storage_model
- \c verbose-exit: \ref options_generic_exit

- \c vm/model: \ref options_vm_model

\subsection options_index_smpi_coll Index of SMPI collective algorithms options

TODO: All available collective algorithms will be made available via the ``smpirun --help-coll`` command.

\section options_model Configuring the platform models

\anchor options_storage_model
\anchor options_vm_model
\subsection options_model_select Selecting the platform models

SimGrid comes with several network, CPU and storage models built in, and you
can change the used model at runtime by changing the passed
configuration. The three main configuration items are given below.
For each of these items, passing the special \c help value gives
you a short description of all possible values. Also, \c --help-models
should provide information about all models for all existing resources.
   - \b network/model: specify the used network model
   - \b cpu/model: specify the used CPU model
   - \b host/model: specify the used host model
   - \b storage/model: specify the used storage model (there is currently only one such model - this option is hence only useful for future releases)
   - \b vm/model: specify the model for virtual machines (there is currently only one such model - this option is hence only useful for future releases)

As of writing, the following network models are accepted. Over
the time new models can be added, and some experimental models can be
removed; check the values on your simulators for an uptodate
information. Note that the CM02 model is described in the research report
<a href="ftp://ftp.ens-lyon.fr/pub/LIP/Rapports/RR/RR2002/RR2002-40.ps.gz">A
Network Model for Simulation of Grid Application</a> while LV08 is
described in
<a href="http://mescal.imag.fr/membres/arnaud.legrand/articles/simutools09.pdf">Accuracy Study and Improvement of Network Simulation in the SimGrid Framework</a>.

  - \b LV08 (default one): Realistic network analytic model
    (slow-start modeled by multiplying latency by 10.4, bandwidth by
    .92; bottleneck sharing uses a payload of S=8775 for evaluating RTT)
  - \anchor options_model_select_network_constant \b Constant: Simplistic network model where all communication
    take a constant time (one second). This model provides the lowest
    realism, but is (marginally) faster.
  - \b SMPI: Realistic network model specifically tailored for HPC
    settings (accurate modeling of slow start with correction factors on
    three intervals: < 1KiB, < 64 KiB, >= 64 KiB). See also \ref
    options_model_network_coefs "this section" for more info.
  - \b IB: Realistic network model specifically tailored for HPC
    settings with InfiniBand networks (accurate modeling contention
    behavior, based on the model explained in
    http://mescal.imag.fr/membres/jean-marc.vincent/index.html/PhD/Vienne.pdf).
    See also \ref options_model_network_coefs "this section" for more info.
  - \b CM02: Legacy network analytic model (Very similar to LV08, but
    without corrective factors. The timings of small messages are thus
    poorly modeled)
  - \b Reno: Model from Steven H. Low using lagrange_solve instead of
    lmm_solve (experts only; check the code for more info).
  - \b Reno2: Model from Steven H. Low using lagrange_solve instead of
    lmm_solve (experts only; check the code for more info).
  - \b Vegas: Model from Steven H. Low using lagrange_solve instead of
    lmm_solve (experts only; check the code for more info).

If you compiled SimGrid accordingly, you can use packet-level network
simulators as network models (see \ref pls_ns3). In that case, you have
two extra models, described below, and some 
\ref options_pls "specific additional configuration flags".
  - \b NS3: Network pseudo-model using the NS3 tcp model

Concerning the CPU, we have only one model for now:
  - \b Cas01: Simplistic CPU model (time=size/power)

The host concept is the aggregation of a CPU with a network
card. Three models exists, but actually, only 2 of them are
interesting. The "compound" one is simply due to the way our internal
code is organized, and can easily be ignored. So at the end, you have
two host models: The default one allows to aggregate an
existing CPU model with an existing network model, but does not allow
parallel tasks because these beasts need some collaboration between
the network and CPU model. That is why, ptask_07 is used by default
when using SimDag.
  - \b default: Default host model. Currently, CPU:Cas01 and
    network:LV08 (with cross traffic enabled)
  - \b compound: Host model that is automatically chosen if
    you change the network and CPU models
  - \b ptask_L07: Host model somehow similar to Cas01+CM02 but
    allowing "parallel tasks", that are intended to model the moldable
    tasks of the grid scheduling literature.

\subsection options_generic_plugin Plugins

SimGrid supports the use of plugins; currently, no known plugins
can be activated but there are use-cases where you may want to write
your own plugin (for instance, for logging).

Plugins can for instance define own classes that inherit from
existing classes (for instance, a class "CpuEnergy" inherits from
"Cpu" to assess energy consumption).

The plugin connects to the code by registering callbacks using
``signal.connect(callback)`` (see file ``src/surf/plugins/energy.cpp`` for
details).

\verbatim
    --cfg=plugin:Energy
\endverbatim

\note
    This option is case-sensitive: Energy and energy are not the same!

\subsection options_model_optim Optimization level of the platform models

The network and CPU models that are based on lmm_solve (that
is, all our analytical models) accept specific optimization
configurations.
  - items \b network/optim and \b cpu/optim (both default to 'Lazy'):
    - \b Lazy: Lazy action management (partial invalidation in lmm +
      heap in action remaining).
    - \b TI: Trace integration. Highly optimized mode when using
      availability traces (only available for the Cas01 CPU model for
      now).
    - \b Full: Full update of remaining and variables. Slow but may be
      useful when debugging.
  - items \b network/maxmin-selective-update and
    \b cpu/maxmin-selective-update: configure whether the underlying
    should be lazily updated or not. It should have no impact on the
    computed timings, but should speed up the computation.

It is still possible to disable the \c maxmin-selective-update feature
because it can reveal counter-productive in very specific scenarios
where the interaction level is high. In particular, if all your
communication share a given backbone link, you should disable it:
without \c maxmin-selective-update, every communications are updated
at each step through a simple loop over them. With that feature
enabled, every communications will still get updated in this case
(because of the dependency induced by the backbone), but through a
complicated pattern aiming at following the actual dependencies.

\subsection options_model_precision Numerical precision of the platform models

The analytical models handle a lot of floating point values. It is
possible to change the epsilon used to update and compare them through
the \b maxmin/precision item (default value: 0.00001). Changing it
may speedup the simulation by discarding very small actions, at the
price of a reduced numerical precision.

\subsection options_concurrency_limit Concurrency limit

The maximum number of variables per resource can be tuned through
the \b maxmin/concurrency-limit item. The default value is -1, meaning that
there is no such limitation. You can have as many simultaneous actions per
resources as you want. If your simulation presents a very high level of
concurrency, it may help to use e.g. 100 as a value here. It means that at
most 100 actions can consume a resource at a given time. The extraneous actions
are queued and wait until the amount of concurrency of the considered resource
lowers under the given boundary.

Such limitations help both to the simulation speed and simulation accuracy
on highly constrained scenarios, but the simulation speed suffers of this
setting on regular (less constrained) scenarios so it is off by default.

\subsection options_model_network Configuring the Network model

\subsubsection options_model_network_gamma Maximal TCP window size

The analytical models need to know the maximal TCP window size to take
the TCP congestion mechanism into account. This is set to 20000 by
default, but can be changed using the \b network/TCP-gamma item.

On linux, this value can be retrieved using the following
commands. Both give a set of values, and you should use the last one,
which is the maximal size.\verbatim
cat /proc/sys/net/ipv4/tcp_rmem # gives the sender window
cat /proc/sys/net/ipv4/tcp_wmem # gives the receiver window
\endverbatim

\subsubsection options_model_network_coefs Correcting important network parameters

SimGrid can take network irregularities such as a slow startup or
changing behavior depending on the message size into account.
You should not change these values unless you really know what you're doing.

The corresponding values were computed through data fitting one the
timings of packet-level simulators.

See
<a href="http://mescal.imag.fr/membres/arnaud.legrand/articles/simutools09.pdf">Accuracy Study and Improvement of Network Simulation in the SimGrid Framework</a>
for more information about these parameters.

If you are using the SMPI model, these correction coefficients are
themselves corrected by constant values depending on the size of the
exchange. Again, only hardcore experts should bother about this fact.

InfiniBand network behavior can be modeled through 3 parameters, as explained in
<a href="http://mescal.imag.fr/membres/jean-marc.vincent/index.html/PhD/Vienne.pdf">this PhD thesis</a>.
These factors can be changed through the following option:

\verbatim
smpi/IB-penalty-factors:"βe;βs;γs"
\endverbatim

By default SMPI uses factors computed on the Stampede Supercomputer at TACC, with optimal
deployment of processes on nodes.

\subsubsection options_model_network_crosstraffic Simulating cross-traffic

As of SimGrid v3.7, cross-traffic effects can be taken into account in
analytical simulations. It means that ongoing and incoming
communication flows are treated independently. In addition, the LV08
model adds 0.05 of usage on the opposite direction for each new
created flow. This can be useful to simulate some important TCP
phenomena such as ack compression.

For that to work, your platform must have two links for each
pair of interconnected hosts. An example of usable platform is
available in <tt>examples/platforms/crosstraffic.xml</tt>.

This is activated through the \b network/crosstraffic item, that
can be set to 0 (disable this feature) or 1 (enable it).

Note that with the default host model this option is activated by default.

\subsubsection options_model_network_sendergap Simulating sender gap

(this configuration item is experimental and may change or disapear)

It is possible to specify a timing gap between consecutive emission on
the same network card through the \b network/sender-gap item. This
is still under investigation as of writting, and the default value is
to wait 10 microseconds (1e-5 seconds) between emissions.

\subsubsection options_model_network_asyncsend Simulating asyncronous send

(this configuration item is experimental and may change or disapear)

It is possible to specify that messages below a certain size will be sent
as soon as the call to MPI_Send is issued, without waiting for the
correspondant receive. This threshold can be configured through the
\b smpi/async-small-thresh item. The default value is 0. This behavior can also be
manually set for MSG mailboxes, by setting the receiving mode of the mailbox
with a call to \ref MSG_mailbox_set_async . For MSG, all messages sent to this
mailbox will have this behavior, so consider using two mailboxes if needed.

This value needs to be smaller than or equals to the threshold set at
\ref options_model_smpi_detached , because asynchronous messages are
meant to be detached as well.

\subsubsection options_pls Configuring packet-level pseudo-models

When using the packet-level pseudo-models, several specific
configuration flags are provided to configure the associated tools.
There is by far not enough such SimGrid flags to cover every aspects
of the associated tools, since we only added the items that we
needed ourselves. Feel free to request more items (or even better:
provide patches adding more items).

When using NS3, the only existing item is \b ns3/TcpModel,
corresponding to the ns3::TcpL4Protocol::SocketType configuration item
in NS3. The only valid values (enforced on the SimGrid side) are
'NewReno' or 'Reno' or 'Tahoe'.

\subsection options_model_storage Configuring the Storage model

\subsubsection option_model_storage_maxfd Maximum amount of file descriptors per host

Each host maintains a fixed-size array of its file descriptors. You
can change its size (1024 by default) through the \b
storage/max_file_descriptors item to either enlarge it if your
application requires it or to reduce it to save memory space.

\section options_modelchecking Configuring the Model-Checking

To enable the SimGrid model-checking support the program should
be executed using the simgrid-mc wrapper:
\verbatim
simgrid-mc ./my_program
\endverbatim

Safety properties are expressed as assertions using the function
\verbatim
void MC_assert(int prop);
\endverbatim

\subsection options_modelchecking_liveness Specifying a liveness property

If you want to specify liveness properties (beware, that's
experimental), you have to pass them on the command line, specifying
the name of the file containing the property, as formatted by the
ltl2ba program.

\verbatim
--cfg=model-check/property:<filename>
\endverbatim

\subsection options_modelchecking_steps Going for stateful verification

By default, the system is backtracked to its initial state to explore
another path instead of backtracking to the exact step before the fork
that we want to explore (this is called stateless verification). This
is done this way because saving intermediate states can rapidly
exhaust the available memory. If you want, you can change the value of
the <tt>model-check/checkpoint</tt> variable. For example, the
following configuration will ask to take a checkpoint every step.
Beware, this will certainly explode your memory. Larger values are
probably better, make sure to experiment a bit to find the right
setting for your specific system.

\verbatim
--cfg=model-check/checkpoint:1
\endverbatim

\subsection options_modelchecking_reduction Specifying the kind of reduction

The main issue when using the model-checking is the state space
explosion. To counter that problem, several exploration reduction
techniques can be used. There is unfortunately no silver bullet here,
and the most efficient reduction techniques cannot be applied to any
properties. In particular, the DPOR method cannot be applied on
liveness properties since it may break some cycles in the exploration
that are important to the property validity.

\verbatim
--cfg=model-check/reduction:<technique>
\endverbatim

For now, this configuration variable can take 2 values:
 * none: Do not apply any kind of reduction (mandatory for now for
   liveness properties)
 * dpor: Apply Dynamic Partial Ordering Reduction. Only valid if you
   verify local safety properties (default value for safety checks).

\subsection options_modelchecking_visited model-check/visited, Cycle detection

In order to detect cycles, the model-checker needs to check if a new explored
state is in fact the same state than a previous one. In order to do this,
the model-checker can take a snapshot of each visited state: this snapshot is
then used to compare it with subsequent states in the exploration graph.

The \b model-check/visited is the maximum number of states which are stored in
memory. If the maximum number of snapshotted state is reached some states will
be removed from the memory and some cycles might be missed.

By default, no state is snapshotted and cycles cannot be detected.

\subsection options_modelchecking_termination model-check/termination, Non termination detection

The \b model-check/termination configuration item can be used to report if a
non-termination execution path has been found. This is a path with a cycle
which means that the program might never terminate.

This only works in safety mode.

This options is disabled by default.

\subsection options_modelchecking_dot_output model-check/dot-output, Dot output

If set, the \b model-check/dot-output configuration item is the name of a file
in which to write a dot file of the path leading the found property (safety or
liveness violation) as well as the cycle for liveness properties. This dot file
can then fed to the graphviz dot tool to generate an corresponding graphical
representation.

\subsection options_modelchecking_max_depth model-check/max_depth, Depth limit

The \b model-checker/max-depth can set the maximum depth of the exploration
graph of the model-checker. If this limit is reached, a logging message is
sent and the results might not be exact.

By default, there is not depth limit.

\subsection options_modelchecking_timeout Handling of timeout

By default, the model-checker does not handle timeout conditions: the `wait`
operations never time out. With the \b model-check/timeout configuration item
set to \b yes, the model-checker will explore timeouts of `wait` operations.

\subsection options_modelchecking_comm_determinism Communication determinism

The \b model-check/communications-determinism and
\b model-check/send-determinism items can be used to select the communication
determinism mode of the model-checker which checks determinism properties of
the communications of an application.

\subsection options_modelchecking_sparse_checkpoint Per page checkpoints

When the model-checker is configured to take a snapshot of each explored state
(with the \b model-checker/visited item), the memory consumption can rapidly
reach GiB ou Tib of memory. However, for many workloads, the memory does not
change much between different snapshots and taking a complete copy of each
snapshot is a waste of memory.

The \b model-check/sparse-checkpoint option item can be set to \b yes in order
to avoid making a complete copy of each snapshot: instead, each snapshot will be
decomposed in blocks which will be stored separately.
If multiple snapshots share the same block (or if the same block
is used in the same snapshot), the same copy of the block will be shared leading
to a reduction of the memory footprint.

For many applications, this option considerably reduces the memory consumption.
In somes cases, the model-checker might be slightly slower because of the time
taken to manage the metadata about the blocks. In other cases however, this
snapshotting strategy will be much faster by reducing the cache consumption.
When the memory consumption is important, by avoiding to hit the swap or
reducing the swap usage, this option might be much faster than the basic
snapshotting strategy.

This option is currently disabled by default.

\subsection options_mc_perf Performance considerations for the model checker

The size of the stacks can have a huge impact on the memory
consumption when using model-checking. By default, each snapshot will
save a copy of the whole stacks and not only of the part which is
really meaningful: you should expect the contribution of the memory
consumption of the snapshots to be \f$ \mbox{number of processes}
\times \mbox{stack size} \times \mbox{number of states} \f$.

The \b model-check/sparse-checkpoint can be used to reduce the memory
consumption by trying to share memory between the different snapshots.

When compiled against the model checker, the stacks are not
protected with guards: if the stack size is too small for your
application, the stack will silently overflow on other parts of the
memory (see \ref options_virt_guard_size).

\subsection options_modelchecking_hash Hashing of the state (experimental)

Usually most of the time of the model-checker is spent comparing states. This
process is complicated and consumes a lot of bandwidth and cache.
In order to speedup the state comparison, the experimental \b model-checker/hash
configuration item enables the computation of a hash summarizing as much
information of the state as possible into a single value. This hash can be used
to avoid most of the comparisons: the costly comparison is then only used when
the hashes are identical.

Currently most of the state is not included in the hash because the
implementation was found to be buggy and this options is not as useful as
it could be. For this reason, it is currently disabled by default.

\subsection options_modelchecking_recordreplay Record/replay (experimental)

As the model-checker keeps jumping at different places in the execution graph,
it is difficult to understand what happens when trying to debug an application
under the model-checker. Event the output of the program is difficult to
interpret. Moreover, the model-checker does not behave nicely with advanced
debugging tools such as valgrind. For those reason, to identify a trajectory
in the execution graph with the model-checker and replay this trajcetory and
without the model-checker black-magic but with more standard tools
(such as a debugger, valgrind, etc.). For this reason, Simgrid implements an
experimental record/replay functionnality in order to record a trajectory with
the model-checker and replay it without the model-checker.

When the model-checker finds an interesting path in the application execution
graph (where a safety or liveness property is violated), it can generate an
identifier for this path. In order to enable this behavious the
\b model-check/record must be set to \b yes. By default, this behaviour is not
enabled.

This is an example of output:

<pre>
[  0.000000] (0:@) Check a safety property
[  0.000000] (0:@) **************************
[  0.000000] (0:@) *** PROPERTY NOT VALID ***
[  0.000000] (0:@) **************************
[  0.000000] (0:@) Counter-example execution trace:
[  0.000000] (0:@) Path = 1/3;1/4
[  0.000000] (0:@) [(1)Tremblay (app)] MC_RANDOM(3)
[  0.000000] (0:@) [(1)Tremblay (app)] MC_RANDOM(4)
[  0.000000] (0:@) Expanded states = 27
[  0.000000] (0:@) Visited states = 68
[  0.000000] (0:@) Executed transitions = 46
</pre>

This path can then be replayed outside of the model-checker (and even in
non-MC build of simgrid) by setting the \b model-check/replay item to the given
path. The other options should be the same (but the model-checker should
be disabled).

The format and meaning of the path may change between different releases so
the same release of Simgrid should be used for the record phase and the replay
phase.

\section options_virt Configuring the User Process Virtualization

\subsection options_virt_factory Selecting the virtualization factory

In SimGrid, the user code is virtualized in a specific mechanism
that allows the simulation kernel to control its execution: when a user
process requires a blocking action (such as sending a message), it is
interrupted, and only gets released when the simulated clock reaches
the point where the blocking operation is done. This is explained
graphically in the [relevant tutorial, available online](http://simgrid.gforge.inria.fr/tutorials/simgrid-simix-101.pdf).

In SimGrid, the containers in which user processes are virtualized are
called contexts. Several context factory are provided, and you can
select the one you want to use with the \b contexts/factory
configuration item. Some of the following may not exist on your
machine because of portability issues. In any case, the default one
should be the most effcient one (please report bugs if the
auto-detection fails for you). They are approximately sorted here from
the slowest to the most efficient:

 - \b thread: very slow factory using full featured threads (either
   pthreads or windows native threads). They are slow but very
   standard. Some debuggers or profilers only work with this factory.
 - \b java: Java applications are virtualized onto java threads (that
   are regular pthreads registered to the JVM)
 - \b ucontext: fast factory using System V contexts (Linux and FreeBSD only)
 - \b boost: This uses the [context implementation](http://www.boost.org/doc/libs/1_59_0/libs/context/doc/html/index.html)
   of the boost library for a performance that is comparable to our
   raw implementation.\nInstall the relevant library (e.g. with the
   libboost-contexts-dev package on Debian/Ubuntu) and recompile
   SimGrid. Note that our implementation is not compatible with recent
   implementations of the library, and it will be hard to fix this since
   the library's author decided to hide an API that we were using.
 - \b raw: amazingly fast factory using a context switching mechanism
   of our own, directly implemented in assembly (only available for x86
   and amd64 platforms for now) and without any unneeded system call.

The main reason to change this setting is when the debugging tools get
fooled by the optimized context factories. Threads are the most
debugging-friendly contextes, as they allow to set breakpoints
anywhere with gdb and visualize backtraces for all processes, in order
to debug concurrency issues. Valgrind is also more comfortable with
threads, but it should be usable with all factories (but the callgrind
tool that really don't like raw and ucontext factories).

\subsection options_virt_stacksize Adapting the used stack size

Each virtualized used process is executed using a specific system
stack. The size of this stack has a huge impact on the simulation
scalability, but its default value is rather large. This is because
the error messages that you get when the stack size is too small are
rather disturbing: this leads to stack overflow (overwriting other
stacks), leading to segfaults with corrupted stack traces.

If you want to push the scalability limits of your code, you might
want to reduce the \b contexts/stack-size item. Its default value
is 8192 (in KiB), while our Chord simulation works with stacks as small
as 16 KiB, for example. For the thread factory, the default value
is the one of the system but you can still change it with this parameter.

The operating system should only allocate memory for the pages of the
stack which are actually used and you might not need to use this in
most cases. However, this setting is very important when using the
model checker (see \ref options_mc_perf).

\subsection options_virt_guard_size Disabling stack guard pages

A stack guard page is usually used which prevents the stack of a given
actor from overflowing on another stack. But the performance impact
may become prohibitive when the amount of actors increases.  The
option \b contexts:guard-size is the number of stack guard pages used.
By setting it to 0, no guard pages will be used: in this case, you
should avoid using small stacks (\b stack-size) as the stack will
silently overflow on other parts of the memory.

When no stack guard page is created, stacks may then silently overflow
on other parts of the memory if their size is too small for the
application. This happens:

- on Windows systems;
- when the model checker is enabled;
- and of course when guard pages are explicitely disabled (with \b contexts:guard-size=0).

\subsection options_virt_parallel Running user code in parallel

Parallel execution of the user code is only considered stable in
SimGrid v3.7 and higher, and mostly for MSG simulations. SMPI
simulations may well fail in parallel mode. It is described in
<a href="http://hal.inria.fr/inria-00602216/">INRIA RR-7653</a>.

If you are using the \c ucontext or \c raw context factories, you can
request to execute the user code in parallel. Several threads are
launched, each of them handling as much user contexts at each run. To
actiave this, set the \b contexts/nthreads item to the amount of
cores that you have in your computer (or lower than 1 to have
the amount of cores auto-detected).

Even if you asked several worker threads using the previous option,
you can request to start the parallel execution (and pay the
associated synchronization costs) only if the potential parallelism is
large enough. For that, set the \b contexts/parallel-threshold
item to the minimal amount of user contexts needed to start the
parallel execution. In any given simulation round, if that amount is
not reached, the contexts will be run sequentially directly by the
main thread (thus saving the synchronization costs). Note that this
option is mainly useful when the grain of the user code is very fine,
because our synchronization is now very efficient.

When parallel execution is activated, you can choose the
synchronization schema used with the \b contexts/synchro item,
which value is either:
 - \b futex: ultra optimized synchronisation schema, based on futexes
   (fast user-mode mutexes), and thus only available on Linux systems.
   This is the default mode when available.
 - \b posix: slow but portable synchronisation using only POSIX
   primitives.
 - \b busy_wait: not really a synchronisation: the worker threads
   constantly request new contexts to execute. It should be the most
   efficient synchronisation schema, but it loads all the cores of your
   machine for no good reason. You probably prefer the other less
   eager schemas.

\section options_tracing Configuring the tracing subsystem

The \ref outcomes_vizu "tracing subsystem" can be configured in several
different ways depending on the nature of the simulator (MSG, SimDag,
SMPI) and the kind of traces that need to be obtained. See the \ref
tracing_tracing_options "Tracing Configuration Options subsection" to
get a detailed description of each configuration option.

We detail here a simple way to get the traces working for you, even if
you never used the tracing API.


- Any SimGrid-based simulator (MSG, SimDag, SMPI, ...) and raw traces:
\verbatim
--cfg=tracing:yes --cfg=tracing/uncategorized:yes --cfg=triva/uncategorized:uncat.plist
\endverbatim
    The first parameter activates the tracing subsystem, the second
    tells it to trace host and link utilization (without any
    categorization) and the third creates a graph configuration file
    to configure Triva when analysing the resulting trace file.

- MSG or SimDag-based simulator and categorized traces (you need to declare categories and classify your tasks according to them)
\verbatim
--cfg=tracing:yes --cfg=tracing/categorized:yes --cfg=triva/categorized:cat.plist
\endverbatim
    The first parameter activates the tracing subsystem, the second
    tells it to trace host and link categorized utilization and the
    third creates a graph configuration file to configure Triva when
    analysing the resulting trace file.

- SMPI simulator and traces for a space/time view:
\verbatim
smpirun -trace ...
\endverbatim
    The <i>-trace</i> parameter for the smpirun script runs the
simulation with --cfg=tracing:yes and --cfg=tracing/smpi:yes. Check the
smpirun's <i>-help</i> parameter for additional tracing options.

Sometimes you might want to put additional information on the trace to
correctly identify them later, or to provide data that can be used to
reproduce an experiment. You have two ways to do that:

- Add a string on top of the trace file as comment:
\verbatim
--cfg=tracing/comment:my_simulation_identifier
\endverbatim

- Add the contents of a textual file on top of the trace file as comment:
\verbatim
--cfg=tracing/comment-file:my_file_with_additional_information.txt
\endverbatim

Please, use these two parameters (for comments) to make reproducible
simulations. For additional details about this and all tracing
options, check See the \ref tracing_tracing_options.

\section options_msg Configuring MSG

\subsection options_msg_debug_multiple_use Debugging MSG

Sometimes your application may try to send a task that is still being
executed somewhere else, making it impossible to send this task. However,
for debugging purposes, one may want to know what the other host is/was
doing. This option shows a backtrace of the other process.

Enable this option by adding

\verbatim
--cfg=msg/debug-multiple-use:on
\endverbatim

\section options_smpi Configuring SMPI

The SMPI interface provides several specific configuration items.
These are uneasy to see since the code is usually launched through the
\c smiprun script directly.

\subsection options_smpi_bench smpi/bench: Automatic benchmarking of SMPI code

In SMPI, the sequential code is automatically benchmarked, and these
computations are automatically reported to the simulator. That is to
say that if you have a large computation between a \c MPI_Recv() and a
\c MPI_Send(), SMPI will automatically benchmark the duration of this
code, and create an execution task within the simulator to take this
into account. For that, the actual duration is measured on the host
machine and then scaled to the power of the corresponding simulated
machine. The variable \b smpi/host-speed allows to specify the
computational speed of the host machine (in flop/s) to use when
scaling the execution times. It defaults to 20000, but you really want
to update it to get accurate simulation results.

When the code is constituted of numerous consecutive MPI calls, the
previous mechanism feeds the simulation kernel with numerous tiny
computations. The \b smpi/cpu-threshold item becomes handy when this
impacts badly the simulation performance. It specifies a threshold (in
seconds) below which the execution chunks are not reported to the
simulation kernel (default value: 1e-6).

\note
    The option smpi/cpu-threshold ignores any computation time spent
    below this threshold. SMPI does not consider the \a amount of these
    computations; there is no offset for this. Hence, by using a
    value that is too low, you may end up with unreliable simulation
    results.

In some cases, however, one may wish to disable simulation of
application computation. This is the case when SMPI is used not to
simulate an MPI applications, but instead an MPI code that performs
"live replay" of another MPI app (e.g., ScalaTrace's replay tool,
various on-line simulators that run an app at scale). In this case the
computation of the replay/simulation logic should not be simulated by
SMPI. Instead, the replay tool or on-line simulator will issue
"computation events", which correspond to the actual MPI simulation
being replayed/simulated. At the moment, these computation events can
be simulated using SMPI by calling internal smpi_execute*() functions.

To disable the benchmarking/simulation of computation in the simulated
application, the variable \b smpi/simulate-computation should be set to no.

\note
    This option just ignores the timings in your simulation; it still executes
    the computations itself. If you want to stop SMPI from doing that,
    you should check the SMPI_SAMPLE macros, documented in the section
    \ref SMPI_adapting_speed.

Solution                           | Computations actually executed? | Computations simulated ?
---------------------------------- | ------------------------------- | ------------------------
--cfg=smpi/simulate-computation:no | Yes                             | No, never
--cfg=smpi/cpu-threshold:42        | Yes, in all cases               | Only if it lasts more than 42 seconds
SMPI_SAMPLE() macro                | Only once per loop nest (see @ref SMPI_adapting_speed "documentation") | Always

\subsection options_model_smpi_adj_file smpi/comp-adjustment-file: Slow-down or speed-up parts of your code.

This option allows you to pass a file that contains two columns: The first column
defines the section that will be subject to a speedup; the second column is the speedup.

For instance:

\verbatim
"start:stop","ratio"
"exchange_1.f:30:exchange_1.f:130",1.18244559422142
\endverbatim

The first line is the header - you must include it.
The following line means that the code between two consecutive MPI calls on
line 30 in exchange_1.f and line 130 in exchange_1.f should receive a speedup
of 1.18244559422142. The value for the second column is therefore a speedup, if it is
larger than 1 and a slow-down if it is smaller than 1. Nothing will be changed if it is
equal to 1.

Of course, you can set any arbitrary filenames you want (so the start and end don't have to be
in the same file), but be aware that this mechanism only supports @em consecutive calls!

\note
    Please note that you must pass the \b -trace-call-location flag to smpicc
    or smpiff, respectively! This flag activates some macro definitions in our
    mpi.h / mpi.f files that help with obtaining the call location.

\subsection options_model_smpi_bw_factor smpi/bw-factor: Bandwidth factors

The possible throughput of network links is often dependent on the
message sizes, as protocols may adapt to different message sizes. With
this option, a series of message sizes and factors are given, helping
the simulation to be more realistic. For instance, the current
default value is

\verbatim
65472:0.940694;15424:0.697866;9376:0.58729;5776:1.08739;3484:0.77493;1426:0.608902;732:0.341987;257:0.338112;0:0.812084
\endverbatim

So, messages with size 65472 and more will get a total of MAX_BANDWIDTH*0.940694,
messages of size 15424 to 65471 will get MAX_BANDWIDTH*0.697866 and so on.
Here, MAX_BANDWIDTH denotes the bandwidth of the link.

\note
    The SimGrid-Team has developed a script to help you determine these
    values. You can find more information and the download here:
    1. http://simgrid.gforge.inria.fr/contrib/smpi-calibration-doc.html
    2. http://simgrid.gforge.inria.fr/contrib/smpi-saturation-doc.html

\subsection options_smpi_timing smpi/display-timing: Reporting simulation time

\b Default: 0 (false)

Most of the time, you run MPI code with SMPI to compute the time it
would take to run it on a platform. But since the
code is run through the \c smpirun script, you don't have any control
on the launcher code, making it difficult to report the simulated time
when the simulation ends. If you set the \b smpi/display-timing item
to 1, \c smpirun will display this information when the simulation ends. \verbatim
Simulation time: 1e3 seconds.
\endverbatim

\subsection options_smpi_temps smpi/keep-temps: not cleaning up after simulation

\b Default: 0 (false)

Under some conditions, SMPI generates a lot of temporary files.  They
usually get cleaned, but you may use this option to not erase these
files. This is for example useful when debugging or profiling
executions using the dlopen privatization schema, as missing binary
files tend to fool the debuggers.

\subsection options_model_smpi_lat_factor smpi/lat-factor: Latency factors

The motivation and syntax for this option is identical to the motivation/syntax
of smpi/bw-factor, see \ref options_model_smpi_bw_factor for details.

There is an important difference, though: While smpi/bw-factor \a reduces the
actual bandwidth (i.e., values between 0 and 1 are valid), latency factors
increase the latency, i.e., values larger than or equal to 1 are valid here.

This is the default value:

\verbatim
65472:11.6436;15424:3.48845;9376:2.59299;5776:2.18796;3484:1.88101;1426:1.61075;732:1.9503;257:1.95341;0:2.01467
\endverbatim

\note
    The SimGrid-Team has developed a script to help you determine these
    values. You can find more information and the download here:
    1. http://simgrid.gforge.inria.fr/contrib/smpi-calibration-doc.html
    2. http://simgrid.gforge.inria.fr/contrib/smpi-saturation-doc.html

\subsection options_smpi_papi_events smpi/papi-events: Trace hardware counters with PAPI

\warning 
    This option is experimental and will be subject to change.
    This feature currently requires superuser privileges, as registers are queried.
    Only use this feature with code you trust! Call smpirun for instance via
        smpirun -wrapper "sudo " <your-parameters>
    or run sudo sh -c "echo 0 > /proc/sys/kernel/perf_event_paranoid"
    In the later case, sudo will not be required.

\note
    This option is only available when SimGrid was compiled with PAPI support.

This option takes the names of PAPI counters and adds their respective values
to the trace files. (See Section \ref tracing_tracing_options.)

It is planned to make this feature available on a per-process (or per-thread?) basis.
The first draft, however, just implements a "global" (i.e., for all processes) set
of counters, the "default" set.

\verbatim
--cfg=smpi/papi-events:"default:PAPI_L3_LDM:PAPI_L2_LDM"
\endverbatim

\subsection options_smpi_privatization smpi/privatization: Automatic privatization of global variables

MPI executables are usually meant to be executed in separated processes, but SMPI is
executed in only one process. Global variables from executables will be placed
in the same memory zone and shared between processes, causing intricate bugs.
Several options are possible to avoid this, as described in the main
<a href="https://hal.inria.fr/hal-01415484">SMPI publication</a>.
SimGrid provides two ways of automatically privatizing the globals,
and this option allows to choose between them.

  - <b>no</b> (default): Do not automatically privatize variables.
  - <b>mmap</b> or <b>yes</b>: Runtime automatic switching of the data segments.\n
    SMPI stores a copy of each global data segment for each process,
    and at each context switch replaces the actual data with its copy
    from the right process. No copy actually occures as this mechanism
    uses mmap for efficiency. As such, it is for now limited to
    systems supporting this functionnality (all Linux and most BSD).\n
    Another limitation is that SMPI only accounts for global variables
    defined in the executable. If the processes use external global
    variables from dynamic libraries, they won't be switched
    correctly. The easiest way to solve this is to statically link
    against the library with these globals (but you should never
    statically link against the simgrid library itself).
  - <b>dlopen</b>: Link multiple times against the binary.\n  
    SMPI loads several copy of the same binary in memory, resulting in
    the natural duplication global variables. Since the dynamic linker
    refuses to link the same file several times, the binary is copied
    in a temporary file before being dl-loaded (it is erased right
    after loading).\n
    Note that this feature is somewhat experimental at time of writing
    (v3.16) but seems to work.\n
    This approach greatly speeds up the context switching, down to
    about 40 CPU cycles with our raw contextes, instead of requesting
    several syscalls with the \c mmap approach. Another advantage is
    that it permits to run the SMPI contexts in parallel, which is
    obviously not possible with the \c mmap approach.\n
    Further work may be possible to alleviate the memory and disk
    overconsumption. It seems that we could 
    <a href="https://lwn.net/Articles/415889/">punch holes</a>
    in the files before dl-loading them to remove the code and
    constants, and mmap these area onto a unique copy. This require
    to understand the ELF layout of the file, but would 
    reduce the disk- and memory- usage to the bare minimum. In
    addition, this would reduce the pressure on the CPU caches (in
    particular on instruction one).

\warning
  This configuration option cannot be set in your platform file. You can only
  pass it as an argument to smpirun.

\subsection options_model_smpi_detached Simulating MPI detached send

This threshold specifies the size in bytes under which the send will return
immediately. This is different from the threshold detailed in  \ref options_model_network_asyncsend
because the message is not effectively sent when the send is posted. SMPI still waits for the
correspondant receive to be posted to perform the communication operation. This threshold can be set
by changing the \b smpi/send-is-detached-thresh item. The default value is 65536.

\subsection options_model_smpi_collectives Simulating MPI collective algorithms

SMPI implements more than 100 different algorithms for MPI collective communication, to accurately
simulate the behavior of most of the existing MPI libraries. The \b smpi/coll-selector item can be used
 to use the decision logic of either OpenMPI or MPICH libraries (values: ompi or mpich, by default SMPI
uses naive version of collective operations). Each collective operation can be manually selected with a
\b smpi/collective_name:algo_name. Available algorithms are listed in \ref SMPI_use_colls .

\subsection options_model_smpi_iprobe smpi/iprobe: Inject constant times for calls to MPI_Iprobe

\b Default value: 0.0001

The behavior and motivation for this configuration option is identical with \a smpi/test, see
Section \ref options_model_smpi_test for details.

\subsection options_model_smpi_iprobe_cpu_usage smpi/iprobe-cpu-usage: Reduce speed for iprobe calls

\b Default value: 1 (no change from default behavior)

MPI_Iprobe calls can be heavily used in applications. To account correctly for the energy
cores spend probing, it is necessary to reduce the load that these calls cause inside
SimGrid.

For instance, we measured a max power consumption of 220 W for a particular application but 
only 180 W while this application was probing. Hence, the correct factor that should
be passed to this option would be 180/220 = 0.81.

\subsection options_model_smpi_init smpi/init: Inject constant times for calls to MPI_Init

\b Default value: 0

The behavior for this configuration option is identical with \a smpi/test, see
Section \ref options_model_smpi_test for details.

\subsection options_model_smpi_ois smpi/ois: Inject constant times for asynchronous send operations

This configuration option works exactly as \a smpi/os, see Section \ref options_model_smpi_os.
Of course, \a smpi/ois is used to account for MPI_Isend instead of MPI_Send.

\subsection options_model_smpi_os smpi/os: Inject constant times for send operations

In several network models such as LogP, send (MPI_Send, MPI_Isend) and receive (MPI_Recv)
operations incur costs (i.e., they consume CPU time). SMPI can factor these costs in as well, but the
user has to configure SMPI accordingly as these values may vary by machine.
This can be done by using smpi/os for MPI_Send operations; for MPI_Isend and
MPI_Recv, use \a smpi/ois and \a smpi/or, respectively. These work exactly as
\a smpi/ois.

\a smpi/os can consist of multiple sections; each section takes three values, for example:

\verbatim
    1:3:2;10:5:1
\endverbatim

Here, the sections are divided by ";" (that is, this example contains two sections).
Furthermore, each section consists of three values.

1. The first value denotes the minimum size for this section to take effect;
   read it as "if message size is greater than this value (and other section has a larger
   first value that is also smaller than the message size), use this".
   In the first section above, this value is "1".

2. The second value is the startup time; this is a constant value that will always
   be charged, no matter what the size of the message. In the first section above,
   this value is "3".

3. The third value is the \a per-byte cost. That is, it is charged for every
   byte of the message (incurring cost messageSize*cost_per_byte)
   and hence accounts also for larger messages. In the first
   section of the example above, this value is "2".

Now, SMPI always checks which section it should take for a given message; that is,
if a message of size 11 is sent with the configuration of the example above, only
the second section will be used, not the first, as the first value of the second
section is closer to the message size. Hence, a message of size 11 incurs the
following cost inside MPI_Send:

\verbatim
    5+11*1
\endverbatim

As 5 is the startup cost and 1 is the cost per byte.

\note
    The order of sections can be arbitrary; they will be ordered internally.

\subsection options_model_smpi_or smpi/or: Inject constant times for receive operations

This configuration option works exactly as \a smpi/os, see Section \ref options_model_smpi_os.
Of course, \a smpi/or is used to account for MPI_Recv instead of MPI_Send.

\subsection options_model_smpi_test smpi/test: Inject constant times for calls to MPI_Test

\b Default value: 0.0001

By setting this option, you can control the amount of time a process sleeps
when MPI_Test() is called; this is important, because SimGrid normally only
advances the time while communication is happening and thus,
MPI_Test will not add to the time, resulting in a deadlock if used as a
break-condition.

Here is an example:

\code{.unparsed}
    while(!flag) {
        MPI_Test(request, flag, status);
        ...
    }
\endcode

\note
    Internally, in order to speed up execution, we use a counter to keep track
    on how often we already checked if the handle is now valid or not. Hence, we
    actually use counter*SLEEP_TIME, that is, the time MPI_Test() causes the process
    to sleep increases linearly with the number of previously failed tests. This 
    behavior can be disabled by setting smpi/grow-injected-times to no. This will
    also disable this behavior for MPI_Iprobe.


\subsection options_model_smpi_shared_malloc smpi/shared-malloc: Factorize malloc()s

\b Default: global

If your simulation consumes too much memory, you may want to modify
your code so that the working areas are shared by all MPI ranks. For
example, in a bloc-cyclic matrix multiplication, you will only
allocate one set of blocs, and every processes will share them.
Naturally, this will lead to very wrong results, but this will save a
lot of memory so this is still desirable for some studies. For more on
the motivation for that feature, please refer to the 
<a href="https://simgrid.github.io/SMPI_CourseWare/topic_understanding_performance/matrixmultiplication/">relevant
section</a> of the SMPI CourseWare (see Activity #2.2 of the pointed
assignment). In practice, change the call to malloc() and free() into
SMPI_SHARED_MALLOC() and SMPI_SHARED_FREE().

SMPI provides 2 algorithms for this feature. The first one, called \c
local, allocates one bloc per call to SMPI_SHARED_MALLOC() in your
code (each call location gets its own bloc) and this bloc is shared
amongst all MPI ranks.  This is implemented with the shm_* functions
to create a new POSIX shared memory object (kept in RAM, in /dev/shm)
for each shared bloc.

With the \c global algorithm, each call to SMPI_SHARED_MALLOC()
returns a new adress, but it only points to a shadow bloc: its memory
area is mapped on a 1MiB file on disk. If the returned bloc is of size
N MiB, then the same file is mapped N times to cover the whole bloc. 
At the end, no matter how many SMPI_SHARED_MALLOC you do, this will
only consume 1 MiB in memory. 

You can disable this behavior and come back to regular mallocs (for
example for debugging purposes) using \c "no" as a value.

If you want to keep private some parts of the buffer, for instance if these
parts are used by the application logic and should not be corrupted, you
can use SMPI_PARTIAL_SHARED_MALLOC(size, offsets, offsets_count).

As an example,

\code{.C}
    mem = SMPI_PARTIAL_SHARED_MALLOC(500, {27,42 , 100,200}, 2);
\endcode

will allocate 500 bytes to mem, such that mem[27..41] and mem[100..199]
are shared and other area remain private.

Then, it can be deallocated by calling SMPI_SHARED_FREE(mem).

When smpi/shared-malloc:global is used, the memory consumption problem
is solved, but it may induce too much load on the kernel's pages table. 
In this case, you should use huge pages so that we create only one
entry per Mb of malloced data instead of one entry per 4k.
To activate this, you must mount a hugetlbfs on your system and allocate
at least one huge page:

\code{.sh}
    mkdir /home/huge
    sudo mount none /home/huge -t hugetlbfs -o rw,mode=0777
    sudo sh -c 'echo 1 > /proc/sys/vm/nr_hugepages' # echo more if you need more
\endcode

Then, you can pass the option --cfg=smpi/shared-malloc-hugepage:/home/huge
to smpirun to actually activate the huge page support in shared mallocs.

\subsection options_model_smpi_wtime smpi/wtime: Inject constant times for calls to MPI_Wtime

\b Default value: 0

By setting this option, you can control the amount of time a process sleeps
when MPI_Wtime() is called; this is important, because SimGrid normally only
advances the time while communication is happening and thus,
MPI_Wtime will not add to the time, resulting in a deadlock if used as a
break-condition.

Here is an example:

\code{.unparsed}
    while(MPI_Wtime() < some_time_bound) {
        ...
    }
\endcode

If the time is never advanced, this loop will clearly never end as MPI_Wtime()
always returns the same value. Hence, pass a (small) value to the smpi/wtime
option to force a call to MPI_Wtime to advance the time as well.


\section options_generic Configuring other aspects of SimGrid

\subsection options_generic_clean_atexit Cleanup before termination

The C / C++ standard contains a function called \b [atexit](http://www.cplusplus.com/reference/cstdlib/atexit/).
atexit registers callbacks, which are called just before the program terminates.

By setting the configuration option clean-atexit to 1 (true), a callback
is registered and will clean up some variables and terminate/cleanup the tracing.

TODO: Add when this should be used.

\subsection options_generic_path XML file inclusion path

It is possible to specify a list of directories to search into for the
\<include\> tag in XML files by using the \b path configuration
item. To add several directory to the path, set the configuration
item several times, as in \verbatim
--cfg=path:toto --cfg=path:tutu
\endverbatim

\subsection options_generic_exit Behavior on Ctrl-C

By default, when Ctrl-C is pressed, the status of all existing
simulated processes is displayed before exiting the simulation. This is very useful to debug your
code, but it can reveal troublesome in some cases (such as when the
amount of processes becomes really big). This behavior is disabled
when \b verbose-exit is set to 0 (it is to 1 by default).

\subsection options_exception_cutpath Truncate local path from exception backtrace

\verbatim
--cfg=exceptions/cutpath:1
\endverbatim

This configuration option is used to remove the path from the
backtrace shown when an exception is thrown. This is mainly useful for
the tests: the full file path makes the tests not reproducible, and
thus failing as we are currently comparing output. Clearly, the path
used on different machines are almost guaranteed to be different and
hence, the output would mismatch, causing the test to fail.

\section options_log Logging Configuration

It can be done by using XBT. Go to \ref XBT_log for more details.

*/